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 ]