BACTERIA ( Gr. bakterion, "little staff"), group of microscopic, unicellular organisms that lack a distinct nucleus and that usually reproduce by cell division. are tiny, ranging from 1 to 10 micrometers (1 micrometer equals 1/25,000 in), and are extremely variable in the ways they obtain energy and nourishment. They can be found in nearly all environments-from air, soil, water, and ice to hot springs; even the hydrothermal vents on the deep ocean floor are the home of sulfur-metabolizing bacteria ( see Marine Life ). Certain types are found in nearly all food products, and bacteria also occur in various forms of symbiosis (q.v.) with most plants and animals and other kinds of life. . the currently used five-kingdom scheme of classification (q.v.) , bacteria constitute the kingdom Monera (q.v.) , also known as Procaryotae-organisms in whose cells the nucleus is not enclosed by a membrane ( see Cell ). About 1600 species are known. Generally, bacteria are classified into species on the basis of characteristics such as shape-cocci (spheres), bacilli (rods), spirochetes (spirals); cell-wall structure; differential staining ( see Gram's Stain ); ability to grow in the presence or absence of air (aerobes and anaerobes, respectively); metabolic or fermentative capabilities; ability to form dormant spores under adverse conditions ( see Spore ); serologic identification of surface components; and nucleic-acid relatedness. most widely used reference for taxonomic classification of bacteria divides them into four major groups based on cell-wall characteristics. The division Gracilicutes encompasses bacteria with thin, gram-negative-type cell walls; the Firmicutes have thick, gram-positive cell walls; the Tenericutes lack cell walls; and the Mendosi-cutes have unusual cell walls made of material other than typical bacterial peptidoglycan. Among the Mendosicutes are the archaebacteria, a group of unusual organisms that includes methanogens, strict anaerobes that produce methane from carbon dioxide and hydrogen; halobacteria, which grow at high salt concentrations; and thermoacidophiles, which are sulfur-dependent extreme thermophiles. It has been argued that the archaebacteria should be classified into a separate kingdom because recent biochemical studies have shown that they are as different from other bacteria as they are from eucaryotes (the nucleii of which are membrane-bound). The four major bacterial divisions are further subdivided into about 30 numbered sections, some of which are divided into orders, families, and genera. Section 1, for example, is made up of spirochetes-long, corkscrew-shaped bacteria with gram-negative cell walls and internal (between the cell wall and cell membrane) filamentous flagella that provide the organisms with motility (ability to move). Treponema pallidum , causing syphilis, is a spirochete, a member of the order Spirochaetales, and the family Spirochaetaceae. all bacteria can move, but the motile ones are generally propelled by screwlike appendages-flagella-that may project from all over the cell or from one or both ends, singly or in tufts. Depending on the direction in which the flagella rotate, the bacteria either move forward or tumble in place. The duration of runs versus tumbling is linked to receptors in the bacterial membrane; variations enable the bacteria to move toward attractants such as food sources and away from unfavorable environmental conditions. In some aquatic bacteria that contain iron-rich particles, locomotion has been found to be oriented to the earth's magnetic field. . genetic material of the bacterial cell is in the form of a circular double strand of DNA ( see Nucleic Acids ). Many bacteria also carry smaller circular DNAs called plasmids, which encode genetic information but are generally not essential for reproduction. Many of these plasmids can be transferred to other bacteria by conjugation (q. v.), a mechanism of genetic exchange. Other mechanisms whereby bacteria can exchange genetic information include transduction, in which bacterial viruses ( see Bacteriophage ) transfer DNA, and transformation, in which DNA is taken into the bacterial cell directly from the environment. Bacterial cells multiply by binary fission (q.v.) ; the genetic material is duplicated and the bacterium elongates, constricts near the middle, and then undergoes complete division, forming two daughter cells essentially identical to the parent cell. Thus, as with higher organisms, a given species of bacteria reproduces only cells of the same species. Some bacteria divide every 20 to 40 minutes. Under favorable conditions, with one division every 30 minutes, after 15 hours a single cell will have produced roughly 1 billion progeny. This mass, called a colony, may be seen with the naked eye. Under adverse conditions some bacteria may undergo a modified division process to produce spores, dormant forms of the cell that can withstand extremes of temperature and humidity. of Bacteria. main groups of bacteria exist: the saprophytes, which live on dead animal and vegetable matter; and the symbionts, which live on or in living animal or vegetable matter. Saprophytes are important because they decompose dead animals and plants into their constituent elements, making them available as food for plants. Symbiotic bacteria are a normal part of many human tissues, including the alimentary canal and the skin, where they may be indispensable to physiological processes. Such a relationship is called mutualistic. Other symbionts gain nutrients from their living host without causing serious damage; this is commensalism. The third type, parasites, can destroy the plants and animals on which they live. See also Parasite . are involved in the spoilage of meat, wine, vegetables, and milk and other dairy products. Bacterial action may render such foods unpalatable by changing their composition. Bacterial growth in foods can also lead to food poisoning such as that caused by Staphylococcus aureus or by Clostridium botulinum ( see Botulism ). On the other hand, bacteria are of great importance in many industries. The fermentative capabilities of various species are manipulated for the production of cheese, yogurt, pickles, and sauerkraut. Bacteria are also important in the production of tanned leather, tobacco, ensilage, textiles, pharmaceuticals and various enzymes, polysaccharides, and detergents. are found in virtually all environments, where they contribute to various biological processes. For example, they may produce light, such as the phosphorescence of dead fish; and they may produce enough heat to induce spontaneous combustion in haystacks or in hop granaries. By decomposing cellulose, certain anaerobic forms evolve marsh gas in stagnant pools; by oxidizing processes, other bacteria assist in forming deposits of bog iron ore, ocher, and manganese ore. See Bioluminescence . have an immense influence on the nature and composition of the soil. One result of their activities is the complete disintegration of organic remains of plants and animals and of inorganic rock particles. This action produces in the aggregate vast quantities of plant food. In addition, the leguminous plants that enrich soil by increasing its nitrogen content do so with the help of Rhizobium radicicola and other bacteria that infect the roots of the plants and cause nitrogen-fixing nodules to grow ( see Nitrogen Fixation ). The photosynthetic process on which plant life itself is based was almost certainly first established in bacteria; the recent discovery of an unusual photosynthesizing bacterium called Heliobacterium chlorum may help in understanding this fundamental development in the history of life. Bacteria. 200 species of bacteria are pathogenic, or disease causing, for humans. Pathogenicity varies widely among various species and is dependent on both the virulence of the particular species and the condition of the host organism. Among the more invasive bacteria responsible for human disease are those that cause cholera, lockjaw, gas gangrene, leprosy, plague, bacillary dysentery, tuberculosis, syphilis, typhoid fever, diphtheria, undulant fever, and several forms of pneumonia. Until the discovery of viruses, bacteria were considered the causative agents of all infectious diseases. pathogenic effects of bacteria on body tissues may be grouped in four classes as follows: (1) effects of the direct local action of the bacteria on the tissues, as in gas gangrene, caused by Clostridium perfringens ; (2) mechanical effects, as when a mass of bacteria blocks a blood vessel, causing an infectious embolus; (3) effects of the body's response to certain bacterial infections on body tissues, as in the forming of lung cavities in tuberculosis, or destruction of heart tissue by the body's own antibodies in rheumatic fever; (4) effects of bacterial-produced toxins ( see Toxin ), chemical substances that act as poisons to certain tissues. Toxins are generally species specific; for example, the toxin responsible for diphtheria is different from the one responsible for cholera. . microorganisms, including certain fungi (q.v.) and some bacteria, produce chemical substances that are toxic to specific bacteria. Such substances, which include penicillin and streptomycin, are known as antibiotics; they either kill the bacteria or prevent them from growing or reproducing. In recent years antibiotics have played an increasingly important role in medicine in the control of bacterial diseases. See Antibiotic . See also Antiseptics ; Bacteriology ; Disease . J.H.N.; A.J.G. & D.M. For further information on this topic, see ~Biblio. Biology, biochemistry , ~Biblio. Viruses, bacteria , ~Biblio. Substance abuse . FIXATION, or industrial process by which molecular atmospheric nitrogen (q.v.) is converted into a chemical compound that is essential for plant growth and is also used in industrial chemical production. Fixation. most widely used and most productive of the soil microorganisms capable of nitrogen fixation are symbiotic bacteria of the genus Rhizobium, which colonize and form nodules on the roots of leguminous plants such as clover, alfalfa, and peas ( see Legume ). These bacteria obtain food from the legume, which in turn is supplied with abundant nitrogen compounds. Soils are sometimes inoculated with a particular species of Rhizobium to increase a legume crop, which is often planted to replenish the nitrogen depleted by other crops. smaller amounts of nitrogen are fixed in the soil by nonsymbiotic (free-living) bacteria such as the aerobes, which function in the presence of oxygen, and bacteria of the genera Klebsiella and Bacillus, which function without oxygen. Some forms of blue-green algae also fix nitrogen, such as the alga Anabaena, which, in symbiosis with the water fern Azolla pinnata, is said to markedly increase rice yields, as was the case in paddies in the Thai Binh region of northern Vietnam. The need for fixed nitrogen in agriculture today is far greater than can be supplied by natural biological processes, and the production of nitrogen compounds from atmospheric nitrogen is a major chemical industry. Fixation. principal industrial nitrogen-fixation process today is the production of ammonia (q.v.) by passing a mixture of atmospheric nitrogen and hydrogen over a metallic catalyst ( see Catalysis ) at 5000-6000 C (9320-11120 F). Ammonia is then oxidized to form nitric acid, which is in turn combined with ammonia to yield ammonium nitrate, used primarily in explosives and fertilizers ( see Fertilizer ). In another method, cyanamide, which is used as a fertilizer or in the production of cyanides, is produced by passing atmospheric nitrogen over heated calcium carbide in the presence of a catalyst. ( Gr. anti, "against"; bios, "life"), substance produced by one organism that is destructive to another. This process traditionally has been called antibiosis and is the opposite of symbiosis (q.v.) . More specifically, an antibiotic is a type of chemotherapeutic agent, that is, it has a toxic effect on certain types of disease-producing microorganisms without acting dangerously on the patient. Some chemotherapeutic agents differ from antibiotics only in that they are not secreted by microorganisms, as are antibiotics, but rather are made synthetically in a chemical laboratory. Examples are quinine (q. v.), used against malaria; arsphenamine, used against syphilis; the sulfa drugs (q.v.) , used against a wide variety of diseases, notably pneumonia; and the quinolones, used against hospital-derived infections (zoonoses). A few antibiotics, among them penicillin (q.v.) and chloramphenicol, have now been produced synthetically also. . first observation of what would now be called an antibiotic effect was made in the 19th century by the French chemist Louis Pas teur, who discovered that certain saprophytic bacteria can kill anthrax germs. About 1900 the German bacteriologist Rudolf von Emmerich (1852-1914) isolated a substance called pyocyanase, which can kill the germs of cholera and diphtheria in the test tube. It was not useful, however, in curing disease. In the 1920s the British bacteriologist Sir Alexander Fleming, who later discovered penicillin, found a substance called lysozyme in many of the secretions of the body, such as tears and sweat, and in certain other plant and animal substances. Lysozyme has strong antimicrobial activity, but mainly against harmless bacteria. , the archetype of antibiotics, was discovered by accident in 1928 by Fleming, who showed its effectiveness in laboratory cultures against many disease-producing bacteria, such as those that cause gonorrhea and certain types of meningitis and bacteremia (blood poisoning); however, he performed no experiments on animals or humans. Penicillin was first used on humans by the British scientists Sir Howard Florey and Ernst Chain during the 1940-41 winter. first antibiotic to be used in the treatment of human diseases was tyrothricin (one of the purified forms of which was called gramicidin), isolated from certain soil bacteria by the American bacteriologist Rene Dubos in 1939. This substance is too toxic for general use, but it is employed in the external treatment of certain infections. Other antibiotics produced by actinomycetes (filamentous and branching bacteria) occurring in soil have proved more successful. One of these, streptomycin (q.v.) , discovered in 1944 by the American microbiologist Selman Waksman and his associates, is effective against many diseases, including several in which penicillin is useless, especially tuberculosis. Use. then, such antibiotics as chloramphenicol, the tetracyclines, erythromycin, neomycin, nystatin, amphotericin, cephalosporins, and kanamycin have been developed and may be used in the treatment of infections caused by some bacteria, fungi, viruses, rickettsia, and other microorganisms. In clinical treatment of infections, the causative organism must be identified and the antibiotics to which it is sensitive must be determined in order to select an antibiotic with the greatest probability of killing the infecting organism. Developments. of bacteria have arisen that are resistant to commonly used antibiotics; for example, gonorrhea-causing bacteria that high doses of penicillin are not able to destroy may transfer this resistance to other bacteria by exchange of genetic structures called plasmids ( see Conjugation ). Some bacteria have become simultaneously resistant to two or more antibiotics by this mechanism. New antibiotics that circumvent this problem, such as the quinolones, are being developed. The cephalosporins, for instance, kill many of the same organisms that penicillin does, but they also kill strains of those bacteria that have become resistant to penicillin. Often the resistant organisms arise in hospitals, where antibiotics are used most often, especially to prevent infections from surgery. problem in hospitals is that many old and very ill persons develop infections from organisms that are not pathogenic in healthy persons, such as the common intestinal bacterium Escherichia coli. New antibiotics have been synthesized to combat these organisms. Fungus infections (q.v.) have also become more common with the increasing use of chemotherapeutic agents to fight cancer, and more effective antifungal drugs are being sought. search for new antibiotics continues in general, as researchers examine soil molds for possible agents. Among those found in the 1980s, for example, are the monobactams, which may also prove useful against hospital infections. Antibiotics are found in other sources as well, such as the family of magainins discovered (in the late 1980s) in frogs; although untested in humans as yet, they hold broad possibilities. have also been used effectively to foster growth in animals. Concern has arisen, however, that this widespread use of antibiotics in animal feed can foster the emergence of antibiotic-resistant organisms that may then be transmitted to human beings. An instance of one such transfer was documented in the U.S. in 1984. S.A.W. For further information on this topic, see ~Biblio. Substance abuse . , derived from the mold Penicillium notatum. The action of this antibiotic was first observed in 1928 by the British bacteriologist Alexander Fleming, but it was another ten years before penicillin was concentrated and studied by the British biochemist Ernst Chain, the British pathologist Sir Howard Florey, and other scientists. acts both by killing bacteria and by inhibiting their growth. It does not kill organisms in the resting stage but only those that are growing and reproducing. It is effective against a wide range of disease-bearing microorganisms, including pneumococci, staphylococci, streptococci, gonococci, meningococci, the clostridium of tetanus, and the syphilis spirochete. The drug has been successfully employed to treat such deadly diseases as subacute bacterial endocarditis, septicemia, and gas gangrene, and also gonorrhea, scarlet fever, and osteomyelitis. Toxic symptoms produced by penicillin are limited largely to allergic reactions which may be determined by scratch tests before administration of the drug. In 1980 a group of physicians announced that they had successfully desensitized several penicillin-allergic patients with a procedure that took only three hours; tests of the method on a wider scale were instituted. Penicillin. the effectiveness displayed by penicillin in curing a wide range of diseases, infections caused by certain strains of staphylococci could not be cured by the antibiotic as a result of the ability of the organism to produce an enzyme, penicillinase, capable of destroying the antibiotic. In addition, enterococci and many gramnegative bacilli known to cause respiratory and urinary-tract infections were found to be intrinsically resistant to the action of penicillin. Appropriate chemical treatment of a biological precursor to penicillin, isolated from bacterial cultures, resulted in the formation of a number of so-called semisynthetic penicillins. The most important of these are Methicillin and Ampicillin, the former remarkably effective against penicillinase-producing staphylococci and the latter not only active against all organisms normally killed by penicillin, but also inhibiting enterococci and most gram-negative bacilli. . strength and dosage of penicillin are measured in terms of international units. Each of these units is equal to 0.0006 g of the crystalline fraction of penicillin called penicillin G. In the early days of penicillin therapy, the drug was administered every three hours in small doses. More recently a preparation called benzathine penicillin G has been produced that provides detectable levels of antibiotic for as long as four weeks after a single intramuscular injection. It is useful for treatment of syphilis. S.A.W. , or chemical agents that prevent putrefaction, infection, and analogous changes in food and living tissue by destroying or arresting the development of microorganisms. Since ancient times food has been preserved by the use of antiseptic agents such as heat in cooking; niter, salt, and vinegar in corning and pickling; and wood smoke (containing creosote, chemically similar to carbolic acid) in the smoking of meats. In modern times the principal antiseptic agents used in the preservation of food are heat and cold in such processes as canning, pasteurization, and refrigeration. Irradiation is being investigated as a means of preserving food. practice of using antiseptics in the care and treatment of wounds was begun by the English surgeon Joseph Lister in 1868. Basing his work on the findings of the German physiologist Theodor Schwann and the French biochemist Louis Pasteur, Lister disinfected surgical and accidental wounds with a solution of carbolic acid, and in five years reduced the death rate from major amputations from 45 percent to about 12 percent. Many other antiseptics have come into use, among which the most important are bichloride of mercury, iodine, boric acid, alcohol, the hypochlorites, mercurochrome, and Merthio-late. Chlorine is used in the sterilization of water, especially in public water systems ( see Water Supply and Waterworks ) and swimming pools. , of bacteria (q.v.) , 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. . 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 spon taneous 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 Monera (q.v.) . 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. 1880, immunity against bacterial diseases has been systematically studied. In that year, Pasteur discovered by accident that Bacillus anthracis, cultivated at a temperature of 420 to 430 C (1080 to 1100 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 (q.v.) , one of the most important modern medical advances. See Antitoxin . 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). . fundamental method of studying bacteria is by culturing them in liquid media or on the surface of media that have been solidified by agar (q.v.) . 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 (q.v.) 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. 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. . 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 . Examination. 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 (1843-1905) 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. Research. recent years, bacteriology has been greatly expanded from its concentration on disease-causing pathogens. 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 . A.J.G. & D.M. For further information on this topic, see ~Biblio. Viruses, bacteria . ENGINEERING, of changing the inherited characteristics of an organism in a predetermined way by altering its genetic material. This is often done to cause microorganisms, such as bacteria or viruses, to synthesize increased yields of compounds, to form entirely new compounds, or to adapt to different environments. Other uses of this technology, which is also called recombinant DNA technology, include gene therapy, which is the supply of a functional gene (see Gene Therapy below) to a person with a genetic disorder ( see Genetic Disorders )or with other diseases, such as acquired immune deficiency syndrome (AIDS; q.v.) or cancer. engineering involves the manipulation of deoxyribonucleic acid, or DNA ( see Nucleic Acids ). Important tools in this process are so-called restriction enzy
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