bacteria Bacteria are distinguished from all other life forms by their prokaryotic (literally, ‘before the nucleus’) cell structure. These microscopic organisms, typically 1–5 micrometres (μm) long, are distinguished by the absence of sub-cellular organelles, such as a nucleus, mitochondria, and chloroplasts. These organelles occur in the eukaryotic cells of higher organisms. Bacteria are ancient organisms, being the first to evolve some 3.8 billion years (Ga) ago and they were the sole type of life for 70 per cent of the history of life on Earth. They are not, however, primitive. They are well adapted to their large range of habitats; they are the most numerous organisms on Earth, and their distribution defines the limits of the biosphere.

Although the variety of different bacterial cell types is limited (there are, for example, rods, cocci, spirilla, and filaments), this belies their vast metabolic diversity and their ability to grow under a remarkably wide range of conditions (for example, at −5 to 113 °C, at pH values ranging from 0 to 11, in near-vacuum or at pressures 1000 times greater than atmospheric, and in distilled water or in saturated salt solution).

Different types of bacteria obtain energy in different ways.(1) Some bacteria obtain energy from photosynthesis, by processes similar to those used by green plants. Blue-green algae, which are actually bacteria, are thought to have been the first to have evolved oxygen-forming photosynthesis; their activity was responsible for the formation of our oxygenated atmosphere and hence the evolution of complex metazoans (plants, animals), which require oxygen.(2) Some photosynthetic bacteria, however, neither produce nor require oxygen; these anoxygenic photoautotrophs represent a more primitive form of photosynthesis. They use reduced compounds such as hydrogen sulphide and ferrous (Fe²⁺) iron, oxidizing them respectively to sulphur and ferric iron. The anaerobic formation of ferric (Fe³⁺) iron may have been important for the early formation of geological banded iron deposits.(3) Like animals, some bacteria can conduct aerobic respiration. These types are termed aerobic heterotrophs.(4) Unlike animals, however, some heterotrophic bacteria are not restricted to using oxygen for respiration and for many of these anaerobic bacteria oxygen is poisonous. Other bacteria can use instead the oxygen in other compounds such as nitrates (NO₃⁻), sulphate (SO₄²⁻), carbon dioxide (CO₂), and even from metal oxides (for example, iron and manganese).(5) Other heterotrophic bacteria do not require any respiratory compound, but instead gain energy from splitting organic compounds into a reduced and an oxidized product, a processes called fermentation. Fermentation products are of considerable commercial importance; for example, citric acid is used widely in foods and beverages for flavouring, and itaconic acid is used in the production of acrylic resins.

Heterotrophic bacteria (types (3)–(5) above) are the detrital specialists. Working together with other micro-organisms they degrade the organic compounds from dead plants and animals extremely efficiently and in this way return nutrients that are essential for further photosynthesis. They similarly drive the biogeochemical cycles of the major elements carbon, sulphur, and nitrogen. These microbial processes are optimized in sewage treatment plants, which enable humans to live at high population densities without contaminating each other and their local environment.(6) Some bacteria specialize in obtaining energy from inorganic rather than organic sources. This unique metabolism includes the oxidation of reduced metals and minerals, generally by using oxygen directly. Relatively little energy is obtained from these reactions, and these bacteria therefore have to process a large amount of material. During mining, minerals are exposed to oxygen. Bacterial oxidation can then be a major problem, especially in abandoned mines. In these circumstances, high concentrations of metals can be produced, together with inorganic acids (such as sulphuric acid), because bacteria oxidize sulphide minerals such as pyrite (FeS₂) to sulphate and ferrous iron. The acid waters are referred to as acid mine drainage. Groundwater and local streams can be made very acidic (pH less than 2), which can kill most wildlife. These conditions are, however, optimal for the bacterial ‘miners’. Microbial mining reactions can, on the other hand, be turned to commercial advantage to extract metals from low-grade ores. Reduced metals and sulphides are also a major source of energy for bacterial communities at hydrothermal vents at ocean ridges. Reduced hydrothermal fluids are geothermal products, and these bacteria and the animal communities that feed on them are unique ecosystems.(7) An even stranger inorganic metabolism, inorganic fermentation, is conducted by some bacteria.

In the environment, bacteria tend to work as an interacting team. Although large bacterial populations are commonly present (about 2000 million bacteria per cubic centimetre in soil, for example), a much smaller number may be active. Small environmental changes can produce conditions that are more suitable to a portion of the non-active bacterial population. There can consequently be rapid changes in the bacterial community to maintain efficient processing of energy sources, and hence stable biogeochemical cycles. In addition, bacterial growth rates can be very rapid (the fastest are about one cell division every 20 minutes), providing further opportunity for bacterial populations to adapt to changing conditions. Associated with these high growth rates are high mutation rates, and hence the possibility for genetic modification and adaptation. Consideration of the extensive metabolic activity of bacteria and their capacity to adapt and evolve resulted in the concept of ‘microbial infallibility’. This concept implied that bacteria would adapt to degrade any chemicals that were artificially introduced into the environment, such as pesticides and herbicides (xenobiotics, strange to life), and hence few precautions would be required in their use. This proved to be a great oversimplification, because although bacteria can degrade many xenobiotics, some are directly toxic to bacteria and some produce toxic break-down products; others provide insufficient energy to support degradation.

Bacteria have adapted effectively to inhabit other organisms, both external and internal. They are commonly present in large numbers in the digestive systems of animals, particularly herbivores, where they assist in the breakdown of decay-resistant compounds such as cellulose from plant material. In ruminants, such as cows and sheep, this relationship has developed to such an extent that a separate stomach (the rumen) has evolved to provide space in which large microbial populations can develop and break down the cellulose in grass. The animal does not have the enzymes to break down cellulose, but survives by absorbing the microbial cellulose degradation products and digesting the bacterial cells that are produced. The importance of microbes to cows is demonstrated by the size of its rumen, which is between 100 and 150 litres. A similar symbiosis occurs in the roots of plants, such as peas and beans, where bacteria develop in small nodules. These bacteria are fed by the plant, and in return they supply the plant with ammonia, an essential nutrient. The bacteria obtain this ammonia from nitrogen in air by nitrogen fixation, in a process unique to bacteria. Bacterial interactions with higher organisms are not, however, always benevolent, for some bacteria are major pathogens that cause a variety of diseases, some of which can be fatal. Fortunately, micro-organisms, including bacteria, have provided a source of antibotics with which to combat these diseases. Many bacterial pathogens have, however, developed resistance to antibiotics, and some bacterial diseases are now increasing in their prevalence. For example, tuberculosis, caused by Mycobacterium tuberculosis, kills three million people every year.

Molecular genetic analysis has demonstrated two distinct types of prokaryotes: Bacteria (formerly Eubacteria) and Archaea (formerly Archaebacteria). Both of these represent the highest order of life, Domains. All eukaryotes, including plants and animals, exist in a single Domain, Eukarya. Two out of the three Domains of life thus contain exclusively prokaryotic organisms, and this underlines their great diversity. Interestingly, the organelles of eukaryotic cell, mitochondria and chloroplasts, belong to the Domain Bacteria, not Eukarya. This demonstrates that these organelles were originally free-living bacteria that have evolved a stable endosymbiotic relationship with eukaryotic cells. Bacteria are thus an integral component of all of us.

R. John Parkes

Bibliography

Madigan, M. T.,, Martinko, J. M.,, and and Parker, J. (1997) Brock biology of microorganisms (8th edn) Prentice Hall, London.

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 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. 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).

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.

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.

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

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.

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

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.

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.

bacteria Forms of VIRUS. Bacteria are programs which replicate themselves and then start executing: carrying out some processor- or memory-intensive task such as reading from a file. Each replicated virus then reproduces itself again. Eventually the number of these viruses reaches the point where the computer is unable to cope with their demands and CRASHes or exhibits extremely sluggish behaviour. They are also known as RABBITS.

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

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

bacterium
1. A single bacterial cell.

2. A particular prokaryotic organism (see PROKARYOTE).

Bacteria The domain comprising the kingdom Eubacteria.

Bacteria The only kingdom in the domain Eubacteria.

bacteria The only kingdom in the domain Eubacteria.

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

bacteria •barrier, 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, Syria •Laurier, 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