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Bacteria

Bacteria

Bacteria are very small organisms, usually consisting of one cell, that lack chlorophyll (a green pigment found in plants that allows for the production of food). Except for viruses, they are the smallest living things on Earth. Many bacteria are so small that a million of them, laid end-to-end, would measure no more than about five centimeters (two inches). The term bacteria is the plural form of the word bacterium, which represents a single organism.

Bacteria are found everywhere, in the air, soil, water, and inside your body and on your skin. They tend to multiply very rapidly under favorable conditions, forming colonies of millions or even billions of organisms within a space as small as a drop of water.

The Dutch merchant and amateur scientist Anton van Leeuwenhoek (16321723) was the first person to observe bacteria and other microorganisms. Using single-lens microscopes of his own design, he described bacteria and other microorganisms (calling them "animalcules") in a series of letters to the Royal Society of London between 1674 and 1723.

Today, bacteria are classified in the kingdom Procaryotae. This term refers to the fact that bacteria consist of prokaryotic cells, cells that do not contain a nucleus. (A nucleus is a structure that controls a cell's functions and contains genes. Genes carry the deoxyribonucleic acid [DNA] that determines the characteristics passed on from one generation to the next.) The genetic material of bacteria is contained, instead, within a single, circular chain of DNA.

Characteristics of bacteria

Bacteria are generally classified into three groups based on their shape. They are described as spherical (coccus), rodlike (bacillus), or spiral or corkscrew (spirochete [pronounced SPY-ruh-keet] or spirilla). Some bacteria also have a shape like that of a comma and are known as vibrio.

Words to Know

Aerobic bacteria: Bacteria that need oxygen in order to live and grow.

Anaerobic bacteria: Bacteria that do not require oxygen in order to live and grow.

Bacillus: A type of bacterium with a rodlike shape.

Capsule: A thick, jelly-like material that surrounds the surface of some bacteria cells.

Coccus: A type of bacterium with a spherical (round) shape.

Decomposers: Bacteria that break down dead organic matter.

Fimbriae: Short, hairlike projections that may form on the outer surface of a bacterial cell.

Fission: A form of reproduction in which a single cell divides to form two new cells.

Flagella: Whiplike projections on the surface of bacterial cells that make movement possible.

Pasteurization: A process by which bacteria in food are killed by heating the food to a particular temperature for some given period of time.

Pili: Projections that join pairs of bacteria together, making possible the transfer of genetic material between them.

Prokaryote: A cell that has no distinct nucleus.

Spirilla: A type of bacterium with a spiral shape.

Spirochetes: A type of bacterium with a spiral shape.

Toxin: A poisonous chemical.

Vibrio: A type of bacterium with a comma-like shape.

As the drawing of the anatomy of a typical bacterium shows, the cytoplasm of all bacteria is enclosed within a cell membrane that is itself surrounded by a rigid cell wall. Bacteria also produce a thick, jelly-like material on the surface of the cell wall. When that material forms a distinct outside layer, it is known as a capsule.

Many rod, spiral, and comma-shaped bacteria have whiplike limbs, known as flagella, attached to the outside of their cells. They use these flagella for movement by waving them back and forth. Other bacteria move simply by wiggling their whole cell back and forth. Some bacteria are unable to move at all.

Two other kinds of projections found on bacterial surfaces include fimbriae and pili. Fimbriae (pronounced FIM-bree-ay) are tiny bristles that allow bacteria to attach themselves to other objects or to surfaces.

Pili are tiny whiskers that allow bacterial cells to exchange genetic material with each other.

Bacterial growth

The term bacterial growth generally refers to the growth of a group of bacteria rather than a single cell. Single cells generally do not get larger in size, so the term growth refers to the reproduction of cells.

Bacteria most commonly reproduce by fission, the process by which a single cell divides to produce two new cells. The process of fission may take anywhere from 15 minutes to 16 hours, depending on the type of bacterium.

A number of factors influence the rate at which bacterial growth occurs, the most important of which are moisture, temperature, and pH. Bacteria

are about 80 to 90 percent water. If too much water passes into or out of a bacterial cell, the cell dies. The bacterial cell wall provides protection against the gain or loss of water in most ordinary circumstances. But conditions may be such as to produce an unusually large gain or loss of water.

For example, if a bacterial cell is placed in a highly concentrated solution of salt water, water begins to pass out of a cell and into the salt water. The cell begins to shrink and is unable to carry on normal life functions. It cannot grow and will eventually die. On the other hand, an excess of water can be harmful to bacteria also. If water flows into a bacterial cell, the cell begins to swell and may eventually burst, resulting in the death of the cell.

All bacteria have a particular temperature range at which they can survive. For a specific type of bacteria, that range can be very high, very low, or somewhere in between, although it is always a narrow range. Most bacteria thrive at temperatures close to that of the human body (37°C or 98.6°F). But some bacteria prefer cold temperatures as low as freezing (0°C or 32°F), and others require very hot temperatures such as those found in hot springs (50°C to 90°C or 120°F to 200°F). The most extreme conditions in which bacteria have been found are around the hydrothermal vents near the Galapagos Islands. The temperatures near these cracks in the ocean floor is about 350°C (660°F), an environment just right, apparently, for the bacteria that live there.

Another factor affecting bacterial growth is pH, the acidity of a solution. Most bacteria require a pH of 6.7 to 7.5 (slightly more or less acidic than pure water). Other bacteria, however, can survive at a pH more severe than that of battery acid.

Finally, bacteria may or may not require oxygen to grow. Those that do need oxygen are called aerobic bacteria, while those that do not are known as anaerobic bacteria. Anaerobic bacteria have evolved ways of using substances other than oxygen, such as compounds of nitrogen, to obtain the energy they need to survive and grow.

Harmless, beneficial, and harmful bacteria

Bacteria can also be classified according to the effects they have on human life. Some bacteria are used to supply products that improve human life, others cause disease, while still others have no overall affect at all on human life.

Helpful bacteria. Bacteria make possible the digestion of foods in many kinds of animals. Cows, deer, sheep, and other ruminants, for example, have a large organ known as the rumen in which bacteria live and help break down cellulose fibers and other tough plant materials. In humans, bacteria known as Escherichia coli (E. coli ) occur everywhere in the digestive system, aiding in the breakdown of many kinds of foods. Bacteria are also responsible for the production of vitamin K and certain B vitamins.

Certain kinds of bacteria are also essential in the decay and decomposition of waste materials. Such bacteria are known as decomposers. Decomposers attack dead materials and break them down into simpler forms that can be used as nutrients by plants.

Finally, bacteria are involved in the production of many foods eaten humans. For example, bacteria that cause milk to become sour are used in the production of cottage cheese, buttermilk, and yogurt. Vinegar and sauerkraut are also produced by the action of bacteria on ethyl alcohol and cabbage, respectively.

Harmful bacteria. It seems likely, however, that most people know bacteria best because of the diseases they cause. Some of these diseases are produced when bacteria attack directly the tissues in a plant or animal. For example, fruits and vegetables that become discolored as they are growing may be under attack by bacteria.

Bacteria also attack organisms by releasing chemicals that are poisonous to plants and animals. Such poisons are known as toxins. A familiar toxin-producing bacterium is Clostridium tetani, responsible for the disease known as tetanus. Tetanus is a condition in which one's muscles are paralyzed, explaining its common name of lockjaw. A related bacterium, Clostridium botulinum, releases a toxin that causes the most severe form of food poisoning, botulism.

Some forms of dangerous bacteria live on the human skin, but cause no harm unless they are able to enter the blood stream through a break in the skin. Among these bacteria is Staphylococcus, responsible for the potentially fatal toxic shock syndrome. And although E. coli is helpful within the digestive system, if it is ingested and enters the bloodstream it causes severe cramping, diarrhea, and possibly even death.

Most forms of food preservation, such as freezing and drying, are designed to kill or inactivate bacteria that would otherwise damage food or cause disease. One of the most common methods of destroying bacteria in foods is pasteurization. Pasteurization is the process of heating a food product to a particular temperature for some given period of time. The temperature and time are selected to be sure that all bacteria in the food are killed by the process. The pasteurization of milk has made it possible to insure safe supplies of one of the most popular of all human foods.

Hardy survivors

In October 2000, a team of biologists claimed to have revived a bacterium that existed 250 million years ago, well before the age of the dinosaurs. They found the bacterium in a drop of fluid trapped in a crystal of rock salt that had been excavated from an air duct supplying a radioactive waste dump 1,850 feet (564 meters) below Earth's surface near Carlsbad, New Mexico. When the biologists drilled into the pocket of fluid in the crystal and mixed nutrients with the fluid, bacteria soon appeared. However, other scientists quickly suggested that the bacteria that grew was simply modern bacteria that had infected the crystal sample. The questioning scientists also pointed out that it would be impossible for the bacterium's DNA (a complex molecule that stores and transmits genetic information) to have survived more than a few thousand years, at best.

Regardless of the debate, bacteria have been around since the dawn of life on Earth, and they have continued to evolve. A major problem facing the medical community today is the ability of disease-causing bacteria to develop a resistance to antibiotics and other antibacterial drugs. These types of bacteria have been able to change their forms or have even been able to secrete enzymes that destroy the antibiotics. Since the

development and use of antibiotics in the 1940s, most known bacterial diseases have developed a resistance to at least one type of antibiotic.

[See also Antibiotics; Antiseptics; Fermentation ]

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bacteria

bacteria [pl. of bacterium], microscopic unicellular prokaryotic organisms characterized by the lack of a membrane-bound nucleus and membrane-bound organelles. Once considered a part of the plant kingdom, bacteria were eventually placed in a separate kingdom, Monera. Bacteria fall into one of two groups, Archaebacteria (ancient forms thought to have evolved separately from other bacteria) and Eubacteria. A recently proposed system classifies the Archaebacteria, or Archaea, and the Eubacteria, or Bacteria, as major groupings (sometimes called domains) above the kingdom level.

Bacteria were the only form of life on earth for 2 billion years. They were first observed by Antony van Leeuwenhoek in the 17th cent.; bacteriology as an applied science began to develop in the late 19th cent. as a result of research in medicine and in fermentation processes, especially by Louis Pasteur and Robert Koch.

Bacteria are remarkably adaptable to diverse environmental conditions: they are found in the bodies of all living organisms and on all parts of the earth—in land terrains and ocean depths, in arctic ice and glaciers, in hot springs, and even in the stratosphere. Our understanding of bacteria and their metabolic processes has been expanded by the discovery of species that can live only deep below the earth's surface and by species that thrive without sunlight in the high temperature and pressure near hydrothermal vents on the ocean floor. There are more bacteria, as separate individuals, than any other type of organism; there can be as many as 2.5 billion bacteria in one gram of fertile soil.

Characteristics

Bacteria are grouped in a number of different ways. Most bacteria are of one of three typical shapes—rod-shaped (bacillus), round (coccus, e.g., streptococcus), and spiral (spirillum). An additional group, vibrios, appear as incomplete spirals. The cytoplasm and plasma membrane of most bacterial cells are surrounded by a cell wall; further classification of bacteria is based on cell wall characteristics (see Gram's stain). They can also be characterized by their patterns of growth, such as the chains formed by streptococci. Many bacteria, chiefly the bacillus and spirillum forms, are motile, swimming about by whiplike movements of flagella; other bacteria have rigid rodlike protuberances called pili that serve as tethers.

Some bacteria (those known as aerobic forms) can function metabolically only in the presence of free or atmospheric oxygen; others (anaerobic bacteria) cannot grow in the presence of free oxygen but obtain oxygen from compounds. Facultative anaerobes can grow with or without free oxygen; obligate anaerobes are poisoned by oxygen.

Reproduction

In bacteria the genetic material is organized in a continuous strand of DNA. This circle of DNA is localized in an area called the nucleoid, but there is no membrane surrounding a defined nucleus as there is in the eukaryotic cells of protists, fungi, plants, and animals (see eukaryote). In addition to the nucleoid, the bacterial cell may include one or more plasmids, separate circular strands of DNA that can replicate independently, and that are not responsible for the reproduction of the organism. Drug resistance is often conveyed via plasmid genes.

Reproduction is chiefly by binary fission, cell division yielding identical daughter cells. Some bacteria reproduce by budding or fragmentation. Despite the fact that these processes should produce identical generations, the rapid rate of mutation possible in bacteria makes them very adaptable. Some bacteria are capable of specialized types of genetic recombination, which involves the transfer of nucleic acid by individual contact (conjugation), by exposure to nucleic acid remnants of dead bacteria (transformation), by exchange of plasmid genes, or by a viral agent, the bacteriophage (transduction). Under unfavorable conditions some bacteria form highly resistant spores with thickened coverings, within which the living material remains dormant in altered form until conditions improve. Others, such as the radioactivity-resistant Deinococcus radiodurans, can withstand serious damage by repairing their own DNA.

Nutrition

Most bacteria are heterotrophic, living off other organisms. Most of these are saprobes, bacteria that live off dead organic matter. The bacteria that cause disease are heterotrophic parasites. There are also many non-disease-causing bacterial parasites, many of which are helpful to their hosts. These include the "normal flora" of the human body.

Autotrophic bacteria manufacture their own food by the processes of photosynthesis and chemosynthesis (see autotroph). The photosynthetic bacteria include the green and purple bacteria and the cyanobacteria. Many of the thermophilic archaebacteria are chemosynthetic autotrophs.

Beneficial Bacteria

Harmless and beneficial bacteria far outnumber harmful varieties. Thousands of bacterial species live commensally in humans, and many provide health benefits to humans, aiding in digestion, for example, or helping to prevent the establishment of colonies of pathogenic bacteria. Because they are capable of producing so many enzymes necessary for the building up and breaking down of organic compounds, bacteria are employed extensively by humans—for soil enrichment with leguminous crops (see nitrogen cycle), for preservation by pickling, for fermentation (as in the manufacture of alcoholic beverages, vinegar, and certain cheeses), for decomposition of organic wastes (in septic tanks, in some sewage disposal plants, and in agriculture for soil enrichment) and toxic wastes, and for curing tobacco, retting flax, and many other specialized processes. Bacteria frequently make good objects for genetic study: large populations grown in a short period of time facilitate detection of mutations, or rare variations.

Pathogenic Bacteria

Bacterial parasites that cause disease are called pathogens. Among bacterial plant diseases are leaf spot, fire blight, and wilts; animal diseases caused by bacteria include tuberculosis, cholera, syphilis, typhoid fever, and tetanus. Some bacteria attack the tissues directly; others produce poisonous substances called toxins. Natural defense against harmful bacteria is provided by antibodies (see immunity). Certain bacterial diseases, e.g., tetanus, can be prevented by injection of antitoxin or of serum containing antibodies against specific bacterial antigens; immunity to some can be induced by vaccination; and certain specific bacterial parasites are killed by antibiotics.

New strains of more virulent bacterial pathogens, many of them resistant to antibiotics, have emerged in recent years. Many believe this to be due to the overuse of antibiotics, both in prescriptions for minor, self-limiting ailments and as growth enhancers in livestock; such overuse increases the likelihood of bacterial mutations. For example, a variant of the normally harmless Escherichia coli has caused serious illness and death in victims of food poisoning. See also drug resistance.

Bibliography

See P. Singleton, Introduction to Bacteria (1992); W. Biddle, A Field Guide to Germs (1995).

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bacteria

bacteria A diverse group of ubiquitous microorganisms all of which consist of only a single cell that lacks a distinct nuclear membrane and has a cell wall of a unique composition (see illustration). Bacteria constitute the prokaryotic organisms of the living world. However, their classification is a controversial issue. It is now recognized, on the basis of differences in ribosomal RNA structure and nucleotide sequences (see molecular systematics), that prokaryotes form two evolutionarily distinct groups. Traditionally these were placed in a single kingdom, variously named Bacteria or Prokaryotae, which was divided into two subkingdoms: Archaea (archaebacteria), including the descendants of ancient bacterial groups; and Eubacteria, representing the vast majority of present-day bacteria. However, it is now recognized that these groups of prokaryotes are so distinct that they should each be raised to the status of domain: Archaea (the archaebacteria, containing a variable number of kingdoms) and Bacteria (containing a single kingdom, Eubacteria). Generally speaking, the term ‘bacteria’ includes both archaebacteria and eubacteria.

Bacteria can be characterized in a number of ways, for example by their reaction with Gram's stain or on the basis of their metabolic requirements (e.g. whether or not they require oxygen: see aerobic respiration; anaerobic respiration) and shape. A bacterial cell may be spherical (see coccus), rodlike (see bacillus), spiral (see spirillum), comma-shaped (see vibrio), corkscrew-shaped (see spirochaete), or filamentous, resembling a fungal cell. The majority of bacteria range in size from 0.5 to 5 μm. Many are motile, bearing flagella, possess an outer slimy capsule, and produce resistant spores (see endospore). In general bacteria reproduce only asexually, by simple division of cells, but a few groups undergo a form of sexual reproduction (see conjugation). Bacteria are largely responsible for decay and decomposition of organic matter, producing a cycling of such chemicals as carbon (see carbon cycle), oxygen, nitrogen (see nitrogen cycle), and sulphur (see sulphur cycle). A few bacteria obtain their food by means of photosynthesis, including the Cyanobacteria; some are saprotrophs; and others are parasites, causing disease. The symptoms of bacterial infections are produced by toxins.

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bacteria

bacteria Unicellular micro‐organisms, ranging between 0.5 to 5 μm in size. They may be classified on the basis of their shape: spherical (coccus), rodlike (bacilli), spiral (spirillum), comma‐shaped (vibrio), corkscrew‐shaped (spirochaetes), or filamentous. Other classifications are based on whether or not they are: stained by Gram's stain; aerobic or anaerobic; and autotrophic or heterotrophic. Some form spores that are resistant to heat and sterilizing agents.

Bacteria are responsible for much food spoilage, and for disease (pathogenic bacteria), but they are also made use of, for example in the pickling process and fermentation of milk, as well as in the manufacture of vitamins and amino acids and a variety of enzymes and hormones.

Between 45 and 85% of the dry matter of bacteria is protein, and some can be grown on petroleum residues or methanol for use in animal feed.

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bacteria

bacteria Simple, unicellular, microscopic organisms. They lack a clearly defined nucleus and most are without chlorophyll. Many are motile, swimming by means of whip-like flagella. Most multiply by fission. In adverse conditions, many can remain dormant inside highly resistant spores with thick protective coverings. Bacteria may be aerobic or anaerobic. Although pathogenic bacteria are a major cause of human disease, many bacteria are harmless or even beneficial to humans by providing an important link in food chains, by decomposing plant and animal tissue, or converting free nitrogen and sulphur into amino acids and other compounds that plants and animals can use. Some contain a form of chlorophyll and carry out photosynthesis. Bacteria belong to the kingdom Prokaryotae. See also Archaebacteria; Eubacteria

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bacteria

bacteria (bak-teer-iă) pl. n. (sing. bacterium) a group of microorganisms all of which lack a distinct nuclear membrane and most of which have a cell wall of unique composition. Most bacteria are unicellular; the cells may be spherical (see coccus), rodlike (see bacillus), spiral (see Spirillum), comma-shaped (see Vibrio) or corkscrew-shaped (see spirochaete). Generally, they range in size between 0.5 and 5 μm. Motile species bear one or more fine hairs (flagella) arising from their surface. Bacteria reproduce asexually by simple division of cells. They live in soil, water, or air or as parasites of humans, animals, and plants. Some parasitic bacteria cause diseases by producing poisons (see endotoxin, exotoxin).
bacterial adj.

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bacteria

bacteria From the Greek ‘small rod’, these are single-celled microscopic organisms found everywhere in the environment and on the human body. They are visible with a light microscope. They are usually harmless, but some are the cause of disease in plants, animals, and humans. They may be broadly split into ‘Gram positive’ and ‘Gram negative’, depending on their reaction to certain stains (described by Gram, a Danish physician in 1884); or into rod-shaped (bacilli) and spherical (cocci). They replicate by fission (splitting), and most have a complex cell wall structure.

Angharad Puw Davies


See microorganisms.

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Bacteria

32. Bacteria

bacteriology
the branch of biology that studies and classifies bacteria. bacteriologist, n. bacteriologic, bacteriological , adj.
chromatophobia
a strong resistance by bacteria to absorbing stains. chromatophobic, adj.
hemophile, haemophile
a bacterium that grows well in the presence of hemoglobin. hemophilic, adj.
microbiology
the branch of biology that studies microorganisms, including bacteria, viruses, fungi, and pathogenic protozoa. microbiologist, n.
microphobia, microbiophobia
an abnormal fear of microorganisms. microphobic, adj.

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bacterium

bac·te·ri·um / bakˈti(ə)rēəm/ • n. (pl. -te·ri·a / -ˈti(ə)rēə/ ) a member of a large group of unicellular microorganisms that have cell walls but lack organelles and an organized nucleus, including some that can cause disease. DERIVATIVES: bac·te·ri·al / -ˈti(ə)rēəl/ adj. ORIGIN: mid 19th cent.: modern Latin, from Greek baktērion, diminutive of baktēria ‘staff, cane’ (because the first ones to be discovered were rod-shaped). Compare with bacillus.

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bacterium

bacterium XIX. — modL. — Gr. baktḗrion, dim. of baktēríā staff, cane. Cf. bacillus.

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bacteria

bac·te·ri·a / bakˈti(ə)rēə/ • plural form of bacterium.

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bacterium

bacterium
1. A single bacterial cell.

2. A particular prokaryotic organism (see PROKARYOTE).

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Bacteria

Bacteria The domain comprising the kingdom Eubacteria.

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Bacteria

Bacteria The only kingdom in the domain Eubacteria.

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bacteria

bacteria The only kingdom in the domain Eubacteria.

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bacterium

bacterium (bak-teer-iŭm) n. see bacteria.

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bacteria

bacteriabarrier, carrier, farrier, harrier, tarrier •Calabria, Cantabria •Andrea • Kshatriya • Bactria •Amu Darya, aria, Zaria •Alexandria •Ferrier, terrier •destrier •aquaria, area, armamentaria, Bavaria, Bulgaria, caldaria, cineraria, columbaria, filaria, frigidaria, Gran Canaria, herbaria, honoraria, malaria, pulmonaria, rosaria, sacraria, Samaria, solaria, tepidaria, terraria •atria, gematria •Assyria, Illyria, Styria, SyriaLaurier, warrior •hypochondria, mitochondria •Austria •auditoria, ciboria, conservatoria, crematoria, emporia, euphoria, Gloria, moratoria, phantasmagoria, Pretoria, sanatoria, scriptoria, sudatoria, victoria, Vitoria, vomitoria •Maurya •courier, Fourier •currier, furrier, spurrier, worrier •Cumbria, Northumbria, Umbria •Algeria, anterior, bacteria, Bashkiria, cafeteria, criteria, cryptomeria, diphtheria, exterior, hysteria, Iberia, inferior, interior, Liberia, listeria, Nigeria, posterior, Siberia, superior, ulterior, wisteria •Etruria, Liguria, Manchuria, Surya

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Bacteria

Bacteria

Definition

Bacteria are prokaryotes (unicellular organisms with no membrane-enclosed nucleus) with simple structures that typically range in size from about 0.5 to 20 micrometers.

Description

Classification

Bacteria are named according to the binomial (two-name) system of nomenclature first used by Swedish botanist Carolus Linnaeus in the eighteenth century. The first name, or generic name, indicates the genus of the bacteria (a group of closely related species). The second name, or specific name, indicates the species (a group of bacteria that share a number of characteristics). Examples of generic names include Staphylococcus and Escherichia aureus and coli are examples of specific names.

The genus and species names of bacteria often reflect their shape; for example, the Bacillus family of bacteria are bacilli- or rod-shaped. Others are named for their founders (e.g. Yersinia pestis, the causative agent of bubonic plague, is named for Alexandre Yersin) or for their preferred habitat (e.g., Thermoplasma prefer temperatures up to 149°F, or 65°C).

Under a microscope, different families of bacteria have different shapes. Typical cell shapes are straight rods (bacilli), spheres (cocci), bent or curved rods (vibrios), spirals (spirochetes), or thin filaments. Some bacteria exist as single cells, while others form clusters of various shape and complexity. Acetobacter aceti, for example, excretes a substance called cellulose that surrounds the cells to form a skinlike layer. Staphylococcus aureus forms grapelike clusters of cells.

Many groups of bacteria have a cell wall, a structure surrounding the cell. Peptidoglycan (a chemical composed of carbohydrates and proteins ) is a major component of the cell wall, although the exact composition of peptidoglycan varies according to bacteria group. Gram-positive organisms have a relatively thick layer of peptidoglycan and stain violet when applied with certain dyes; gram-negative organisms have a thin layer of peptidoglycan covered by an outer membrane and stain red under the same application of dyes. Gram staining is therefore an important method for identifying bacteria.

Bacteria may be classified by their biochemical composition, and analysis of the protein and lipid content of an organism is often a means of identification. Growth requirements are often used as a means of classification: Mycobacterium tuberculosis, for example, is an obligate aerobe and therefore requires oxygen for growth, while the presence of oxygen is toxic to the anaerobe Clostridium tetani.

The most precise method of classification, however, is genetic analysis. Each species of bacteria has a unique genetic makeup, and therefore a unique sequence of deoxyribonucleic acid (DNA) bases. Some sequences remain constant by genus or species, while others vary considerably. These distinguishing factors are used to trace genetic relatedness and are often used for identification of unknown organisms.

Bacterial structure

Although prokaryotic organisms such as bacteria are considered to be "simple" in structure, each cell is equipped with all of the structures and macromolecules necessary for growth and survival. Complex functions such as energy production, synthesis of biomolecules, and assembly of new structures take place in a highly organized manner.

The genetic material of bacteria is carried on double-stranded molecules of DNA (deoxyribonucleic acid), which is arranged into a circular structure called the chromosome. The region of the cytoplasm where the chromosome is located is called the nucleoid and is not enclosed by a membrane, as in eukaryotic cells. Many bacteria carry additional genetic information (often required for survival in specific environments) on structures called plasmids, smaller circular strands of DNA that are independent of the chromosome. Also found in the cytoplasm are ribosomes, small cellular components important in the process of translating genetic information into proteins. The total collection of genes is called the bacterial genome.

Directly bordering the cytoplasm of the bacteria is the cytoplasmic membrane, which is important for various functions such as energy production and transport of materials in and out of the cell. The cell wall surrounds the cytoplasmic membrane of most bacteria. In gram-negative organisms, the cell wall is composed of a thin layer of peptidoglycan enclosed by an outer membrane. Lipopolysaccharide (LPS; also called endotoxin) is a major constituent of the outer membrane. The cell wall of gram-positive bacteria is distinctly different. Multiple peptidoglycan layers envelop the cytoplasmic membrane, and no outer membrane is present. The peptidoglycan layers form a meshlike shell around the cell that is important for maintaining structure, for replication, and for protection in extreme or toxic conditions. Proteins, lipids, and polysaccharides may also be found in the cell wall.

Many bacteria have additional means of protection in hostile conditions. Some have the ability to form a capsule, layers of polysaccharides and proteins attached to the cell wall, that provides protection against toxic substances and helps inhibit host immune response. Spores are made by some gram-positive bacteria. Under favorable conditions, the cells exist in a vegetative state; but when introduced to a hostile environment, the cells convert to a spore state and become dormant, awaiting conditions in which they may once again prosper.

The flagellum is a filamentous structure attached to many prokaryotes that provides motility. With flagella, bacteria may move toward food and away from toxins, a process called chemotaxis. Pili are hairlike structures shorter than flagella found on many bacteria; they are used for adherence to other bacteria and surfaces such as host cells.

Function

Energy requirements for growth

All living organisms must find in their environment a source of energy to fuel cellular processes. Bacteria are no different. Phototrophs are organisms that use light as an energy source; those that require organic carbon are called heterotrophs. Autotrophs use carbon dioxide. Lithotrophs oxidize inorganic compounds such as hydrogen or ammonia for energy.

Many bacteria have structures and processes that allow them to adapt to hostile environments, and they can exist under an enormous range of conditions. Those that require oxygen for growth are called obligate aerobes. In contrast, obligate anaerobes will not grow in the presence of oxygen. Acidophiles are bacteria that grow optimally under acidic conditions (pH of less than 7.0), while alkaphiles prefer alkaline or basic conditions (pH of greater than 7.0). Organisms that require a temperature near 98.6°F (37°C) (the body temperature of warm-blooded animals) for growth are called mesophiles; those that grow at temperatures above 113°F (45°C) are called thermophiles; and psychrophiles are able to grow at temperatures near 32°F (0°C). Halophiles require sodium chloride (salt) for growth; osmophiles are able to grow in environments high in sugar; and xerophiles grow under dry conditions.

Binary fission and the growth curve

Bacteria grow and replicate in a process known as binary fission. In this process, one parent cell divides to produce two daughter cells. The process begins with the growth of the parent cell; the chromosome unwinds and replicates, each copy moving to opposite ends of the cell. The cell is then partitioned in half by the production of a dividing wall (called the septum). The cell is cleaved at the septum, and the two daughter cells are freed. The daughter cells then go on to reproduce as parent cells (i.e., if necessary nutrients and energy sources are present).

The dynamics of a population of bacteria change during binary fission. The doubling time, or time required for one parent cell to produce two daughter cells, varies by bacteria species and strain and also by the environmental conditions. All bacteria exhibit a characteristic pattern of growth when introduced to a new medium; this is known as the growth curve. There are four phases of the growth curve:

  • During the lag phase, bacteria are adapting to the medium and begin to produce the cellular components necessary for cell division. There is no increase in cell population during the lag phase.
  • Cell division occurs at a maximal rate during the log or exponential phase. The doubling time remains constant, so the number of cells increases exponentially.
  • Cells stop growing exponentially and therefore remain constant during the stationary phase. This occurs when the medium begins to run out of the nutrients necessary for growth or when toxic products accumulate.
  • The number of cells begins to decrease during the death phase as cells begin to die, usually due to toxic conditions or lack of nutrients.

Role in human health

Normal flora

Only a small percentage of the vast population of bacteria is pathogenic (disease-causing) to humans. Many species of bacteria colonize the human body and are called the normal flora. Organisms of the normal flora are normally found on surface tissues (i.e., the skin, mucous membranes, and the gastrointestinal system). It is when bacteria enter normally sterile areas of the body (e.g., the brain, blood, muscle, etc.) that disease may result.

Some organisms of the normal flora neither harm nor provide benefit to the human body; this relationship is called commensalism. Normal commensals are bacteria that can always be found on or in healthy individuals and rarely cause disease. Bacteria that occasionally colonize the human body without causing disease are called occasional commensals. Although a human fetus is sterile in utero, colonization with normal flora bacteria begins with birth when the baby comes into contact with the mother's vaginal bacteria; this continues with breastfeeding and subsequent contact with the environment.

Many other types of bacteria interact with the human body in a relationship called mutualism, from which both organisms benefit. There are a number of ways that bacteria benefit the human host:

  • Normal flora bacteria on the skin such as Staphylococcus epidermidis protect against colonization by pathogenic bacteria, through a process called microbial competition.
  • Bacteria in the vagina (e.g., Lactobacillus acidophilus) help to establish an acidic environment that inhibits colonization of pathogenic bacteria and yeast.
  • The normal flora in the gastrointestinal (GI) tract (e.g., Escherichia coli) secrete vitamins such as K and B12 that are essential for humans. The development of some GI tissues is stimulated by normal flora bacteria.
  • Ruminants (animals with a four-chambered stomach ) rely on enzymes secreted by bacteria such as Ruminococcus albus to digest cellulose (a major component of plant cell walls).

Pathogenic bacteria

Although normal flora bacteria are not normally pathogenic, disease may result from invasion of normal flora into normally sterile areas or if the host immune system is deficient. When bacteria that normally reside in the GI tract (such as E. coli) are introduced to the urinary tract, for example, a urinary-tract infection may result. This is considered an endogenous infection.

Exogenous infections result from invasion of noncommensal organisms (i.e., those not normally found on the human body). Transmission of exogenous bacteria may occur by various routes, including inhalation of aerosolized organisms, ingestion (e.g., contaminated food or utensils), or direct contact of a wound or mucous membrane with organisms.

When bacteria first enter the body, local inflammation may be the first sign of infection. Physical symptoms such as pain, erythema (redness), edema (swelling), or pus formation result from the response of the immune system against the invading bacteria. If the bacteria spread to the bloodstream (bacteremia), they may disseminate to and colonize at various sites in the body.

VIRULENCE FACTORS. Bacteria have developed numerous mechanisms that allow them to invade a host and colonize an otherwise inhospitable site to cause disease. Many of these mechanisms enhance their ability to cause disease in humans; such traits are called virulence factors. Some common virulence factors include:

  • Bacterial growth. The byproducts of normal bacterial growth may cause tissue destruction if colonization has occurred in a normally sterile site. For example, Clostridium perfringens is a normal flora bacteria of the GI tract but may cause gas gangrene if it infects a wound or trauma site.
  • Release of toxins. Some pathogenic bacteria produce proteins (toxins) that are inevitably toxic to the host. An endotoxin is composed of lipopolysaccharides found in the outer membrane of gram-negative bacteria. Exotoxins are proteins produced intracellularly and secreted by either gram-negative or grampositive bacteria.
  • Capsule formation. The polysaccharide layers of a capsule form a protective shield around a bacteria and help the cell to evade immune response.
  • Internalization. Some bacteria are able to escape intracellular killing when internalized by phagosomes and go on to survive in the cytoplasm (e.g., Mycobacterium tuberculosis). In this way they are protected from antibody-mediated immune responses.
  • Granuloma formation. A granuloma is a lesion formed in response to infection by some intracellular pathogens. Viable bacteria are walled off in the granuloma and thus prevented from further colonization.
  • Antigenic mimicry. A bacterial cell may be able to trick the immune system by presenting antigens (molecules recognized by antibodies) that are similar to host antigens. Immunological cells therefore have difficulty distinguishing between the bacterium and a host cell.

Antibiotic resistance

The emergence of bacterial strains that are resistant to treatment by current antibiotics is an important public-health concern. Antibiotics are chemical substances produced by microorganisms that inhibit bacterial growth or kill bacterial cells. Narrow-spectrum antibiotics target only a limited variety of bacteria, while broad-spectrum antibiotics have the ability to inhibit or kill a wide variety of bacteria.

Bacteria can resist the action of antibiotics using one or more of four basic mechanisms:

  • inactivation or modification of the drug
  • modifying the drug's target binding site
  • decreasing uptake of the drug into the cell
  • altering the biochemical pathway that the drug is targeting

In many developing countries, antibiotics are freely distributed as over-the-counter drugs, leading to their widespread use for viral or noninfectious illnesses. This practice, coupled with the overuse of antibiotics in veterinary medicine, farming, and plant culture, has contributed to the spread of antibiotic resistant bacteria.

Common diseases and disorders

The following list describes some of the most common bacteria that are pathogenic to humans.

KEY TERMS

Antibiotics— Chemicals produced by microorganisms that inhibit bacterial growth or kill bacterial cells.

Binary fission— The process by which a single parent cell divides to produce two identical daughter cells.

Flagellum— A filamentous structure attached to some bacteria that provides motility.

Gram-negative— A class of bacteria that have a cell wall composed of a thin layer of peptidoglycan surrounded by an outer membrane made of polysaccharides and proteins.

Gram-positive— A class of bacteria that have a cell wall composed of a thick layer of peptidoglycan.

Growth curve— A characteristic growth pattern of bacteria when introduced into a new medium; it includes four phases (lag, log, stationary, and death).

Normal flora— Species of bacteria that colonize the human body and do not normally cause disease.

Nucleoid— Cytosolic region of a bacterial cell in which the chromosome is located.

Pathogen— A disease-causing microbe.

Peptidoglycan— A chemical composed of carbohydrates and proteins that is a major component of the bacterial cell wall.

Prokaryote— A unicellular organism lacking a membrane-enclosed nucleus.

Ribosome— A cytosolic structure important in the translating of genetic information into proteins.

Toxins— Proteins produced by bacteria that are toxic to host cells. Endotoxin is a component of the cell wall of gram-negative bacteria; exotoxin is secreted by both gram-negative and gram-positive bacteria.

  • Staphylococcus. Staphylococci are gram-positive bacteria found as part of the normal flora of most individuals. S. aureus is the causative agent of many infections, including toxic shock syndrome (TSS), staphylococcal food poisoning, impetigo, and furuncles (boils ). S. saprophyticus causes urinarytract infections in sexually active women. S. epidermidis may infect damaged or artificial heart valves and cause a condition called endocarditis.
  • Streptococcus. Streptococci are gram-positive bacteria that commonly colonize the oropharynx (the area of the throat at the back of the mouth). Example syndromes include pharyngitis (sore throat ), scarlet fever, necrotizing fasciitis (streptococci are popularly known as the "flesh-eating bacteria"), and rheumatic fever. S. pneumoniae is a common cause of bacterial pneumonia and meningitis.
  • Neisseria. N. gonorrhoeae is the causative agent of gonorrhea, a leading sexually transmitted disease (STD). N. meningitidis is a leading cause of adult meningitis.
  • Escherichia. E. coli is the most commonly encountered species of this genus. The bacteria is a common cause of gastroenteritis (inflammation of the lining of the stomach and intestines) but also causes urinarytract infections and neonatal meningitis.
  • Salmonella. Most Salmonella infections result from ingestion of contaminated food and lead to enteritis. The febrile (fever-inducing) illness typhoid fever is caused by S. typhi.
  • Vibrio. The most commonly known Vibrio infection is cholera, caused by V. cholerae. Spread by ingestion of contaminated food or water, cholera infection is an important cause of diarrheal disease in developing countries.
  • Clostridium. C. perfringens causes a variety of human diseases, including myonecrosis (gas gangrene), clostridial food poisoning, and soft-tissue infections (cellulitis and fasciitis). Tetanus (also known as lockjaw) is caused by C. tetani; C. botulinum causes food-borne botulism.
  • Mycobacterium. Tuberculosis, caused by infection with M. tuberculosis, is a highly prevalent pulmonary disease. Hansen's disease (also known as leprosy) is caused by M. leprae.
  • Chlamydia. Chlamydiae, once thought to be viruses because of their small size, cause numerous human diseases. C. trachomatis is the causative agent of conjunctivitis (inflammation of the outer surface of the eye), infant pneumonia, and urogenital chlamydia. Bronchitis, pneumonia, and sinusitis are often caused by C. pneumoniae.

Resources

BOOKS

Bergquist, L. M., and B. Pogosian. Microbiology Principles and Health Science Applications. Philadelphia: W. B. Saunders, 2000.

Lengeler, J. W., G. Drews, and H. G. Schlegel, editors. Biology of the Prokaryotes. Malden, MA: Blackwell Science, 1999.

Murray, P. R., K. S. Rosenthal, G. S. Kobayashi, and M. A. Pfaller. Medical Microbiology. St. Louis: Mosby, 1998.

Williams, P., J. Ketley, and G. Salmond, editors. Bacterial Pathogenesis. San Diego: Academic Press, 1998.

PERIODICALS

Hawkey, P. M. "The Origins and Molecular Basis of Antibiotic Resistance." British Medical Journal (September 5, 1998): 657-660.

Mitchell, T. J. "Molecular Basis of Virulence." Archives of Diseases in Childhood (March 1998): 197-199.

ORGANIZATIONS

American Society for Microbiology. 1752 N Street NW, Washington, DC 20036. (202) 737-3600. 〈http://www.asmusa.org〉.

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Bacteria

Bacteria

Characteristics of bacteria

Bacterial growth

Aerobic and anaerobic bacteria

The role of bacteria in fermentation

Identifying and classifying bacteria

Bacteria and disease

Bacterial adaptation

Resources

Bacteria are mostly unicellular (single-celled) organisms that lack chlorophyll and are among the smallest living things on Earthonly viruses are smaller. Multiplying rapidly under favorable conditions, bacteria can aggregate into colonies of millions or even billions of organisms within a space as small as a drop of water.

The Dutch merchant and amateur scientist Anton van Leeuwenhoek (16321723) was the first to observe bacteria and other microorganisms. Using single-lens microscopes of his own design, he described bacteria and other microorganisms (calling them animacules) in a series of letters to the Royal Society of London between 1674 and 1723.

Bacteria are classified as prokaryotes. Broadly, this taxonomic ranking reflects the fact that the genetic material of bacteria is contained in a single, circular chain of deoxyribonucleic acid (DNA) that is not enclosed within a nuclear membrane. The word prokaryote is derived from Greek meaning prenucleus. Moreover, the DNA of prokaryotes is not associated with the special chromosome proteins called histones, which are found in higher organisms. In addition, prokaryotic cells lack other membrane-bounded organ-elles, such as mitochondria. Prokaryotes belong to the kingdom Monera. Some scientists have proposed splitting this designation into the kingdoms Eubacteria and Archaebacteria. Eubacteria, or true bacteria, consist of more common species, while Archaebacteria (with the prefix archaemeaning ancient) represent microorganisms that are bacteria-like in appearance that inhabit very hostile environments. Scientists believe the latter microorganisms are most closely related to the bacteria that lived when Earth was very young. Examples of archaebacteria are halophiles, which live in extremely salty environments, and thermophiles, which can tolerate and even thrive in near boiling waters of geysers and geothermal vents of the ocean floor.

Characteristics of bacteria

Although all bacteria share certain structural, genetic, and metabolic characteristics, important biochemical differences exist among the many species of bacteria. These differences permit bacteria to live in many different, and sometimes extreme, environments. For example, some bacteria recycle nitrogen and carbon from decaying organic matter, then release these gases into the atmosphere to be reused by other living things. Other bacteria cause diseases in humans and animals, help digest sewage in treatment plants, or produce the alcohol in wine, beer, and liquors. Still others are used by humans to break down toxic waste chemicals in the environment, a process called bioremediation.

The cytoplasm of all bacteria is enclosed within a cell membrane surrounded by a rigid cell wall whose polymers, with few exceptions, include peptidoglycanslarge, structural molecules made of protein carbohydrate.

Some bacteria can also secrete a viscous, gelatinous polymer (called the glycocalyx) on their cell surfaces. This polymer, composed either of polysaccharide, polypeptide, or both, is called a capsule when it occurs as an organized layer firmly attached to the cell wall. Capsules increase the disease-causing ability (virulence) of bacteria by inhibiting immune system cells called phagocytes from engulfing them. One such bacterium, Streptococcus pneumoniae, is the cause of pneumonia.

The shape of bacterial cells are classified as spherical (coccus), rodlike (bacillus), spiral (spirochete), helical (spirilla) and comma-shaped (vibrio) cells. Many bacilli and vibrio bacteria have whiplike appendages (called flagella) protruding from the cell surface. Flagella are composed of tight, helical rotors made of chains of globular protein called flagellin, and act as tiny propellers, making the bacteria very mobile.

Flagella may be arranged in any of four ways, depending on the species of bacteria. There is the monotrichous condition (single flagellum at one end), the amphitrichous (single flagellum at each end of the bacterium), the lophotrichous (two or more flagella at either or both ends of the bacterium), and the peritrichous condition (flagella distributed over the entire cell).

Spirochetes are spiral-shaped bacteria that live in contaminated water, sewage, soil and decaying organic matter, as well as inside humans and animals. Spirochetes move by means of axial filaments, which consist of bundles of fibrils arising at each end of the cell beneath an outer sheath. The fibrils, which spiral around the cell, rotate, causing an opposite movement of the outer sheath that propels the spirochetes forward, like a turning corkscrew. The best known spirochete is Treponema pallidum, the organism that causes syphilis.

On the surface of some bacteria are short, hairlike, proteinaceous projections that may arise at the ends of the cell or over the entire surface. These projections, called fimbriae, let the bacteria adhere to surfaces. For example, fimbriae on the bacterium Neisseria gonorrhoea, which causes gonorrhea, allow these organisms to attach to mucous membranes.

Other proteinaceous projections, called pili, occur singly or in pairs, and join pairs of bacteria together, facilitating transfer of DNA between them.

During periods of harsh environmental conditions some bacteria, such as those of the genera Clostridium and Bacillus, produce within themselves a dehydrated, thick-walled endospore. These endo-spores can survive extreme temperatures, dryness, and exposure to many toxic chemicals and to radiation. Endospores can remain dormant for long periods (hundreds of years in some cases) before being reactivated by the return of favorable conditions.

A primitive form of exchange of genetic material between bacteria involving plasmids does occur. Plasmids are small, circular, extrachromosomal DNA molecules that are capable of replication and are known to be capable of transferring genes among bacteria. For example, resistance plasmids carry genes for resistance to antibiotics from one bacterium to another, while other plasmids carry genes that confer pathogenicity. In addition, the transfer of genes via bacteriophagesviruses that specifically parasitize bacteriaalso serves as a means of genetic recombination. Corynebacterium diphtheriae, for example, produces the diphtheria toxin only when infected by a phage that carries the diphtherotoxin gene.

The above examples of genetic information exchange between bacterial cells occurs regularly in nature. This natural exchange, or lateral gene transfer, can be mimicked artificially in the laboratory. Bioengineering uses sophisticated techniques to purposely transfer DNA from one organism to another in order to give the second organism some new characteristic it did not have previously. For example, in a process called transformation, antibiotic-susceptible bacteria that are induced to absorb manipulated plasmids placed in their environment, can acquire resistance to that antibiotic substance due to the new genes they have incorporated. Similarly, in a process called transfection, specially constructed viruses are used to artificially inject bioengineered DNA into bacteria, giving infected cells some new characteristic.

Bacteria synthesize special DNA-cutting enzymes (known as restriction enzymes) that destroy the DNA of phages that do not normally infect them. Purified restriction enzymes are used in the laboratory to slice pieces of DNA from one organism and insert them into the genetic material of another organism as mentioned above.

Bacterial growth

The term bacterial growth generally refers to growth of a population of bacteria, rather than of an individual cell. Individual cells usually reproduce asexually by means of binary fission, in which one cell divides into two cells. Thus, bacterial growth of the population is a geometric progression of numbers of cells, with division occurring in regular intervals, called generation time, ranging from 15 minutes to 16 hours, depending upon the type of bacterium. In addition, some filamentous bacteria (actinomycetes) reproduce by producing chains of spores at their tips, while other filamentous species fragment into new cells.

Under ideal conditions, the growth of a population of bacteria occurs in several stages termed lag, log, stationary, and death. During the lag phase, active metabolic activity occurs involving synthesis of DNA and enzymes, but no growth. Geometric population growth occurs during the log, or exponential phase, when metabolic activity is most intense and cell reproduction exceeds cell death. Following the log phase, the growth rate slows and the production of new cells equals the rate of cell death. This period, known as the stationary phase, involves the establishment of an equilibrium in population numbers and a slowing of the metabolic activities of individual cells. The stationary phase reflects a change in growing conditionfor example, a lack of nutrients and/or the accumulation of waste products.

When the rate of cell deaths exceeds the number of new cells formed, the population equilibrium shifts to a net reduction in numbers and the population enters the death phase, or logarithmic decline phase. The population may diminish until only a few cells remain, or the population may die out entirely.

The physical and chemical requirements for growth can vary widely among different species of bacteria, and some are found in environments as extreme as cold polar regions and hot, acid springs.

In general, the physical requirements for bacteria include proper temperature, pH, and osmotic pressure. Most bacteria thrive only within narrow ranges of these conditions, however extreme those ranges may be.

The lowest temperature at which a particular species will grow is the minimum growth temperature, while the maximum growth temperature is the highest temperature at which they will grow. The temperature at which their growth is optimal is called the optimum growth temperature. In general, the maximum and minimum growth temperatures of any particular type of bacteria are about 30°F (1°C) apart.

Most bacteria thrive at temperatures at or around that of the human body 98.6°F (37°C), and some, such as Escherichia coli, are normal parts of the human intestinal flora. These organisms are mesophiles (moderate-temperature-loving), with an optimum growth temperature between 77°F (25°C) and 104°F (40°C). Mesophiles have adapted to thrive in temperatures close to that of their host.

Psychrophiles, which prefer cold temperatures, are divided into two groups. One group has an optimal growth temperature of about 59°F (15°C), but can grow at temperatures as low as 32°F (0°C). These organisms live in ocean depths or Arctic regions. Other psychrophiles that can also grow at 32°F (0°C) have an optimal growth temperature between 68°F (20°C) and 86°F (30°C). These organisms, sometimes called psychrotrophs, are often those associated with food spoilage under refrigeration.

Thermophiles thrive in very hot environments, many having an optimum growth temperature between 122°F (50°C) and 140°F (60°C), similar to that of hot springs in Yellowstone National Park. Such organisms thrive in compost piles, where temperatures can rise as high as 140°F (60°C). Extreme thermophiles grow at temperatures above 195°F (91°C). Along the sides of hydrothermal vents on the ocean bottom 217 mi (350 km) north of the Galapagos Islands, for example, bacteria grow in temperatures that can reach 662°F (350°C).

Like temperature, pH also plays a role in determining the ability of bacteria to grow or thrive in particular environments. Most commonly, bacteria grow optimally within a narrow range of pH between 6.7 and 7.5.

Acidophiles, however, prefer acidic conditions. For example, Thiobacillus ferrooxidans, which occurs in drainage water from coal mines, can survive at pH 1. Other bacteria, such as Vibrio cholera, the cause of cholera, can thrive at a pH as high as 9.0.

Osmotic pressure is another limiting factor in the growth of bacteria. Bacteria are about 80-90% water; they require moisture to grow because they obtain most of their nutrients from their aqueous environment.

Cell walls protect prokaryotes against changes in osmotic pressure over a wide range. However, sufficiently hypertonic media at concentrations greater than those inside the cell (such as 20% sucrose) cause water loss from the cell by osmosis. Fluid leaves the bacteria causing the cell to contract, which, in turn, causes the cell membrane to separate from the overlying cell wall. This process of cell shrinkage is called plasmolysis.

Because plasmolysis inhibits bacterial cell growth, the addition of salts or other solutes to a solution inhibits food spoilage by bacteria, as occurs when meats or fish is salted.

Some types of bacteria, called extreme or obligate halophiles, are adapted toand requirehigh salt concentrations, such as found in the Dead Sea, where salt concentrations can reach 30%. Facultative halophiles do not require high salt environments to survive, but are capable of tolerating these conditions. Halophiles can grow in salt concentrations up to 2%, a level that would inhibit the growth of other bacteria. However, some facultative halophiles, such as Halobacterium halobium grow in salt lakes, salt flats, and other environments where the concentration of salts is up to seven times greater than that of the oceans.

When bacteria are placed in hypotonic media with concentrations weaker than the inside of the cell, water tends to enter by osmosis. The accumulation of this water causes the cell to swell and then to burst, a process called osmotic lysis.

Carbon, nitrogen, and other growth factors

In addition to water and the correct salt balance, bacteria also require a wide variety of elements, especially carbon, hydrogen, and nitrogen, sulfur and phosphorus, potassium, iron, magnesium, and calcium. Growth factors, such as vitamins and pyrimi-dines and purines (the building blocks of DNA), are also necessary.

Carbon is the fundamental building block of all the organic compounds needed by living things, including nucleic acids, carbohydrates, proteins, and fats.

Chemoheterotrophs are bacteria that use organic compounds such as proteins, carbohydrates, and lipids as their carbon source, and which use electrons from organic compounds as their energy source. Most bacteria (as well as all fungi, protozoans and animals) are chemoheterotrophs. Chemoautotrophs (for example hydrogen, sulfur, iron, and nitrifying bacteria) use carbon dioxide as their carbon source and electrons from inorganic compounds as their energy source.

Saprophytes are heterotrophs that obtain their carbon from decaying dead organic matter. Many different soil bacteria release plant nitrogen as ammonia (ammonification). Other bacteria, the Nitrosomonas, convert ammonia to nitrite, while Nitrobacter convert nitrite to nitrate. Other bacteria, especially Pseudomonas, convert nitrate to nitrogen gas. These bacteria complement the activity of nitrogen-fixing bacteria (for example, Rhizobium), which fix nitrogen from the atmosphere and make it available to leguminous plants, and Azotobacter, which are also found in fresh and marine waters. Together, the activity of these bacteria underlies the nitrogen cycle, by which the gas is taken up by living organisms, used to make proteins and other organic compounds, returned to the soil during decay, then released into the atmosphere to be reused by living things.

Phototrophs use light as their primary source of energy, but may differ in their carbon sources. Photoheterotrophs (purple nonsulfur and green non-sulfur bacteria) use organic compounds as their carbon source, while photoautotrophs (for example, photosynthetic green sulfur and purple sulfur bacteria) use carbon dioxide as a source of carbon.

Aerobic and anaerobic bacteria

Oxygen may or may not be a requirement for a particular species of bacteria, depending on the type of metabolism used to extract energy from food (aerobic or anaerobic). In all cases, the initial breakdown of glucose to pyruvic acid occurs during glycolysis, which produces a net gain of two molecules of the energy-rich moleculeadenosine triphosphate (ATP).

Aerobic bacteria use oxygen to break down pyruvic acid, releasing much more ATP than is produced during glycolysis during the process known as aerobic respiration. In addition, aerobic bacteria have enzymes such as superoxide dismutase capable of breaking down toxic forms of oxygen, such as super-oxide free radicals, which are also formed by aerobic respiration.

During aerobic respiration, enzymes remove electrons from the organic substrate and transfer them to the electron transport chain, which is located in the membrane of the mitochondrion. The electrons are transferred along a chain of electron carrier molecules. At the final transfer position, the electrons combine with atoms of oxygenthe final electron acceptor which in turn combines with protons (H+) to produce water molecules. Energy, in the form of ATP, is also made here. Along the chain of electron carriers, protons that are pumped across the mitochondrial membrane re-enter the mitochondrion. This flow of electrons across the membrane fuels oxidative phosphorylation, the chemical reaction that adds a phosphate group to adenosine diphosphate (ADP) to produce ATP.

Obligate aerobes must have oxygen in order to live. Facultative aerobes can also exist in the absence of oxygen by using fermentation or anaerobic respiration. Anaerobic respiration and fermentation occur in the absence of oxygen, and produce substantially less ATP than aerobic respiration.

Anaerobic bacteria use inorganic substances other than oxygen as a final electron acceptor. For example, Pseudomonas and Bacillus reduce nitrate ion (NO3-) to nitrite ion (NO2-), nitrous oxide (N2 O) or nitrogen gas (N2). Clostridium species, which include those that cause tetanus and botulism, are obligate anaerobes. That is, they are not only unable to use molecular oxygen to produce ATP, but are harmed by toxic forms of oxygen formed during aerobic respiration. Unlike aerobic bacteria, obligate anaerobes lack the ability synthesize enzymes to neutralize these toxic forms of oxygen.

The role of bacteria in fermentation

Fermentation bacteria are anaerobic, but use organic molecules as their final electron acceptor to produce fermentation end-products. Streptococcus, Lactobacillus, and Bacillus, for example, produce lactic acid, while Escherichia and Salmonella produce ethanol, lactic acid, succinic acid, acetic acid, CO2, and H2.

Fermenting bacteria have characteristic sugar fermentation patterns, i.e., they can metabolize some sugars but not others. For example, Neisseria meningitidis ferments glucose and maltose, but not sucrose and lactose, while Neisseria gonorrhoea ferments glucose, but not maltose, sucrose or lactose. Such fermentation patterns can be used to identify and classify bacteria.

During the 1860s, the French microbiologist Louis Pasteur (182295) studied fermenting bacteria. He demonstrated that fermenting bacteria could contaminate wine and beer during manufacturing, turning the alcohol produced by yeast into acetic acid (vinegar). Pasteur also showed that heating the beer and wine to kill the bacteria preserved the flavor of these beverages. The process of heating, now called pasteurization in his honor, is still used to kill bacteria in some alcoholic beverages, as well as milk.

Pasteur described the spoilage by bacteria of alcohol during fermentation as being a disease of wine and beer. His work was thus vital to the later idea that human diseases could also be caused by microorganisms, and that heating can destroy them.

Identifying and classifying bacteria

The most fundamental technique for classifying bacteria is the gram stain, developed in 1884 by Danish scientist Christian Gram (18531938). It is called a differential stain because it differentiates among bacteria and can be used to distinguish among them, based on differences in their cell wall.

In this procedure, bacteria are first stained with crystal violet, then treated with a mordanta solution that fixes the stain inside the cell (e.g., iodine-KI mixture). The bacteria are then washed with a decolorizing agent, such as alcohol, and counterstained with safranin, a light red dye.

The walls of gram-positive bacteria (for example, Staphylococcus aureus ) have more peptidoglycans (the large molecular network of repeating disaccharides attached to chains of four or five amino acids) than do gram-negative bacteria. Thus, gram-positive bacteria retain the original violet dye and cannot be counterstained.

Gram-negative bacteria (e.g., Escherichia coli ) have thinner walls, containing an outer layer of lipopolysaccharide, which is disrupted by the alcohol wash. This permits the original dye to escape, allowing the cell to take up the second dye, or counterstain. Thus, gram-positive bacteria stain violet, and gram-negative bacteria stain pink.

The gram stain works best on young, growing populations of bacteria, and can be inconsistent in older populations maintained in the laboratory.

Microbiologists have accumulated and organized the known characteristics of different bacteria in a reference book called Bergeys Manual of Systematic Bacteriology (the first edition of which was written primarily by David Hendricks Bergey of the University of Pennsylvania in 1923).

The identification schemes of Bergeys Manual are based on morphology (e.g., coccus, bacillus), staining (gram-positive or negative), cell wall composition (e.g., presence or absence of peptidoglycan), oxygen requirements (e.g., aerobic, facultatively anaerobic) and biochemical tests (e.g., which sugars are aerobically metabolized or fermented).

In addition to the gram stain, other stains include the acid-fast stain, used to distinguish Mycobacterium species (for example, Mycobacterium tuberculosis, the cause of tuberculosis); endospore stain, used to detect the presence of endospores; negative stain, used to demonstrate the presence of capsules; and flagella stain, used to demonstrate the presence of flagella.

Another important identification technique is based on the principles of antigenicitythe ability to stimulate the formation of antibodies by the immune system. Commercially available solutions of antibodies against specific bacteria (antisera) are used to identify unknown organisms in a procedure called a slide agglutination test. A sample of unknown bacteria in a drop of saline is mixed with antisera that has been raised against a known species of bacteria. If the antisera causes the unknown bacteria to clump (agglutinate), then the test positively identifies the bacteria as being identical to that against which the antisera was raised. The test can also be used to distinguish between strains, slightly different bacteria belonging to the same species.

Phage typing, like serological testing, identifies bacteria according to their response to the test agent, in this case viruses. Phages are viruses that infect specific bacteria. Bacterial susceptibility to phages is determined by growing bacteria on an agar plate, to which solutions of phages that infect only a specific species of bacteria are added. Areas that are devoid of visible bacterial growth following incubation of the plate represent organisms susceptible to the specific phages.

Because a specific bacterium might be susceptible to infection by two or more different phages, it may be necessary to perform several tests to definitively identify a specific bacterium.

The evolutionary relatedness of different species can also be determined by laboratory analysis. For example, analysis of the amino acid sequences of proteins from different bacteria disclose how similar the proteins are. In turn, this reflects the similarity of the genes coding for these proteins.

Protein analysis compares the similarity or extent of differences between the entire set of protein products of each bacterium. Using a technique called electrophoresis, the entire set of proteins of each bacterium is separated according to size by an electrical charge applied across gel. The patterns produced when the gel is stained to show the separate bands of proteins reflects the genetic makeup, and relatedness, of the bacteria.

The powerful techniques of molecular biology have given bacteriologists other tools to determine the identity and relatedness of bacteria.

Taxonomists interested in studying the relatedness of bacteria compare the ratio of nucleic acid base pairs in the DNA of microorganisms, that is, the number of guanosine-cytosine pairs (G-C pairs) in the DNA. Because each guanosine on a double-stranded molecule of DNA has a complementary cyto-sine on the opposite strand, comparing the number of G-C pairs in one bacterium, with that in another bacterium, provides evidence for the extent of their relatedness.

Determining the percentage of G-C pairs making up the DNA also discloses the percentage of adenosine-thymine (A-T)the other pair of complementary nucleic acids making up DNA (100% [% G-C] = % A-T).

The closer the two percentages are, the more closely related the bacteria may be, although other lines of evidence are needed to make a definitive determination regarding relationships.

The principle of complementarity is also used to identify bacteria by means of nucleic acid hybridization. The technique assumes that if two bacteria are closely related, they will have long stretches of identical DNA. First, one bacteriums DNA is isolated and gently heated to break the bonds between the two complementary strands. Specially prepared DNA probes representing short segments of the other organisms DNA are added to this solution of single-stranded DNA. The greater degree to which the probes combine with (hybridize) complementary stretches of the single stranded DNA, the greater the relatedness of the two organisms.

In addition to helping bacteriologists better classify bacteria, the various laboratory tests are valuable tools for identifying disease-causing organisms. This is especially important when physicians must determine which antibiotic or other medication to use to treat an infection.

Bacteria and disease

The medical community did not accept the concept that bacteria can cause disease until well into the nineteenth century. Joseph Lister (18271912), an English surgeon, applied the so-called germ theory to medical practice in the 1860s. Lister soaked surgical dressing in carbolic acid (phenol), which reduced the rate of post-surgical infections so dramatically that the practice spread.

In 1876, the German physician Robert Koch (18431910) identified Bacillus anthracis as the cause of anthrax, and in so doing, developed a series of laboratory procedures for proving that a specific organism can cause a specific disease. These procedures, called Kochs postulates, are still generally valid. Briefly, they state that, to prove an organism causes a specific disease, the investigator must systematically:

  • find the same pathogenic microorganism in every case of the disease;
  • isolate the pathogen from the diseased patient or experimental animal and grow it in pure culture;
  • demonstrate that the pathogen from the pure culture causes the disease when it is injected into a healthy laboratory animal, and;
  • isolate the pathogen from the inoculated animal and demonstrate that it is the original organism injected into the animal.

The ability to isolate, study, and identify bacteria has greatly enhanced the understanding of their disease-causing role in humans and animals, and the subsequent development of treatments. Part of that understanding derives from the realization that since bacteria are ubiquitous and are found in large numbers in and on humans, they can cause a wide variety of diseases.

The skin and the nervous, cardiovascular, respiratory, digestive, and genitourinary systems are common sites of bacterial infections, as are the eyes and ears.

The skin is the bodys first line of defense against infection by bacteria and other microorganisms, although it supports enormous numbers of bacteria itself, especially Staphylococcus and Streptococcus species. Sometimes these bacteria are only dangerous if they enter a break in the skin or invade a wound, for example, the potentially fatal staphylococcal toxic shock syndrome. Among other common bacterial skin ailments are acne, caused by Propionibacterium acnes and superficial infection of the outer ear canal, caused by Pseudomonas aeruginosa.

Among the neurological diseases are meningitis, an inflammation of the brains membranes caused by Neisseria meningitidis and Hemophilus influenzae.

Many medically important bacteria produce toxins, poisonous substances that have effects in specific areas of the body. Exotoxins are proteins produced during bacterial growth and metabolism and released into the environment. Most of these toxin-producing bacteria are gram positive.

Among the gram positive toxin-producing bacteria are Clostridium tetani, which causes tetanus, an often fatal paralytic disease of muscles; Clostridium botulinum, which causes botulism, a form of potentially lethal food poisoning; and Staphylococcus aureus, which also causes a form of food poisoning (gastroenteritis).

Most gram-negative bacteria (for example, Salmonella typhi, the cause of typhoid fever) produce endotoxins, toxins that are part of the bacterial cell wall.

As the role of bacteria in causing disease became understood, entire industries developed that addressed the public health issues of these diseases.

As far back as 1810, the French confectioner Nicholas Appert (17501841) proved that food stored in glass bottles and heated to high temperatures could be stored for long periods of time without spoiling. Appert developed tables that instructed how long such containers should be boiled, depending upon the type of food and size of the container. Today, the food preservation industry includes not only canning, but also freezing and freeze-drying. An important benefit to food preservation is the ability to destroy potentially lethal contamination by Clostridium botulinum spores.

Even as concepts of prevention of bacterial diseases were being developed, scientists were looking for specific treatments. Early in the twentieth century, the German medical researcher Paul Ehrlich (18541915) theorized about producing a magic bullet that would destroy pathogenic organisms without harming the host.

In 1928, the discovery by Scottish bacteriologist Alexander Fleming (18811955) that the mold Penicillium notatum inhibited growth of Staphylococcus aureus ushered in the age of antibiotics. Subsequently, English scientists Howard Florey (18981968) and Ernst Chain (190679), working at Oxford University in England, demonstrated the usefulness of penicillin, the antibacterial substance isolated from P. notatum in halting growth of this bacterium. This inhibitory effect of penicillin on bacteria is an example of antibiosis, and from this term is derived the word antibiotic, which refers to a substance produced by microorganisms that inhibits other microorganisms.

Beginning in the 1930s, the development of synthetic antibacterial compounds called sulfa drugs further stimulated the field of antibacterial drug research. The many different anti-bacterial drugs available today work in a variety of ways, such as the inhibition of synthesis of cell walls, of proteins, or of DNA or RNA.

As of 2006 medical science and the multi-billion dollar pharmaceutical industry are facing the problem of bacterial resistance to drugs, even as genetically engineered bacteria are being used to produce important medications for humans.

Bacterial adaptation

Bacteria have been designed to be adaptable. Their surrounding layers and the genetic information for these and other structures associated with a bacterium are capable of alteration. Some alterations are reversible, disappearing when the particular pressure is lifted. Other alterations are maintained and can even be passed on to succeeding generations of bacteria.

The first antibiotic was discovered in 1929. Since then, a myriad of naturally occurring and chemically synthesized antibiotics have been used to control bacteria. Introduction of an antibiotic is frequently followed by the development of resistance to the agent. Resistance is an example of the adaptation of the bacteria to the antibacterial agent.

Antibiotic resistance can develop swiftly. For example, resistance to penicillin (the first antibiotic discovered) was recognized almost immediately after introduction of the drug. As of the mid-1990s, almost 80% of all strains of Staphylococcus aureus were resistant to penicillin. Meanwhile, other bacteria remain susceptible to penicillin. An example is provided by Group A Streptococcus pyogenes, another gram-positive bacteria.

The adaptation of bacteria to an antibacterial agent such as an antibiotic can occur in two ways. The first method is known as inherent (or natural) resistance. Gram-negative bacteria are often naturally resistant to penicillin, for example. This is because these bacteria have another outer membrane, which makes the penetration of penicillin to its target more difficult. Sometimes when bacteria acquire resistance to an antibacterial agent, the cause is a membrane alteration that has made the passage of the molecule into the cell more difficult.

The second category of adaptive resistance is called acquired resistance. This resistance is almost always due to a change in the genetic make-up of the bacterial genome. Acquired resistance can occur because of mutation or as a response by the bacteria to the selective pressure imposed by the antibacterial agent. Once the genetic alteration that confers resistance is present, it can be passed on to subsequent generations. Acquired adaptation and resistance of bacteria to some clinically important antibiotics has become a great problem in the last decade of the twentieth century.

Bacteria adapt to other environmental conditions as well. These include adaptations to changes in temperature, pH, concentrations of ions such as sodium, and the nature of the surrounding support. An example of the latter is the response shown by Vibrio parahaemolyticus to growth in a watery environment versus a more viscous environment. In the more viscous setting, the bacteria adapt by forming what are called swarmer cells. These cells adopt a different means of movement, which is more efficient for moving over a more solid surface. This adaptation is under tight genetic control, involving the expression of multiple genes.

Bacteria react to a sudden change in their environment by expressing or repressing the expression of a whole lost of genes. This response changes the properties of both the interior of the organism and its surface chemistry. A well-known example of this adaptation is the so-called heat shock response of Escherichia coli. The name derives from the fact that the response was first observed in bacteria suddenly shifted to a higher growth temperature.

One of the adaptations in the surface chemistry of Gram-negative bacteria is the alteration of a molecule called lipopolysaccharide. Depending on the growth conditions or whether the bacteria are growing on an artificial growth medium or inside a human, as examples, the lipopolysaccharide chemistry can become more or less water-repellent. These changes can profoundly affect the ability of antibacterial agents or immune components to kill the bacteria.

Another adaptation exhibited by Vibrio parahaemolyticus, and a great many other bacteria as well, is the formation of adherent populations on solid surfaces. This mode of growth is called a biofilm. Adoption of a biofilm mode of growth induces a myriad of changes, many involving the expression of previously unexpressed genes. In addition, de-activation of actively expressing genes can occur. Furthermore, the pattern of gene expression may not be uniform throughout the biofilm. Bacteria within a biofilm and bacteria found in other niches, such as in a wound where oxygen is limited, grow and divide at a far slower speed than the bacteria found in the test tube in the laboratory. Such bacteria are able to adapt to the slower growth rate, once again by changing their chemistry and gene expression pattern.

A further example of adaptation is the phenomenon of chemotaxis, whereby a bacterium can sense the chemical composition of the environment and either moves toward an attractive compound, or shifts direction and moves away from a compound sensed as being detrimental. Chemotaxis is controlled by more than 40 genes that code for the production of components of the flagella that propels the bacterium along, for sensory receptor proteins in the membrane, and for components that are involved in signaling a bacterium to move toward or away from a compound. The adaptation involved in the chemotactic response must have a memory component, because the concentration of a compound at one moment in time must be compared to the concentration a few moments later.

See also Antisepsis; Biodegradable substances; Biodiversity; Composting; Microbial genetics; Origin of life; Water microbiology; Nitrogen fixation.

KEY TERMS

Capsule A viscous, gelatinous polymer composed either of polysaccharide, polypeptide, or both, that surrounds the surface of some bacteria cells. Capsules increase the disease-causing ability (virulence) of bacteria by inhibiting immune system cells called phagocytes from engulfing them.

Death phase Stage of bacterial growth when the rate of cell deaths exceeds the number of new cells formed and the population equilibrium shifts to a net reduction in numbers. The population may diminish until only a few cells remain, or the population may die out entirely.

Exotoxins Toxic proteins produced during bacterial growth and metabolism and released into the environment.

Fimbriae Short, hairlike, proteinaceous projections that may arise at the ends of the bacterial cell or over the entire surface. These projections let the bacteria adhere to surfaces.

Gram staining A method for classifying bacteria, developed in 1884 by Danish scientist Christian Gram, which is based upon a bacteriums ability or inability to retain a purple dye.

Kochs postulates A series of laboratory procedures, developed by German physician Robert Koch in the late nineteenth century, for proving that a specific organism cause a specific disease.

Lag phase Stage of bacterial growth in which metabolic activity occurs but no growth.

Log phase Stage of bacterial growth when metabolic activity is most intense and cell reproduction exceeds cell death. Also known as exponential phase.

Phage typing A method for identifying bacteria according to their response to bacteriophages, which are viruses that infect specific bacteria.

Pili Proteinaceous projections that occur singly or in pairs and join pairs of bacteria together, facilitating transfer of DNA between them.

Spirochetes Spiral-shaped bacteria which live in contaminated water, sewage, soil and decaying organic matter, as well as inside humans and animals.

Stationary phase Stage of bacterial growth in which the growth rate slows and the production of new cells equals the rate of cell death.

Resources

BOOKS

Alberts, Bruce, et al. Molecular Biology of the Cell. 4th. ed New York: Garland Science, 2002.

Dyer, Betsey Dexter. A Field Guide to Bacteria. Ithaca, NY: Cornell University Press, 2003.

Koehler, T.M. Anthrax. New York: Springer Verlag, 2002. Walsh, Christopher. Antibiotics: Actions, Origins,

Resistance. Washington, DC: American Society for Microbiology Press, 2003.

Marc Kusinitz

Brian Hoyle

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Bacteria

Bacteria

Bacteria are mostly unicellular organisms that lack chlorophyll and are among the smallest living things on earth—only viruses are smaller. Multiplying rapidly under favorable conditions, bacteria can aggregate into colonies of millions or even billions of organisms within a space as small as a drop of water .

The Dutch merchant and amateur scientist Anton van Leeuwenhoek was the first to observe bacteria and other microorganisms . Using single-lens microscopes of his own design, he described bacteria and other microorganisms (calling them "animacules") in a series of letters to the Royal Society of London between 1674 and 1723.

Bacteria are classified as prokaryotes. Broadly, this taxonomic ranking reflects the fact that the genetic material of bacteria is contained in a single, circular chain of deoxyribonucleic acid (DNA) that is not enclosed within a nuclear membrane . The word prokaryote is derived from Greek meaning "prenucleus." Moreover, the DNA of prokaryotes is not associated with the special chromosome proteins called histones, which are found in higher organisms. In addition, prokaryotic cells lack other membrane-bounded organelles, such as mitochondria. Prokaryotes belong to the kingdom Monera. Some scientists have proposed splitting this designation into the kingdoms Eubacteria and Archaebacteria . Eubacteria, or true bacteria, consist of more common species , while Archaebacteria (with the prefix archae—meaning ancient) represent strange bacteria that inhabit very hostile environments. Scientists believe these bacteria are most closely related to the bacteria which lived when the earth was very young. Examples of archaebacteria are those bacteria which currently live in extremely salty environments or extremely hot environments, like geothermal vents of the ocean floor.

Characteristics of bacteria

Although all bacteria share certain structural, genetic, and metabolic characteristics, important biochemical differences exist among the many species of bacteria. These differences permit bacteria to live in many different, and sometimes extreme, environments. For example, some bacteria recycle nitrogen and carbon from decaying organic matter , then release these gases into the atmosphere to be reused by other living things. Other bacteria cause diseases in humans and animals, help digest sewage in treatment plants, or produce the alcohol in wine, beer and liquors. Still others are used by humans to break down toxic waste chemicals in the environment, a process called bioremediation .

The cytoplasm of all bacteria is enclosed within a cell membrane surrounded by a rigid cell wall whose polymers, with few exceptions, include peptidoglycans—large, structural molecules made of protein carbohydrate .

Bacteria also secrete a viscous, gelatinous polymer (called the glycocalyx) on their cell surfaces. This polymer, composed either of polysaccharide, polypeptide, or both, is called a capsule when it occurs as an organized layer firmly attached to the cell wall. Capsules increase the disease-causing ability (virulence) of bacteria by inhibiting immune system cells called phagocytes from engulfing them. One such bacterium, Streptococcus pneumoniae, is the cause of pneumonia .

The shape of bacterial cells are classified as spherical (coccus), rodlike (bacillus), spiral (spirochete), helical (spirilla) and comma-shaped (vibrio) cells. Many bacilli and vibrio bacteria have whiplike appendages (called flagella ) protruding from the cell surface. Flagella are composed of tight, helical rotors made of chains of globular protein called flagellin, and act as tiny propellers, making the bacteria very mobile.

Flagella may be arranged in any of four ways, depending on the species of bacteria. There is the monotrichous condition (single flagellum at one end), the amphitrichous (single flagellum at each end of the bacterium), the lophotrichous (two or more flagella at either or both ends of the bacterium), and the peritrichous condition (flagella distributed over the entire cell).

Spirochetes are spiral-shaped bacteria that live in contaminated water, sewage, soil and decaying organic matter, as well as inside humans and animals. Spirochetes move by means of axial filaments, which consist of bundles of fibrils arising at each end of the cell beneath an outer sheath. The fibrils, which spiral around the cell, rotate, causing an opposite movement of the outer sheath that propels the spirochetes forward, like a turning corkscrew. The best known spirochete is Treponema pallidum, the organism that causes syphilis.

On the surface of some bacteria are short, hairlike, proteinaceous projections that may arise at the ends of the cell or over the entire surface. These projections, called fimbriae, let the bacteria adhere to surfaces. For example, fimbriae on the bacterium Neisseria gonorrhoea, which causes gonorrhea, allow these organisms to attach to mucous membranes.

Other proteinaceous projections, called pili, occur singly or in pairs, and join pairs of bacteria together, facilitating transfer of DNA between them.

During periods of harsh environmental conditions some bacteria, such as those of the genera Clostridium and Bacillus, produce within themselves a dehydrated, thick-walled endospore. These endospores can survive extreme temperatures, dryness, and exposure to many toxic chemicals and to radiation . Endospores can remain dormant for long periods (hundreds of years in some cases) before being reactivated by the return of favorable conditions.

A primitive form of exchange of genetic material between bacteria involving plasmids does occur. Plasmids are small, circular, extrachromosomal DNA molecules that are capable of replication and are known to be capable of transferring genes among bacteria. For example, resistance plasmids carry genes for resistance to antibiotics from one bacterium to another, while other plasmids carry genes that confer pathogenicity. In addition, the transfer of genes via bacteriophages—viruses that specifically parasitize bacteria—also serves as a means of genetic recombination. Corynebacterium diphtheriae, for example, produces the diphtheria toxin only when infected by a phage that carries the diphtherotoxin gene .

The above examples of genetic information exchange between bacterial cells occurs regularly in nature. This natural exchange, or lateral gene transfer, can be mimicked artificially in the laboratory. Bioengineering uses sophisticated techniques to purposely transfer DNA from one organism to another in order to give the second organism some new characteristic it did not have previously. For example, in a process called transformation, antibiotic susceptible bacteria that are induced to absorb manipulated plasmids placed in their environment, can acquire resistance to that antibiotic substance due to the new genes they have incorporated. Similarly, in a process called transfection, specially constructed viruses are used to artificially inject bioengineered DNA into bacteria, giving infected cells some new characteristic.

Bacteria synthesize special DNA-cutting enzymes (known as restriction enzymes) that destroy the DNA of phages that do not normally infect them. Purified restriction enzymes are used in the laboratory to slice pieces of DNA from one organism and insert them into the genetic material of another organism as mentioned above.

Bacterial growth

The term "bacterial growth" generally refers to growth of a population of bacteria, rather than of an individual cell. Individual cells usually reproduce asexually by means of binary fission, in which one cell divides into two cells. Thus, bacterial growth of the population is a geometric progression of numbers of cells, with division occurring in regular intervals, called generation time , ranging from 15 minutes to 16 hours, depending upon the type of bacterium. In addition, some filamentous bacteria (actinomycetes) reproduce by producing chains of spores at their tips, while other filamentous species fragment into new cells.


Stages of bacterial growth

Under ideal conditions, the growth of a population of bacteria occurs in several stages termed lag, log, stationary, and death.

During the lag phase, active metabolic activity occurs involving synthesis of DNA and enzymes, but no growth. Geometric population growth occurs during the log, or exponential phase, when metabolic activity is most intense and cell reproduction exceeds cell death . Following the log phase, the growth rate slows and the production of new cells equals the rate of cell death. This period, known as the stationary phase, involves the establishment of an equilibrium in population numbers and a slowing of the metabolic activities of individual cells. The stationary phase reflects a change in growing condition—for example, a lack of nutrients and/or the accumulation of waste products.

When the rate of cell deaths exceeds the number of new cells formed, the population equilibrium shifts to a net reduction in numbers and the population enters the death phase, or logarithmic decline phase. The population may diminish until only a few cells remain, or the population may die out entirely.


Physical and chemical requirements for bacterial growth

The physical and chemical requirements for growth can vary widely among different species of bacteria, and some are found in environments as extreme as cold polar regions and hot, acid springs.

In general, the physical requirements for bacteria include proper temperature , pH and osmotic pressure . Most bacteria thrive only within narrow ranges of these conditions, however extreme those ranges may be.


Temperature and bacteria

The lowest temperature at which a particular species will grow is the minimum growth temperature, while the maximum growth temperature is the highest temperature at which they will grow. The temperature at which their growth is optimal is called the optimum growth temperature. In general, the maximum and minimum growth temperatures of any particular type of bacteria are about 30°F (-1°C) apart.

Most bacteria thrive at temperatures at or around that of the human body 98.6°F (37°C), and some, such as Escherichia coli , are normal parts of the human intestinal flora . These organisms are mesophiles (moderate-temperature-loving), with an optimum growth temperature between 77°F (25°C) and 104°F (40°C). Mesophiles have adapted to thrive in temperatures close to that of their host.

Psychrophiles, which prefer cold temperatures, are divided into two groups. One group has an optimal growth temperature of about 59°F (15°C), but can grow at temperatures as low as 32°F (0°C). These organisms live in ocean depths or Arctic regions. Other psychrophiles that can also grow at 32°F (0°C) have an optimal growth temperature between 68°F (20°C) and 86°F (30°C). These organisms, sometimes called psychrotrophs, are often those associated with food spoilage under refrigeration.

Thermophiles thrive in very hot environments, many having an optimum growth temperature between 122°F (50°C) and 140°F (60°C), similar to that of hot springs in Yellowstone National Park. Such organisms thrive in compost piles, where temperatures can rise as high as 140°F (60°C). Extreme thermophiles grow at temperatures above 195°F (91°C). Along the sides of hydrothermal vents on the ocean bottom 217 mi (350 km) north of the Galapagos Islands, for example, bacteria grow in temperatures that can reach 662°F (350°C).


pH and bacteria

Like temperature, pH also plays a role in determining the ability of bacteria to grow or thrive in particular environments. Most commonly, bacteria grow optimally within a narrow range of pH between 6.7 and 7.5.

Acidophiles, however, prefer acidic conditions. For example, Thiobacillus ferrooxidans, which occurs in drainage water from coal mines, can survive at pH 1. Other bacteria, such as Vibrio cholera, the cause of cholera , can thrive at a pH as high as 9.0.


Osmotic pressure and bacteria

Osmotic pressure is another limiting factor in the growth of bacteria. Bacteria are about 80-90% water; they require moisture to grow because they obtain most of their nutrients from their aqueous environment.

Cell walls protect prokaryotes against changes in osmotic pressure over a wide range. However, sufficiently hypertonic media at concentrations greater than those inside the cell (such as 20% sucrose) cause water loss from the cell by osmosis . Fluid leaves the bacteria causing the cell to contract, which, in turn, causes the cell membrane to separate from the overlying cell wall. This process of cell shrinkage is called plasmolysis.

Because plasmolysis inhibits bacterial cell growth, the addition of salts or other solutes to a solution inhibits food spoilage by bacteria, as occurs when meats or fish is salted.

Some types of bacteria, called extreme or obligate halophiles, are adapted to—and require—high salt concentrations, such as found in the Dead Sea, where salt concentrations can reach 30%. Facultative halophiles do not require high salt environments to survive, but are capable of tolerating these conditions. Halophiles can grow in salt concentrations up to 2%, a level that would inhibit the growth of other bacteria. However, some facultative halophiles, such as Halobacterium halobium grow in salt lakes, salt flats, and other environments where the concentration of salts is up to seven times greater than that of the oceans.

When bacteria are placed in hypotonic media with concentrations weaker than the inside of the cell, water tends to enter by osmosis. The accumulation of this water causes the cell to swell and then to burst, a process called osmotic lysis.


Carbon, nitrogen and other growth factors

In addition to water and the correct salt balance, bacteria also require a wide variety of elements, especially carbon, hydrogen , and nitrogen, sulfur and phosphorus , potassium, iron , magnesium and calcium . Growth factors, such as vitamins and pyrimidines and purines (the building blocks of DNA), are also necessary.

Carbon is the fundamental building block of all the organic compounds needed by living things, including nucleic acids, carbohydrates, proteins and fats.

Chemoheterotrophs are bacteria that use organic compounds such as proteins, carbohydrates and lipids as their carbon source, and which use electrons from organic compounds as their energy source. Most bacteria (as well as all fungi , protozoans and animals) are chemoheterotrophs. Chemoautotrophs (for example hydrogen, sulfur, iron, and nitrifying bacteria) use carbon dioxide as their carbon source and electrons from inorganic compounds as their energy source.

Saprophytes are heterotrophs that obtain their carbon from decaying dead organic matter. Many different soil bacteria release plant nitrogen as ammonia (ammonification ). Other bacteria, the Nitrosomonas, convert ammonia to nitrite, while Nitrobacter convert nitrite to nitrate. Other bacteria, especially Pseudomonas, convert nitrate to nitrogen gas. These bacteria complement the activity of nitrogen-fixing bacteria (for example, Rhizobium), which fix nitrogen from the atmosphere and make it available to leguminous plants, and Azotobacter, which are also found in fresh and marine waters. Together, the activity of these bacteria underlies the nitrogen cycle , by which the gas is taken up by living organisms, used to make proteins and other organic compounds, returned to the soil during decay, then released into the atmosphere to be reused by living things.

Phototrophs use light as their primary source of energy, but may differ in their carbon sources. Photo-heterotrophs (purple nonsulfur and green nonsulfur bacteria) use organic compounds as their carbon source, while photoautotrophs (for example, photosynthetic green sulfur and purple sulfur bacteria) use carbon dioxide as a source of carbon.


Aerobic and anaerobic bacteria

Oxygen may or may not be a requirement for a particular species of bacteria, depending on the type of metabolism used to extract energy from food (aerobic or anaerobic ). In all cases, the initial breakdown of glucose to pyruvic acid occurs during glycolysis , which produces a net gain of two molecules of the energy-rich molecule adenosine triphosphate (ATP).


Aerobic bacteria

Aerobic bacteria use oxygen to break down pyruvic acid, releasing much more ATP than is produced during glycolysis during the process known as aerobic respiration . In addition, aerobic bacteria have enzymes such as superoxide dismutase capable of breaking down toxic forms of oxygen, such as superoxide free radicals, which are also formed by aerobic respiration.

During aerobic respiration, enzymes remove electrons from the organic substrate and transfer them to theelectron transport chain, which is located in the membrane of the mitochondrion. The electrons are transferred along a chain of electron carrier molecules. At the final transfer position, the electrons combine with atoms of oxygen—the final electron acceptor—which in turn combines with protons (H+) to produce water molecules. Energy, in the form of ATP, is also made here. Along the chain of electron carriers, protons that are pumped across the mitochondrial membrane re-enter the mitochondrion. This flow of electrons across the membrane fuels oxidative phosphorylation, the chemical reaction that adds a phosphate group to adenosine diphosphate (ADP) to produce ATP.

Obligate aerobes must have oxygen in order to live. Facultative aerobes can also exist in the absence of oxygen by using fermentation or anaerobic respiration. Anaerobic respiration and fermentation occur in the absence of oxygen, and produce substantially less ATP than aerobic respiration.


Anaerobic bacteria

Anaerobic bacteria use inorganic substances other than oxygen as a final electron acceptor. For example, Pseudomonas and Bacillus reduce nitrate ion (NO3 ) to nitrite ion (NO2 ), nitrous oxide (N2O) or nitrogen gas (N2). Clostridium species, which include those that cause tetanus and botulism , are obligate anaerobes. That is, they are not only unable to use molecular oxygen to produce ATP, but are harmed by toxic forms of oxygen formed during aerobic respiration. Unlike aerobic bacteria, obligate anaerobes lack the ability synthesize enzymes to neutralize these toxic forms of oxygen.



The role of bacteria in fermentation

Fermentation bacteria are anaerobic, but use organic molecules as their final electron acceptor to produce fermentation end-products. Streptococcus, Lactobacillus, and Bacillus, for example, produce lactic acid , while Escherichia and Salmonella produce ethanol , lactic acid, succinic acid, acetic acid , CO2, and H2.

Fermenting bacteria have characteristic sugar fermentation patterns, i.e., they can metabolize some sugars but not others. For example, Neisseria meningitidis ferments glucose and maltose, but not sucrose and lactose, while Neisseria gonorrhoea ferments glucose, but not maltose, sucrose or lactose. Such fermentation patterns can be used to identify and classify bacteria.

During the 1860s, the French microbiologist Louis Pasteur studied fermenting bacteria. He demonstrated that fermenting bacteria could contaminate wine and beer during manufacturing, turning the alcohol produced by yeast into acetic acid (vinegar). Pasteur also showed that heating the beer and wine to kill the bacteria preserved the flavor of these beverages. The process of heating, now called pasteurization in his honor, is still used to kill bacteria in some alcoholic beverages, as well as milk.

Pasteur described the spoilage by bacteria of alcohol during fermentation as being a "disease" of wine and beer. His work was thus vital to the later idea that human diseases could also be caused by microorganisms, and that heating can destroy them.


Identifying and classifying bacteria

The most fundamental technique for classifying bacteria is the gram stain, developed in 1884 by Danish scientist Christian Gram. It is called a differential stain because it differentiates among bacteria and can be used to distinguish among them, based on differences in their cell wall.

In this procedure, bacteria are first stained with crystal violet, then treated with a mordant—a solution that fixes the stain inside the cell (e.g., iodine-KI mixture). The bacteria are then washed with a decolorizing agent, such as alcohol, and counterstained with safranin, a light red dye.

The walls of gram positive bacteria (for example, Staphylococcus aureus) have more peptidoglycans (the large molecular network of repeating disaccharides attached to chains of four or five amino acids) than do gram-negative bacteria. Thus, gram-positive bacteria retain the original violet dye and cannot be counterstained.

Gram negative bacteria (e.g., Escherichia coli) have thinner walls, containing an outer layer of lipopolysaccharide, which is disrupted by the alcohol wash. This permits the original dye to escape, allowing the cell to take up the second dye, or counterstain. Thus, gram-positive bacteria stain violet, and gram-negative bacteria stain pink.

The gram stain works best on young, growing populations of bacteria, and can be inconsistent in older populations maintained in the laboratory.

Microbiologists have accumulated and organized the known characteristics of different bacteria in a reference book called Bergey's Manual of Systematic Bacteriology (the first edition of which was written primarily by David Hendricks Bergey of the University of Pennsylvania in 1923).

The identification schemes of Bergey's Manual are based on morphology (e.g., coccus, bacillus), staining (gram-positive or negative), cell wall composition (e.g., presence or absence of peptidoglycan), oxygen requirements (e.g., aerobic, facultatively anaerobic) and biochemical tests (e.g., which sugars are aerobically metabolized or fermented).

In addition to the gram stain, other stains include the acid-fast stain, used to distinguish Mycobacterium species (for example, Mycobacterium tuberculosis, the cause of tuberculosis ); endospore stain, used to detect the presence of endospores; negative stain, used to demonstrate the presence of capsules; and flagella stain, used to demonstrate the presence of flagella.

Another important identification technique is based on the principles of antigenicity—the ability to stimulate the formation of antibodies by the immune system. Commercially available solutions of antibodies against specific bacteria (antisera) are used to identify unknown organisms in a procedure called a slide agglutination test. A sample of unknown bacteria in a drop of saline is mixed with antisera that has been raised against a known species of bacteria. If the antisera causes the unknown bacteria to clump (agglutinate), then the test positively identifies the bacteria as being identical to that against which the antisera was raised. The test can also be used to distinguish between strains, slightly different bacteria belonging to the same species.

Phage typing, like serological testing, identifies bacteria according to their response to the test agent, in this case viruses. Phages are viruses that infect specific bacteria. Bacterial susceptibility to phages is determined by growing bacteria on an agar plate, to which solutions of phages that infect only a specific species of bacteria are added. Areas that are devoid of visible bacterial growth following incubation of the plate represent organisms susceptible to the specific phages.

Because a specific bacterium might be susceptible to infection by two or more different phages, it may be necessary to perform several tests to definitively identify a specific bacterium.

The evolutionary relatedness of different species can also be determined by laboratory analysis. For example, analysis of the amino acid sequences of proteins from different bacteria disclose how similar the proteins are. In turn, this reflects the similarity of the genes coding for these proteins.

Protein analysis compares the similarity or extent of differences between the entire set of protein products of each bacterium. Using a technique called electrophoresis , the entire set of proteins of each bacterium is separated according to size by an electrical charge applied across gel. The patterns produced when the gel is stained to show the separate bands of proteins reflects the genetic makeup, and relatedness, of the bacteria.

The powerful techniques of molecular biology have given bacteriologists other tools to determine the identity and relatedness of bacteria.

Taxonomists interested in studying the relatedness of bacteria compare the ratio of nucleic acid base pairs in the DNA of microorganisms, that is, the number of guanosine-cytosine pairs in the DNA. Because each guanosine on a double-stranded molecule of DNA has a complementary cytosine on the opposite strand, comparing the number of G-C pairs in one bacterium, with that in another bacterium, provides evidence for the extent of their relatedness.

Determining the percentage of G-C pairs making up the DNA also discloses the percentage of adenosine-thymine (A-T)—the other pair of complementary nucleic acids making up DNA (100% - [ % G-C] = % A-T).

The closer the two percentages are, the more closely related the bacteria may be, although other lines of evidence are needed to make a definitive determination regarding relationships.

The principle of complementarity is also used to identify bacteria by means of nucleic acid hybridization. The technique assumes that if two bacteria are closely related, they will have long stretches of identical DNA. First, one bacterium's DNA is isolated and gently heated to break the bonds between the two complementary strands. Specially prepared DNA probes representing short segments of the other organism's DNA are added to this solution of single-stranded DNA. The greater degree to which the probes combine with (hybridize) complementary stretches of the single stranded DNA, the greater the relatedness of the two organisms.

In addition to helping bacteriologists better classify bacteria, the various laboratory tests are valuable tools for identifying disease-causing organisms. This is especially important when physicians must determine which antibiotic or other medication to use to treat an infection.


Bacteria and disease

The medical community did not accept the concept that bacteria can cause disease until well into the nineteenth century. Joseph Lister, an English surgeon, applied the so-called "germ theory" to medical practice in the 1860s. Lister soaked surgical dressing in carbolic acid (phenol), which reduced the rate of post-surgical infections so dramatically that the practice spread.

In 1876, the German physician Robert Koch identified Bacillus anthracis as the cause of anthrax , and in so doing, developed a series of laboratory procedures for proving that a specific organism can cause a specific disease. These procedures, called Koch's postulates, are still generally valid. Briefly, they state that, to prove an organism causes a specific disease, the investigator must:

  1. Find the same pathogenic microorganism in every case of the disease.
  2. Isolate the pathogen from the diseased patient or experimental animal and grow it in pure culture.
  3. Demonstrate that the pathogen from the pure culture causes the disease when it is injected into a healthy laboratory animal.
  4. Isolate the pathogen from the inoculated animal and demonstrate that it is the original organism injected into the animal.

The ability to isolate, study, and identify bacteria has greatly enhanced the understanding of their diseasecausing role in humans and animals, and the subsequent development of treatments. Part of that understanding derives from the realization that since bacteria are ubiquitous and are found in large numbers in and on humans, they can cause a wide variety of diseases.

The skin and the nervous, cardiovascular, respiratory, digestive and genitourinary systems are common sites of bacterial infections, as are the eyes and ears.

The skin is the body's first line of defense against infection by bacteria and other microorganisms, although it supports enormous numbers of bacteria itself, especially Staphylococcus and Streptococcus species. Sometimes these bacteria are only dangerous if they enter a break in the skin or invade a wound, for example, the potentially fatal staphylococcal toxic shock syndrome . Among other common bacterial skin ailments are acne , caused by Propionibacterium acnes and superficial infection of the outer ear canal, caused by Pseudomonas aeruginosa.

Among the neurological diseases are meningitis , an inflammation of the brain's membranes caused by Neisseria meningitidis and Hemophilus influenzae.

Many medically important bacteria produce toxins, poisonous substances that have effects in specific areas of the body. Exotoxins are proteins produced during bacterial growth and metabolism and released into the environment. Most of these toxin-producing bacteria are gram positive.

Among the gram positive toxin-producing bacteria are Clostridium tetani, which causes tetanus, an often fatal paralytic disease of muscles; Clostridium botulinum, which causes botulism, a form of potentially lethal food poisoning ; and Staphylococcus aureus, which also causes a form of food poisoning (gastroenteritis).

Most gram negative bacteria (for example, Salmonella typhi, the cause of typhoid fever ) produce endotoxins, toxins that are part of the bacterial cell wall.

As the role of bacteria in causing disease became understood, entire industries developed that addressed the public health issues of these diseases.

As far back as 1810, the French confectioner Nicholas Appert proved that food stored in glass bottles and heated to high temperatures could be stored for long periods of time without spoiling. Appert developed tables that instructed how long such containers should be boiled, depending upon the type of food and size of the container. Today, the food preservation industry includes not only canning, but also freezing and freeze-drying. An important benefit to food preservation is the ability to destroy potentially lethal contamination by Clostridium botulinum spores.

Even as concepts of prevention of bacterial diseases were being developed, scientists were looking for specific treatments. Early in the twentieth century, the German medical researcher Paul Ehrlich theorized about producing a "magic bullet" that would destroy pathogenic organisms without harming the host.

In 1928, the discovery by Scottish bacteriologist Alexander Fleming that the mold Penicillium notatum inhibited growth of Staphylococcus aureus ushered in the age of antibiotics. Subsequently, English scientists Howard Florey and Ernst Chain, working at Oxford University in England, demonstrated the usefulness of penicillin, the anti-bacterial substance isolated from P. notatum in halting growth of this bacterium. This inhibitory effect of penicillin on bacteria is an example of antibiosis, and from this term is derived the word antibiotic, which refers to a substance produced by microorganisms that inhibits other microorganisms.

Beginning in the 1930s, the development of synthetic anti-bacterial compounds called sulfa drugs further stimulated the field of anti-bacterial drug research. The many different anti-bacterial drugs available today work in a variety of ways, such as the inhibition of synthesis of cell walls, of proteins, or of DNA or RNA.

Today, medical science and the multi-billion dollar pharmaceutical industry are facing the problem of bacterial resistance to drugs, even as genetically engineered bacteria are being used to produce important medications for humans.


Bacterial adaptation

Bacteria have been designed to be adaptable. Their surrounding layers and the genetic information for these and other structures associated with a bacterium are capable of alteration. Some alterations are reversible, disappearing when the particular pressure is lifted. Other alterations are maintained and can even be passed on to succeeding generations of bacteria.

The first antibiotic was discovered in 1929. Since then, a myriad of naturally occurring and chemically synthesized antibiotics have been used to control bacteria. Introduction of an antibiotic is frequently followed by the development of resistance to the agent. Resistance is an example of the adaptation of the bacteria to the antibacterial agent.

Antibiotic resistance can develop swiftly. For example, resistance to penicillin (the first antibiotic discovered) was recognized almost immediately after introduction of the drug. As of the mid 1990s, almost 80% of all strains of Staphylococcus aureus were resistant to penicillin. Meanwhile, other bacteria remain susceptible to penicillin. An example is provided by Group A Streptococcus pyogenes, another Gram-positive bacteria.


The adaptation of bacteria to an antibacterial agent such as an antibiotic can occur in two ways. The first method is known as inherent (or natural) resistance. Gram-negative bacteria are often naturally resistant to penicillin, for example. This is because these bacteria have another outer membrane, which makes the penetration of penicillin to its target more difficult. Sometimes when bacteria acquire resistance to an antibacterial agent, the cause is a membrane alteration that has made the passage of the molecule into the cell more difficult.

The second category of adaptive resistance is called acquired resistance. This resistance is almost always due to a change in the genetic make-up of the bacterial genome . Acquired resistance can occur because of mutation or as a response by the bacteria to the selective pressure imposed by the antibacterial agent. Once the genetic alteration that confers resistance is present, it can be passed on to subsequent generations. Acquired adaptation and resistance of bacteria to some clinically important antibiotics has become a great problem in the last decade of the twentieth century.

Bacteria adapt to other environmental conditions as well. These include adaptations to changes in temperature, pH, concentrations of ions such as sodium , and the nature of the surrounding support. An example of the latter is the response shown by Vibrio parahaemolyticus to growth in a watery environment versus a more viscous environment. In the more viscous setting, the bacteria adapt by forming what are called swarmer cells. These cells adopt a different means of movement, which is more efficient for moving over a more solid surface. This adaptation is under tight genetic control, involving the expression of multiple genes.

Bacteria react to a sudden change in their environment by expressing or repressing the expression of a whole lost of genes. This response changes the properties of both the interior of the organism and its surface chemistry . A well-known example of this adaptation is the so-called heat shock response of Escherichia coli. The name derives from the fact that the response was first observed in bacteria suddenly shifted to a higher growth temperature.

One of the adaptations in the surface chemistry of Gram-negative bacteria is the alteration of a molecule called lipopolysaccharide. Depending on the growth conditions or whether the bacteria are growing on an artificial growth medium or inside a human, as examples, the lipopolysaccharide chemistry can become more or less water-repellent. These changes can profoundly affect the ability of antibacterial agents or immune components to kill the bacteria.

Another adaptation exhibited by Vibrio parahaemolyticus, and a great many other bacteria as well, is the formation of adherent populations on solid surfaces. This mode of growth is called a biofilm. Adoption of a biofilm mode of growth induces a myriad of changes, many involving the expression of previously unexpressed genes. In addition,l de-activation of actively expressing genes can occur. Furthermore, the pattern of gene expression may not be uniform throughout the biofilm. Bacteria within a biofilm and bacteria found in other niches, such as in a wound where oxygen is limited, grow and divide at a far slower speed than the bacteria found in the test tube in the laboratory. Such bacteria are able to adapt to the slower growth rate, once again by changing their chemistry and gene expression pattern.

A further example of adaptation is the phenomenon of chemotaxis, whereby a bacterium can sense the chemical composition of the environment and either moves toward an attractive compound, or shifts direction and moves away from a compound sensed as being detrimental. Chemotaxis is controlled by more than 40 genes that code for the production of components of the flagella that propels the bacterium along, for sensory receptor proteins in the membrane, and for components that are involved in signaling a bacterium to move toward or away from a compound. The adaptation involved in the chemotactic response must have a memory component, because the concentration of a compound at one moment in time must be compared to the concentration a few moments later.

See also Antisepsis; Biodegradable substances; Biodiversity; Composting; Microbial genetics; Origin of life; Water microbiology; Nitrogen fixation.

Resources

books

Alberts, et al. Molecular Biology of the Cell, 4th. ed New York: Garland Science, 2002.

Cullimore, Roy D. Practical Atlas for Bacterial Determination Boca Raton, FL: CRC Press, 2000.

Dyer, Betsey Dexter. A Field Guide to Bacteria. Ithaca, NY: Cornell University Press, 2003.

Groisman, Eduardo A. Principles of Bacterial Pathogenesis. Burlington, MA: Academic Press, 2000.

Koehler, T.M. Anthrax New York: Springer Verlag, 2002.

Walsh, Christopher. Antibiotics: Actions, Origins, Resistance. Washington, DC: American Society for Microbiology Press, 2003.


other

The Foundation for Bacteriology, New York University. "Virtual Museum of Bacteria" [cited February 5, 2003]. <http://www.bacteriamuseum.org/main1.shtml>.


Marc Kusinitz

Brian Hoyle

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Capsule

—A viscous, gelatinous polymer composed either of polysaccharide, polypeptide, or both, that surrounds the surface of some bacteria cells. Capsules increase the disease-causing ability (virulence) of bacteria by inhibiting immune system cells called phagocytes from engulfing them.

Death phase

—Stage of bacterial growth when the rate of cell deaths exceeds the number of new cells formed and the population equilibrium shifts to a net reduction in numbers. The population may diminish until only a few cells remain, or the population may die out entirely.

Exotoxins

—Toxic proteins produced during bacterial growth and metabolism and released into the environment.

Fimbriae

—Short, hairlike, proteinaceous projections that may arise at the ends of the bacterial cell or over the entire surface. These projections let the bacteria adhere to surfaces.

Gram staining

—A method for classifying bacteria, developed in 1884 by Danish scientist Christian Gram, which is based upon a bacterium's ability or inability to retain a purple dye.

Koch's postulates

—A series of laboratory procedures, developed by German physician Robert Koch in the late nineteenth century, for proving that a specific organism cause a specific disease.

Lag phase

—Stage of bacterial growth in which metabolic activity occurs but no growth.

Log phase

—Stage of bacterial growth when metabolic activity is most intense and cell reproduction exceeds cell death. Also known as exponential phase.

Phage typing

—A method for identifying bacteria according to their response to bacteriophages, which are viruses that infect specific bacteria.

Pili

—Proteinaceous projections that occur singly or in pairs and join pairs of bacteria together, facilitating transfer of DNA between them.

Spirochetes

—Spiral-shaped bacteria which live in contaminated water, sewage, soil and decaying organic matter, as well as inside humans and animals.

Stationary phase

—Stage of bacterial growth in which the growth rate slows and the production of new cells equals the rate of cell death.

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Bacteria

Bacteria


Bacteria are a group of one-celled organisms so small they can only be seen with a microscope. As the most abundant form of life on Earth, they live in every habitat—in the air, soil, and water, as well as within the bodies of living things. As the simplest of all organisms, they reproduce asexually by splitting in two. Although bacteria are often associated with disease, they perform an enormously useful function in the natural world since they are responsible for much of the decay of organic material.

Bacteria were first discovered in 1683 by the Dutch microscopist, Anton van Leeuwenhoek (1632–1723), who called them "little animalcules." However, it was not until the middle of the nineteenth century that biologists began to understand bacteria better. Although they were considered to be animals and then plants, bacteria eventually came to be placed in the Monera kingdom since they do not have a distinct nucleus (a cell's control center). Members of the four other kingdoms (Protista, Fungi, Plantae, and Animalia) all are eukaryotic, meaning their cells have nuclei kept within a membrane. Most bacteria are single-celled and can be grouped according to three definite shapes. Rod-shaped bacteria are called "bacilli." One species of bacillus causes the cattle disease anthrax. Spherical or round bacteria are known as "cocci." Certain "cocci" can cause staph or strep infections. Bacteria that have a helical or coiled shape resembling corkscrews are known as "spirilla." Spirilla are often carried by rats and can cause a form of rat-bite fever. Bacteria can also sometimes be in the shape of a fat comma, and these are called "vibroids." A particularly nasty form of this bacteria causes cholera.

ROBERT KOCH

German bacteriologist (one who specializes in the study of bacteria) Robert Koch (1843–1910) was the first to prove that a specific bacteria (a group of one-celled organisms so small they can only be seen with a microscope) can cause a given disease, thus helping to establish the germ theory of disease. As a pioneer in bacteria and cell culture, he also established the rules, called Koch's postulates, upon which modern bacteriology is built. He won the 1905 Nobel Prize for Physiology and Medicine for his work on tuberculosis.

Robert Koch was born near Hanover, Germany, and was one of thirteen children. Although his father wanted him to be a shoemaker, Koch was able to go to school and eventually received a medical degree in 1866. After serving his country as a surgeon in the Franco-Prussian War, he settled down as a country doctor in Wollstein in 1872. It was there that he began to study anthrax, a cattle disease, since an epidemic had struck the area. No one knew what caused this disease, and Koch set himself the task of finding out. What he would do however, would be not only to teach the world about anthrax, but help bacteriology to develop as a science as well.

Koch first obtained the anthrax bacterium from the spleen of infected cattle and gave the disease to a mouse. He then transferred the infection from that mouse to another, and then to another, until he was sure that he could identify the particular rod-shaped bacterium (called a bacillus) that caused anthrax with absolute certainty. In order to do this, Koch had to essentially invent the techniques of studying microorganisms (organisms that can only be seen with a microscope). For example, he developed a technique in which he spread a liquid gelatin on glass slide plates, which enabled him to examine a pure culture and even to photograph it. What Koch also learned to do was how to cultivate bacteria, or allow it to grow and multiply, outside the living body. Koch used blood serum at body temperature to do this, and was therefore also able to trace the entire life cycle of the anthrax bacterium. The specific techniques that Koch pioneered would serve as a model for others to follow.

Before Koch, many others had put forth the germ theory of disease, arguing that certain diseases are caused and transmitted by specific microorganisms. However, until Koch, no one had been able to prove this theory. With his identification of the anthrax bacillus, Koch not only demonstrated that this theory was true, but he also established the rules for properly identifying the cause of a disease. Called "Koch's postulates," these rules guided many a researcher in the right direction, and they still hold true today.

First, Koch determined that the suspected microorganism must be found in the infected animal. Second, after being cultured, or grown, it must be able to reinfect a healthy animal with the same disease. Third, the exact same microorganism must be found in the second animal that infected the first. Using his own techniques and rules, Koch isolated the cause of cholera epidemics in Egypt and India and discovered the tubercle bacillus that causes the lung disease, tuberculosis.

Until Koch isolated this microorganism, science was baffled by tuberculosis, not knowing how or whether it actually spread or whether it was simply hereditary. Once he identified the bacillus, however, he was able to show that tuberculosis was caused by a germ that could be carried in the air and passed from one person to another. Until Koch had proven the germ theory of disease once and for all, the science of medicine could not really progress much beyond what it was centuries before when people believed in spontaneous generation (the idea that living things can come from nonliving matter, such as maggots from rotting meat). After Koch, the science of microbiology, which is the study of things that can only be seen with a microscope, could really begin to make progress.

PASTEURIZATION KILLS BACTERIA

Most scientists believe that bacteria were the earliest forms of life on Earth. Today, there are more than 10,000 species that have been identified. Bacteria can live in many different environments—even under extreme conditions such as lack of oxygen. Some live in or on other organisms, and many that grow inside the bodies of animals cause disease and are known commonly as "germs." Examples of some of the more common infectious diseases caused by bacteria are strep throat, tuberculosis, typhoid fever, and tooth decay. Certain bacteria can also cause milk and wine to go bad. In the nineteenth century, the French chemist Louis Pasteur (1822–1895) discovered that heat would kill bacteria. When he examined good wine, he noticed that it contained yeast cells that caused the process of fermentation and therefore produced alcohol. However, when he looked at sour wine under a microscope, he saw bacteria as well. His successful heat remedy to kill unwanted bacteria has come to be known as pasteurization. It was Pasteur who also discovered the bacterial origin of certain diseases like anthrax.

THE BENEFITS OF BACTERIA

Many bacteria are harmless and essential to the well-being of certain ecosystems (an area in which living things interact with each other on the environment). Bacteria have always played a vital role in breaking down organic material after they are dead, and because of bacteria, nature is able to recycle that material's basic chemicals back into the environment. It is bacteria that immediately break down an animal's waste products. Without bacteria, the leaves that fall to the ground or the grass that is cut would never rot and get reused by nature. Just as important, the environment would be cluttered and smelly without bacteria decomposing organic matter. Bacteria even help animals digest the food they eat.

Humans have mastered killing unwanted, disease-causing bacteria (usually with antibiotics) and have learned to use bacteria in many helpful ways. Some bacteria produce desirable chemicals such as ethyl alcohol and acetic acid, while others are used in the production of food products such as cheese, butter, coffee, wine, and cocoa. Bacteria are used in the manufacture of silk, cotton, and rubber, and help produce useful medical substances like insulin, antibiotics, and interferon. Certain bacteria help clean up oil spills in the ocean by breaking up the oil into its harmless components. Bacteria have little trouble reproducing, since each bacterium can do so on its own. A bacterium reproduces by a process called "binary fission," meaning that it splits in two, making a pair of identical cells. Under proper conditions, bacteria do this about every twenty minutes.

While certain bacteria are responsible for human misery and even death, others are absolutely essential to the mechanisms of the natural world. Therefore, much of the proper functioning of nature is attributable to vastly abundant but virtually invisible organisms called bacteria.

[See alsoAntibiotics; Fermentation ]

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