Bacteria is the common name for a vast group of one-celled microscopic organisms that encompasses the smallest, simplest, and perhaps first form of \Tcell\t life that evolved. They constitute one of two divisions in the kingdom \TMonera\t. They are unicellular and furnish both the raw material and the chemical machinery for their own reproduction, whereas viruses, for example, do not. The oldest sign of life is a fossilized bacterial cell discovered in a rock in Africa and estimated at about 3 billion years old. The study of bacteria is called bacteriology, which belongs to the broader science of \Tmicrobiology\t, or the study of all types of microorganisms, including one-celled protozoa, yeasts, and algae. Medical microbiology is concerned with the behavior and control of pathogens, which are microorganisms that cause \Tinfectious diseases\t in humans and other animals. SIZE AND HABITAT Thirty trillion bacteria of average size would weigh about 28 g (1 oz). Bacteria are measured in microns (0.001 micrometers, about 0.00004 in) and most types range in size from 0.1 to 4.0 microns in width and 0.2 to 50 microns in length. Bacteria are found everywhere. Approximately 2,000 species have been identified, many of them living in conditions that would destroy any other organism. They have been found in the almost airless reaches of the upper atmosphere, 10 km (6 mi) below the surface of the ocean, living in frozen soil, and attached to rocks in hot springs. Some bacteria produce a resting stage, the endospore, which is the most resistant living thing known and cannot be killed except by boiling in steam under pressure for many hours. CLASSIFICATION Advances in \Tbiology\t during the 19th century indicated that bacteria and certain other organisms were neither \Tplant\t nor \Tanimal\t. Both bacteria and plants have rigid cell walls, but unlike plants, most kinds of bacteria move about and use organic foods for energy and growth; only a few use photosynthesis. Although bacteria are classified as plants in the traditional two-kingdom classification system, in one of the five-kingdom classification systems used today the one-celled \Lprokaryote\ls bacteria and cyanobacteria, or \Tblue-green algae\t) are classified in the kingdom Monera and the one-celled \Leukaryote\ls (protozoa) are placed in the kingdom Protista. According to current definitions, each group of similar strains, or types, of bacteria may be considered a species. A more precise definition is difficult. On the basis of their shapes, bacteria may be grouped into three main types: the rod-shaped bacilli, which often have small whiplike structures known as \Tflagella\t that propel the organism, usually in a rolling or tumbling motion; the spherical cocci (singular coccus), which may grow in chains (\LStreptococc\li, or "strep germs," as in \Tstrep throat\t) or which may clump together like a bunch of grapes (\LStaphylococc\li); and the comma- or spiral-shaped spirilla and \Lspirochete\ls . Another kind of bacteria, called the \LMycoplasma\ls, have no rigid cell walls and consequently are formless. These are the smallest bacteria and are often called pleuropneumonia-like organisms (PPLO), because they cause a contagious form of pneumonia in cows and human beings. Recently, a new classification system has been proposed that arranges species according to genetic make-up. In this new system, \Tarchaebacteria\t, a group of highly specialized bacteria, are differentiated from the eubacteria, a group that includes the vast majority of bacteria, on the basis of the sequence of ribosomal \TRNA\t. An important, widely used technique for identifying bacteria is gram staining, perfected by the Danish bacteriologist Hans Christian Gram in 1884. In this process the bacteria are treated with a special dye, or stain, and other chemicals. The treated bacteria fall into two groups: gram-positive bacteria, which appear deep violet in color, and gram-negative bacteria, which appear red in color. Physicians often use gram staining in choosing the proper \Tantibiotic\t for treating a bacterial infection. Gram-positive bacteria are more susceptible to penicillin, whereas gram-negative bacteria are usually more susceptible to other antibiotics such as streptomycin. The basis of this difference in staining properties is still a mystery, but evidence indicates that the difference lies in the composition of the bacterial cell wall. Although bacterial and plant cells are enclosed by rigid walls, they differ in important aspects of their composition. Plant cell walls derive their strength largely from cellulose, whereas bacterial cell walls are stiffened primarily by murein (a compound made of amino acids and sugar). This important difference is the basis for the selective activity of certain drugs such as penicillin. Nontoxic to plants and animals, penicillin is toxic to growing bacteria because it inhibits formation of murein and thus interferes with bacterial reproduction. PHYSIOLOGY The only organisms comparable to bacteria in size and simplicity are the cyanobacteria, still often known as the blue-green algae, Cyanophyta (see \Talgae\t). The kingdom \TMonera\t is thus composed of only two groups and classified separately from all other forms of cellular life. Other life forms, including human beings, are eucaryotic, that is, their cells contain organelles such as the nucleus, chloroplast, chromoplast, lysosome, mitochondrion, endoplasmic reticulum, and golgi apparatus. Cells of monerans are procaryotic (prenuclear) and lack organelles but, interestingly, not the functions controlled by them. In terms of metabolism, the diversity of life is much more evident in bacteria (and blue-green algae) than in other organisms. Bacteria display a staggering variety of mechanisms for obtaining energy that have no parallel in higher organisms. Bacteria may be classified on the basis of their requirements for free atmospheric oxygen. Those requiring oxygen are aerobes; those which cannot live in the presence of oxygen are obligate \Lanaerobe\ls; and those which do well with oxygen but can survive without it are facultative anaerobes. Many photosynthetic bacteria are anaerobic. The sulfur bacteria that live in oxygen-poor environments produce sulfur instead of the oxygen gas given off by green plants. In the absence of oxygen, anaerobes obtain sufficient energy for all their needs through \Tfermentation\t (breaking down organic molecules with enzymes). Other bacteria are chemosynthetic; unlike photosynthetic bacteria, which use light as an energy source, they use nitrogen- and sulfur-containing compounds to obtain the necessary energy for food manufacture. REPRODUCTION Most bacteria reproduce asexually by binary fission (see \Treproduction\t), in which a single cell divides in two after developing a wall across its width. Many species divide as often as every 20 minutes under favorable conditions. If all the descendants survived, the initial cell would result in about 500,000 new cells after 6 hours. Increases to large numbers in a short period of time help to explain the rapid development of disease, food spoilage, decay, and the speed at which certain chemical processes used in industry take place. Certain bacteria such as E. coli reproduce by conjugation, which resembles sexual reproduction, in that the two bacteria join (mate) and exchange genes. As in true sexual reproduction, the genetic material, or "nuclear" chromosomes, recombine with one another (see \Tgenetics\t). In the process of recombination, a fragment of a chromosome transmitted from one bacterium is incorporated in the chromosome of the recipient. Conjugation and recombination increase the total number of different hereditary characteristics in a population of bacteria, increasing the bacteria's chances of survival. ECONOMIC IMPORTANCE The beneficial role of bacteria is worth emphasizing, as the popular idea that all bacteria are hostile stems from the prominent historical connection between bacteria and disease. Most important bacteria are harmless to humans, and many are essential to the existence of plant and animal life. Only a small fraction of bacteria cause disease; most bacteria attack organic matter only after it is dead. Were it not for bacteria that decompose animal waste matter and the bodies of dead animals and plants, these materials would accumulate almost indefinitely. Bacteria also enrich the soil in various ways. The so-called nitrogen-fixing bacteria take nitrogen gas from the atmosphere and convert it to a form (nitrate) that green plants use for growth. Small nodules in the roots of leguminous plants contain bacteria, genus Rhizobium, that help fix nitrogen in usable form, and a number of cyanobacteria fix atmospheric nitrogen (see \Tnitrogen cycle\t). Bacteria also create \Tfertilizer\t by breaking down compost heaps made of soil and dead plant matter. Bacteria are important industrially in the production of cheese, yogurt, buttermilk, vinegar, and sauerkraut; in the preparation of antibiotics such as Streptomycin, which is extracted from soil bacteria; in the tanning of leather and hides and the curing of tobacco; and in sewage disposal plants to render organic wastes harmless. Cattle, sheep, and goats live on grass; yet without bacteria they would not be able to digest the tough fibers of plant cellulose. Stored foods that have been improperly processed are subject to spoilage by bacteria. Poisonous toxins are sometimes produced by such food-spoiling bacteria as Staphylococci, Streptococci, and \TSalmonella\t. They cause severe illness in humans eating affected food. Clostridium botulinum, growing in canned or smoked foods that have been improperly processed, produces a toxin that causes a frequently fatal disease called \Tbotulism\t. DESTRUCTION OF BACTERIA High temperature usually kills most bacteria. Most disease-producing bacteria in milk, for example, can be killed by maintaining the temperature at 62 deg C (143 deg F) for 30 minutes, a process called \Tpasteurization\t. "Flash pasteurization," done at 71 deg C (160 deg F) for 15 seconds, is now commonly used. Most nonspore-forming bacteria are destroyed by boiling water and can be killed by various disinfectants. \Lantiseptic\ls may kill bacteria or prevent infection by inhibiting their growth. Among the most potent disinfectants are phenol (carbolic acid), chlorine gas (drinking water is treated with chlorine to render it safe), and alcohol in a 50- to 70-percent solution, as in rubbing alcohol. Bichloride of mercury and other mercury-containing compounds (mercurochrome and merthiolate) are often used as disinfectants and antiseptics. The above chemicals are poisonous in the human body and should not be taken internally. Antibiotics, however, are substances produced by living organisms (usually bacteria and \Tmolds\t) that are used internally for inhibiting the growth of bacteria or destroying them; antimicrobial agents are natural or artificial chemicals having the same use. BACTERIA AND DISEASE A century ago in the United States, and even today in the less developed countries, at least 25 percent of the children died of bacterial infections before reaching puberty. In the United States and other Western nations, this figure is now below 5 percent as a result of improved sanitation, hygiene, nutrition, and medical care. The control of \Ttyphoid fever\t alone is perhaps the greatest triumph of organized PREVENTIVE \Tmedicine\t. As late as 1900, the annual death rate from typhoid fever in the United States was more than 30 per 100,000; by 1944, the rate had decreased to 0.4 per 100,000. For the world as a whole, however, typhoid fever remains a major disease. History To account for the spread of certain diseases from person to person, thoughtful men since ancient times postulated the existence of transmissible agents of infection invisible to the naked eye. In his book on contagious diseases, published in 1546, the Italian physician Girolamo \TFracastoro\t described the transmission of disease by "seminaria," or living germs. Although Fracastoro proposed the true germ theory of disease, visualization of the germs could not take place until the microscope had been invented. Bacteria and other microscopic organisms were first seen in 1676 by a Dutch linen-draper, Antoni van \TLeeuwenhoek\t, who made single-lens microscopes with sufficient magnification to observe the major types of bacteria as well as the larger microbes, including protozoa, yeasts, and one-celled algae. Leeuwenhoek is regarded as the father of bacteriology. By keeping secret the methods of making and using his instruments, he remained the sole occupant of the field he had created for the rest of his life. The first important classification of bacteria was made in the early 1800s. In 1829 Christian Gottfried established the genus Bacterium, using a term formed from the Greek word bacterion, signifying a rod. The entire subject of bacteriology has taken its common name from the prominence of rodlike forms of bacteria, now called bacilli. Eventually, bacteria were classified in the plant kingdom, and this remained the dominant view until the 1960s. Historically, living organisms have been classified as either plant or animal. In this traditional classification, bacteria are regarded as the simplest forms of \Tfungi\t and are named Schizomycetes, or "fission fungi," because they multiply by fission (splitting in two). Bacteria are now classified as Monerans, and the name Schizophyta is the scientific designation of these organisms. An experimental science of bacteriology emerged slowly and required the development of special methodology. The key was the use of sterile (germ-free) materials and antiseptic techniques. Although the chemist defines purity in terms of the percentage of contaminating material, a single contaminating cell can ruin an experiment in bacteriology. Only after learning to avoid such contamination could investigators recognize the existing variety of bacteria, their distribution, and their major roles. Bacteria in Disease In Vienna in the mid-1840s, Ignaz Philip \TSemmelweis\t, a Hungarian obstetrician, tried to convince his disbelieving colleagues that the disease that swept through maternity wards and killed hundreds of women each year could be prevented. Childbed fever (puerperal sepsis) was caused by a strain of Streptococcus spread by medical students going from the dissecting room to the patients without first washing. Semmelweiss had the students on his wards wash their hands in disinfectant before each delivery, and this precaution greatly reduced the amount of infection. In spite of the remarkable drop in the number of deaths, he was discredited by his colleagues. Ironically, he died of an infection from a cut received during an \Tautopsy\t. The role of bacteria in a disease was first proved (1876) by the German bacteriologist Robert \TKoch\t for \Tanthrax\t, and was confirmed by Louis \TPasteur\t. Koch meticulously developed the techniques that are used today in culturing bacteria for study and set down rules still used for proving that a given infection is caused by particular bacteria. These rules are called Koch's postulates and may be summarized as follows: the bacteria must be present in the infected tissue in every case of the infection; they must be isolated in pure culture on an artificial medium; inoculation of this culture into experimental animals must cause a similar disease; and the organisms must be recovered from the infected tissue. About ten years before Koch isolated anthrax, Joseph \TLister\t virtually eliminated wound infections by soaking bandages in carbolic acid. This was the start of modern aseptic surgical techniques. Following Koch's initial discovery, medical scientists raced to identify other pathogens. Pasteur, however, devoted himself to developing vaccines, which are materials made from specially treated organisms and inoculated into humans and other animals to develop immunity to a specific infectious disease. Studies of the response of the body's defense mechanisms to bacteria later gave rise to the field of \Timmunology\t. Immunology is thus inseparable from bacteriology, although, strictly speaking, it is a branch of medicine. It is now known that bacteria are transmitted by air, insects, water, food, and direct contact with human beings, animals, and objects. Not all of the bacteria associated with human beings cause disease, however; a natural flora exists in the body, and the E. coli of the large intestine help to control the body's water balance and to provide certain vitamins. Their presence in drinking water may be taken as a measure of contamination, however, suggesting that pathogens may be present. Plasmids Many bacteria contain \Lplasmid\ls, which are tiny pieces of \TDNA\t (see \Tnucleic acid\t) that are much smaller than and independent of chromosomal \TDNA\t and generally carry nonessential bacterial genes. First observed in E. coli, some plasmids carry traits such as resistance to antibiotics. How they acquire their extra \TDNA\t controlling resistance and virulence is not known, although many authorities believe that overuse of antibiotics is the main cause. In \Tgenetic engineering\t, plasmids are isolated, opened up for insertion of pieces of \TDNA\t from other sources, and then resealed; this hybrid \TDNA\t is called recombinant \TDNA\t. The new plasmid is placed into a receptor cell, "infecting" it as if it were a virus. The inserted genes then express themselves along with the normal genetic complement of the cell. A bacterium may be programmed in this way to produce a useful substance; human insulin, for example, is produced when human genes controlling insulin production are placed into a plasmid and then inserted into a bacterium. Transduction Bacteria also exchange chromosomal material through transduction, which occurs when bacteria are infected by viruses called bacteriophages. Transduction involves the accidental transfer of bacterial genes between bacterial cells by a bacteriophage and the incorporation of these genes into the recipient bacterium. This transfer requires that the bacteriophage infect a bacterial strain that is destroyed by the virus and that the recipient strain of bacteria be the one that harbors the virus but that is not usually destroyed by it. This alternation between destruction of bacterial cells by bacteriophage and harboring the virus with no sign of infection is called lysogeny. A process analogous to lysogeny is believed to be the reason viruses cause certain cancers in animals and perhaps humans. BACTERIA IN MODERN BIOLOGY Until the 1950s, bacteriology was a branch of medicine concerned almost solely with pathogenic bacteria. Eventually, however, bacteria were found particularly suitable for studying many basic problems common to all cells, such as metabolism, the molecular aspects of genetics that involve the structure and function of \TDNA\t and the way in which cellular action is regulated, and the synthesis of proteins in the cell. These studies revealed many instances of a resemblance between microbial cells and cells of higher organisms, in their building blocks, enzymes, and metabolic pathways. The advantages of bacteria for such studies include their relatively simple structure; homogenous cell populations (each cell is exactly like the others); extremely rapid growth; and the ease with which billions of individual cells can be cultivated and selected to yield mutants and mutant \Lhybrid\ls. Mutants obtained in this way have permitted scientists to identify the role of various genes and protein molecules in cell actions as well as reasons for bacterial resistance to antibiotics. These developments have led to an interdisciplinary activity known as molecular biology or biochemical genetics. Reviewed by Lawrence J. Crockett Bibliography: Bainbridge, B. W., Genetics of Microbes (1986); Fletcher, M.M., and Floodgate, G.D., eds., Bacteria in Their Natural Environments (1985); Gunsalus, I. C., et al., eds., The Bacteria, 8 vols. (1960-85); Singleton, Paul, and Sainsbury, Diana, Introduction to Bacteria (1981); Sinha, U., and Srivastava, S., An Introduction to Bacteria (1983); VanDemark, P.J., and Batzing, B. L., The Microbes: An Introduction to Their Nature and Importance (1987).