Microbiology, study of bacteria, including their classification and the prevention of diseases that arise from bacterial infection. The subject matter of bacteriology is distributed not only among bacteriologists but also among chemists, biochemists, geneticists, pathologists, immunologists, and public-health physicians.
Bacteria were first observed by the Dutch naturalist Antoni van Leeuwenhoek with the aid of a simple microscope of his own construction. He reported his discovery to the Royal Society of London in 1683, but the science of bacteriology was not firmly established until the middle of the 19th century. For nearly 200 years it was believed that bacteria are produced by spontaneous generation. The efforts of several generations of chemists and biologists were required to prove that bacteria, like all living organisms, arise only from other similar organisms. This fundamental fact was finally established in 1860 by the French scientist Louis Pasteur, who also discovered that fermentation and many infectious diseases are caused by bacteria. The first systematic classification of bacteria was published in 1872 by the German biologist Ferdinand J. Cohn, who placed them in the plant kingdom. They are now usually included in the kingdom Prokaryote. In 1876 Robert Koch, who had devised the method of inoculating bacteria directly into nutrient media as a means of studying them, found that a bacterium was the cause of the disease anthrax.
Since 1880, immunity against bacterial diseases has been systematically studied. In that year, Pasteur discovered by accident that Bacillus anthracis, cultivated at a temperature of 42° to 43° C (108° to 110° F), lost its virulence after a few generations. Later it was found that animals inoculated with these enfeebled bacteria showed resistance to the virulent bacilli. From this beginning date the prevention, modification, and treatment of disease by immunization, one of the most important modern medical advances. See Antitoxin.
Other significant developments in bacteriology were the discoveries of the organisms causing glanders (1862), relapsing fever (1868), typhoid fever (1880), tetanus (1885), tuberculosis (1890), plague (1894), bacillary dysentery (1898), syphilis (1905), and tularemia (1912).
A fundamental method of studying bacteria is by culturing them in liquid media or on the surface of media that have been solidified by agar. Media contain nutrients, varying from simple sugars to complex substances such as meat broth. To purify or isolate a single bacterial species from a mixture of different bacteria, solidified media generally are used. Individual cells dividing on the surface of solidified media do not move away from each other as they do in liquid, and after many rounds of replication they form visible colonies composed of tens of millions of cells all derived by binary fission from a single cell. If a portion of a colony is then transferred to a liquid medium, it will grow as a pure culture free of all other bacteria except the single species that was found in the colony.
Many different species of bacteria so closely resemble one another in appearance that they cannot be differentiated from one another under the microscope. Various culture techniques have been developed to aid in species identification. Some media contain substances to inhibit the growth of many bacteria, but not the species of interest. Others contain sugars that some but not all bacteria can utilize for growth. Some media contain pH indicators that change color to indicate that a constituent of the media has been fermented, yielding acid end products. Gas production as an end product of fermentation can be detected by inoculating bacteria in solidified media in tubes rather than on plates. Sufficient gas production will result in the formation in the agar of bubbles that can easily be seen. Still other media are formulated to identify bacteria that produce certain enzymes that can break down constituents in the media; for example, blood agar plates, which can detect whether bacteria produce an enzyme to lyse, that is, dissolve, red blood cells. The various culture media and culture techniques are essential to the hospital laboratory, whose job it is to identify the cause of various infectious diseases.
Drying or freezing kills many species of bacteria and causes others to become inactive. Heat or moist heat above a certain temperature kills all bacteria. Sterilization of many different objects, such as spacecraft and surgical instruments, are important facets of bacteriological work. See also Antiseptics.
The microscope is one of the most important tools used in studying bacteria. Dyeing or staining bacterial specimens or cultures was introduced in 1871 by the German pathologist Karl Weigert and has greatly helped the bacteriologist in identifying and observing bacteria under the microscope. A bacterial specimen is first placed on a glass slide. After the specimen has dried, it is stained to render the organism easier to observe. Stains also stimulate reactions in certain bacteria. For example, the tuberculosis bacillus can be recognized only on the basis of its reaction to certain stains (see Gram's Stain). Bacteriologists have been greatly aided by the electron microscope (see Microscope), which has far greater magnification powers than ordinary microscopes.
In recent years, bacteriology has been greatly expanded from its concentration on bacterial disease. The discovery that bacteria fix nitrogen in the root nodules of leguminous plants (see Nitrogen Fixation) has led to attempts to inoculate the roots of other plant strains and thereby increase soil fertility and the productivity of food crops. Some bacteria are able to digest petroleum and other hydrocarbons; others absorb phosphorus. These bacteria are being intensively investigated as possible aids in cleaning up oil spills and removing phosphorus from sewage sludge. Other bacteria may be more efficient than yeast at producing alcohol and are being explored in the search for new energy sources. Escherichia coli, a normal inhabitant of the human intestinal tract, is the most thoroughly studied of all organisms. Studies of the mechanisms of genetic exchange and the biology of plasmids and bacteriophages (see Bacteriophage) of E. coli have been crucial in understanding many aspects of DNA replication and the expression of genetic material. These studies have led to the ability to insert DNA from unrelated organisms into E. coli plasmids and bacteriophages and to have that DNA replicated by the bacteria, with the genetic information it contains expressed by the bacteria. It is thus possible for bacteria to become living factories for scarce biological products such as human insulin, interferon, and growth hormone. See Genetic Engineering.