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[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, MoneraMonera,
taxonomic kingdom that comprises the prokaryotes (bacteria and cyanobacteria). Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and usually lack membrane-bound organelles (mitochondria, chloroplasts; see cell, in biology).
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. Bacteria fall into one of two groups, ArchaebacteriaArchaebacteria
, diverse group of bacteria (prokaryotes), sometimes called the archaea and considered a major group unto themselves. Archaebacteria are contrasted with the Eubacteria, from which they differ biochemically in the arrangement of the bases in their ribosomal RNA and
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 (ancient forms thought to have evolved separately from other bacteria) and Eubacteria. A recently proposed system classifies the Archaebacteria, or Archaea, and the Eubacteria, or Bacteria, 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 LeeuwenhoekLeeuwenhoek, Antony van
, 1632–1723, Dutch student of natural history and maker of microscopes, b. Delft. His use of lenses in examining cloth as a draper's apprentice probably led to his interest in lens making.
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 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 PasteurPasteur, Louis
, 1822–95, French chemist. He taught at Dijon, Strasbourg, and Lille, and in Paris at the École normale supérieure and the Sorbonne (1867–89).
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 and Robert KochKoch, Robert
, 1843–1910, German bacteriologist. He studied at Göttingen under Jacob Henle. As a country practitioner in Wollstein, Posen (now Wolsztyn, Poland), he devoted much time to microscopic studies of bacteria, for which he devised not only a method of
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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 ventshydrothermal vent,
crack along a rift or ridge in the deep ocean floor that spews out water heated to high temperatures by the magma under the earth's crust. Some vents are in areas of seafloor spreading, and in some locations water temperatures above 350°C; (660°F;) have
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 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.


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 stainGram's stain,
laboratory staining technique that distinguishes between two groups of bacteria by the identification of differences in the structure of their cell walls. The Gram stain, named after its developer, Danish bacteriologist Christian Gram, has become an important tool
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). 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.


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 eukaryoteeukaryote
, a cell or organism composed of cells that have a membrane-bound nucleus and organelles (mitochondria, chloroplasts; see cell, in biology) and genetic material organized in chromosomes in which the DNA is combined with histone proteins.
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). 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 recombinationrecombination,
process of "shuffling" of genes by which new combinations can be generated. In recombination through sexual reproduction, the offspring's complete set of genes differs from that of either parent, being rather a combination of genes from both parents.
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, 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 bacteriophagebacteriophage
, virus that infects bacteria and sometimes destroys them by lysis, or dissolution of the cell. Bacteriophages, or phages, have a head composed of protein, an inner core of nucleic acid—either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)—and a
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 (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.


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 photosynthesisphotosynthesis
, process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g.
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 and chemosynthesischemosynthesis,
process in which carbohydrates are manufactured from carbon dioxide and water using chemical nutrients as the energy source, rather than the sunlight used for energy in photosynthesis. Most life on earth is fueled directly or indirectly by sunlight.
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 (see autotrophautotroph
, in biology, an organism capable of synthesizing its own organic substances from inorganic compounds. Autotrophs produce their own sugars, lipids, and amino acids using carbon dioxide as a source of carbon, and ammonia or nitrates as a source of nitrogen.
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). The photosynthetic bacteria include the green and purple bacteria and the cyanobacteriacyanobacteria
or blue-green algae,
photosynthetic bacteria that contain chlorophyll. For many years they were classified in the plant kingdom along with algae, but discoveries made possible by the electron microscope and new biochemical techniques have shown them to be
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. Many of the thermophilic archaebacteria are chemosynthetic autotrophs.

Beneficial Bacteria

Harmless and beneficial bacteria far outnumber harmful varieties. Thousands of bacterial species live commensally in humans, and many provide health benefits to humans, aiding in digestion, for example, or helping to prevent the establishment of colonies of pathogenic bacteria. 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 cyclenitrogen cycle,
the continuous flow of nitrogen through the biosphere by the processes of nitrogen fixation, ammonification (decay), nitrification, and denitrification. Nitrogen is vital to all living matter, both plant and animal; it is an essential constituent of amino acids,
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), 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 mutationsmutation,
in biology, a sudden, random change in a gene, or unit of hereditary material, that can alter an inheritable characteristic. Most mutations are not beneficial, since any change in the delicate balance of an organism having a high level of adaptation to its environment
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, or rare variations.

Pathogenic Bacteria

Bacterial parasites that cause disease are called pathogens. Among bacterial plant diseases are leaf spot, fire blightblight,
general term for any sudden and severe plant disease or for the agent that causes it. The term is now applied chiefly to diseases caused by bacteria (e.g., bean blights and fire blight of fruit trees), viruses (e.g., soybean bud blight), fungi (e.g.
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, and wilts; animal diseases caused by bacteria include tuberculosistuberculosis
(TB), contagious, wasting disease caused by any of several mycobacteria. The most common form of the disease is tuberculosis of the lungs (pulmonary consumption, or phthisis), but the intestines, bones and joints, the skin, and the genitourinary, lymphatic, and
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, choleracholera
or Asiatic cholera,
acute infectious disease caused by strains of the bacterium Vibrio cholerae that have been infected by bacteriophages. The bacteria, which are found in fecal-contaminated food and water and in raw or undercooked seafood, produce a
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, syphilissyphilis
, contagious sexually transmitted disease caused by the spirochete Treponema pallidum (described by Fritz Schaudinn and Erich Hoffmann in 1905). Syphilis was not widely recognized until an epidemic in Europe at the end of the 15th cent.
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, typhoid fevertyphoid fever
acute, generalized infection caused by Salmonella typhi. The main sources of infection are contaminated water or milk and, especially in urban communities, food handlers who are carriers.
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, and tetanustetanus
or lockjaw,
acute infectious disease of the central nervous system caused by the toxins of Clostridium tetani. The organism has a widespread distribution and is common in the soil, human and animal feces, and the digestive tracts of animals and humans;
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. Some bacteria attack the tissues directly; others produce poisonous substances called toxins. Natural defense against harmful bacteria is provided by antibodies (see immunityimmunity,
ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity.
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). Certain bacterial diseases, e.g., tetanus, can be prevented by injection of antitoxinantitoxin,
any of a group of antibodies formed in the body as a response to the introduction of poisonous products, or toxins. By introducing small amounts of a specific toxin into the healthy body, it is possible to stimulate the production of antitoxin so that the body's
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 or of serum containing antibodies against specific bacterial antigens; immunity to some can be induced by vaccinationvaccination,
means of producing immunity against pathogens, such as viruses and bacteria, by the introduction of live, killed, or altered antigens that stimulate the body to produce antibodies against more dangerous forms.
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; and certain specific bacterial parasites are killed by antibioticsantibiotic,
any of a variety of substances, usually obtained from microorganisms, that inhibit the growth of or destroy certain other microorganisms. Types of Antibiotics
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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 coliEscherichia coli
, common bacterium that normally inhabits the intestinal tracts of humans and animals, but can cause infection in other parts of the body, especially the urinary tract.
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 has caused serious illness and death in victims of food poisoningfood poisoning,
acute illness following the eating of foods contaminated by bacteria, bacterial toxins, natural poisons, or harmful chemical substances. It was once customary to classify all such illnesses as "ptomaine poisoning," but it was later discovered that ptomaines, the
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. See also drug resistancedrug resistance,
condition in which infecting bacteria can resist the destructive effects of drugs such as antibiotics and sulfa drugs. Drug resistance has become a serious public health problem, since many disease-causing bacteria are no longer susceptible to previously
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See P. Singleton, Introduction to Bacteria (1992); W. Biddle, A Field Guide to Germs (1995).


Extremely small—usually 0.3 to 2.0 micrometers in diameter—and relatively simple microorganisms possessing the prokaryotic type of cell construction. Although traditionally classified within the fungi as Schizomycetes, they show no phylogenetic affinities with the fungi, which are eukaryotic organisms. The only group that is clearly related to the bacteria are the blue-green algae. Bacteria are found almost everywhere, being abundant, for example, in soil, water, and the alimentary tracts of animals. Each kind of bacterium is fitted physiologically to survive in one of the innumerable habitats created by various combinations of space, food, moisture, light, air, temperature, inhibitory substances, and accompanying organisms. Dried but often still living bacteria can be carried into the air. Bacteria have a practical significance for humans. Some cause disease in humans and domestic animals, thereby affecting health and the economy. Some bacteria are useful in industry, while others, particularly in the food, petroleum, and textile industries, are harmful. Some bacteria improve soil fertility. As in higher forms of life, each bacterial cell arises either by division of a preexisting cell with similar characteristics or through a combination of elements from two such cells in a sexual process. See Industrial microbiology

Descriptions of bacteria are preferably based on the studies of pure cultures, since in mixed cultures it is uncertain which bacterium is responsible for observed effects. Pure cultures are sometimes called axenic, a term denoting that all cells had a common origin in being descendants of the same cell, without implying exact similarity in all characteristics. Pure cultures can be obtained by selecting single cells, but indirect methods achieving the same result are more common.

If conditions are suitable, each bacterium grows and divides, using food diffused through the gel, and produces a mass of cells called a colony. Colonies always develop until visible to the naked eye unless toxic products or deficient nutrients limit them to microscopic dimensions. See Culture

The morphology, that is, the shape, size, arrangement, and internal structures, of bacteria can be distinguished microscopically and provides the basis for classifying the bacteria into major groups. Three principal shapes of bacteria exist, spherical (coccus), rod (bacillus), and twisted rod (spirillum). The coccus may be arranged in chains of cocci as in Streptococcus, or in tetrads of cocci as in Sarcina. The rods may be single or in filaments. Stains are used to visualize bacterial structures otherwise not seen, and the stain reaction with Gram's stain provides a characteristic used in classifying bacteria.

Many bacteria are not motile. Of the motile bacteria, however, some move by means of tiny whirling hairlike flagella extending from within the cell. Others are motile without flagella and have a creeping or gliding motion. Many bacteria are enveloped in a capsule, a transparent gelatinous or mucoid layer outside the cell wall. Some form within the cell a heat- and drought-resistant spore, called an endospore. Cytoplasmic structures such as reserve fat, protein, and volutin are occasionally visible within the bacterial cell.

The nucleus of bacteria is prokaryotic, that is, not separated from the rest of the cell by a membrane. It contains the pattern material for forming new cells. This material, deoxyribonucleic acid (DNA), carrying the information for synthesis of cell parts, composes a filament with the ends joined to form a circle. The filament consists of two DNA strands joined throughout their length. The joining imparts a helical form to the double strand. The double-stranded DNA consists of linearly arranged hereditary units, analogous and probably homologous with the “genes” of higher forms of life. During cell division and sexual reproduction, these units are duplicated and a complete set is distributed to each new cell by an orderly mechanism.

The submicroscopic differences that distinguish many bacterial genera and species are due to structures such as enzymes and genes that cannot be seen. The nature of these structures is determined by studying the metabolic activities of the bacteria. Data are accumulated on the temperatures and oxygen conditions under which the bacteria grow, their response in fermentation tests, their pathogenicity, and their serological reactions. There are also modern methods for determining directly the similarity in deoxyribonucleic acids between different bacteria. See Fermentation, Pathogen, Serology

Bacteria are said to be aerobic if they require oxygen and grow best at a high oxygen tension, usually 20% or more. Microaerophilic bacteria need oxygen, but grow best at, or may even require, reduced oxygen tensions, that is, less than 10%. Anaerobic bacteria do not require oxygen for growth. Obligatorily anaerobic bacteria can grow only in the complete absence of oxygen. Some bacteria obtain energy from the oxidation of reduced substances with compounds other than oxygen (O2). The sulfate reducers use sulfate, the denitrifiers nitrate or nitrite, and the methanogenic bacteria carbon dioxide as the oxidizing agents, producing H2S, nitrogen (N2), and methane (CH4), respectively, as reduction products.

Interrelationships may be close and may involve particular species. Examples are the parasitic association of many bacteria with plant and animal hosts, and the mutualistic association of nitrogen-fixing bacteria with leguminous plants, of cellulolytic bacteria with grazing animals, and of luminous bacteria with certain deep-sea fishes. See Population ecology

Endospores are resistant and metabolically dormant bodies produced by the gram-positive rods of Bacillus (aerobic or facultatively aerobic), Clostridia (strictly anaerobic), by the coccus Sporosarcina, and by certain other bacteria. Sporeforming bacteria are found mainly in the soil and water and also in the intestines of humans and animals. Some sporeformers are found as pathogens in insects; others are pathogenic to animals and humans. Endospores seem to be able to survive indefinitely. Spores kept for more than 50 years have shown little loss of their capacity to germinate and propagate by cell division. The mature spore has a complex structure which contains a number of layers. The unique properties of bacterial spores are their extreme resistance to heat, radiation from ultraviolet light and x-rays, organic solvents, chemicals, and desiccation. The capacity of a bacterial cell to form a spore is under genetic control, although the total number of genes specific for sporulation is not known. The actual phenotypic expression of the spore genome depends upon a number of external factors. For each species of sporeforming bacteria, there exist optimum conditions for sporogenesis which differ from the optimal conditions for vegetative growth. These conditions include pH, degree of aeration, temperature, metals, and nutrients. The three processes involved in the conversion of the spore into a vegetative cell are (1) activation (usually by heat or aging), which conditions the spore to germinate in a suitable environment; (2) germination, an irreversible process which results in the loss of the typical characteristics of a dormant spore; and (3) outgrowth, in which new classes of proteins and structures are synthesized so that the spore is converted into a new vegetative cell.



a large group (type) of microscopic, predominantly unicellular organisms having a cell wall, containing a great deal of deoxyribonucleic acid (DNA), with a primitive nucleus, lacking visible chromosomes and membrane, and as a rule not containing chlorophyll or plastids. Bacteria multiply by transversal division (less frequently by elongation and budding). The vast majority of bacteria have a rodlike shape. However, among bacteria there are also microorganisms which have spherelike, threadlike, or coiled shapes. Bacteria vary in their physiology and are biochemically very active. They are found in soil, in water, at the bottom of reservoirs, and so on. Bacteria do not form one single homogeneous group; rather, they have developed along various paths. Certain bacteria (for example, threadlike bacteria and Azotobacter) are close to the blue-green algae; others (the actinomycetes) are related to the radiant fungi, and the spirochetes and several other kinds of bacteria show a resemblance to unicellular animals—the protozoans.

Bacteria participate in the nitrogen cycle in nature. Some of them are responsible for diseases in man, animals and plants; they are also used in various branches of the antibiotics industry. The study of bacteria is called bacteriology. This is a part of a broader discipline—microbiology—whose task it is to study all facets of the life functions not only of bacteria but also of other microorganisms (yeast, mold fungi, and algae).

Man made use of bacteria even before he knew of their existence. With the help of leavens containing bacteria, people made sour milk products, vinegar, dough, and so on. Bacteria were first observed by A. van Leeuwenhoek, the inventor of the microscope, when he was studying vegetable infusions and the film found on teeth. By the turn of the 20th century, a great number of bacteria living in the soil, water, food products, and so on had been distinguished, and many types of pathogenic bacteria had been discovered. The classical experiments of L. Pasteur in the area of the physiology of bacteria served as the basis for further study of their metabolism. A contribution to the study of bacteria was made by the Russian and Soviet scientists S. N. Vinograd-skii, V. L. Omelianskii, and B. L. Isachenko, who elucidated the role of bacteria in the nitrogen cycle in nature which makes life on earth possible. This direction in microbiological research is intimately related to developments in geology, biogeochemistry, soil chemistry, and the studies of V. I. Vernadskii on the biosphere.

Morphology and taxonomy, DIMENSIONS, FORM, STRUCTURE, AND MOBILITY. The diameter of spherical bacteria is usually from 1 to 2 microns; the width of rodlike bacteria varies from 0.4 to 0.8 microns and their length from 2 to 5 microns. Extremely large bacteria are encountered less frequently—for instance, the sulfur bacteria Thiophysa macrophysa, which have a diameter of 20 microns. The threads of other sulfur bacteria (Beggiatoa) are visible with the naked eye. There are also extremely small bacteria—for example, Bdellovibrio—which are parasites on bacteria of normal size. Certain bacteria—for example, those which cause pleuropneumonia in cattle—are so small that they are invisible with an optical microscope. Spherical bacteria are called cocci; if they occur in pairs, they are called dip-lococci. If cocci multiply by transversal division and after division remain linked forming chains, then they are called streptococci. In the case of sarcinas, the cells divide along three mutually perpendicular axes, forming packages of cells. When cocci divide on various surfaces, groups of cells resembling clusters of grapes arise; these are characteristic of staphylococci. Rodlike bacteria that form spores are called bacilli. Rodlike bacteria can have either truncated or bulging ends; they can occur individually or, less frequently, in the form of a chain. Bacteria forming long threads (threadlike bacteria) live predominantly in water. Bacteria in the form of a comma are called vibrio, coiled bacteria with loose spiral turns are called spirilli, and those with several small and regular turns are called spirochetes.

All bacteria have a cell wall. The cell wall is clearly visible if the bacteria are placed in a salt solution. When this is done the contents of the cell contract and move away from the wall, and plasmolysis occurs. In many bacteria the wall is surrounded by a mucoid capsule, the presence of which can be established by placing such bacteria in a solution of india ink. With electron microscopy it is apparent that the cell wall is composed of several layers (usually three). It is made up of muramic acid, amino acids, lipids, glucosamine, and other compounds. The chemical composition of the cell wall varies with different taxonomic groups, and in the case of bacteria it also varies with gram-positive and gram-negative staining. The cytoplasmic membrane which is located below the cell wall plays an important role in the exchange of materials. The numerous enzyme systems of the bacterial cell are concentrated in the membrane. The cytoplasm contains ribo-somes, which are made up of RNA. The amount of nucleic acids in bacteria varies from 10 to 22 percent, with varying proportions of RNA to DNA (in the rods of the intestine the ratio is 2:1). With the aid of the electron microscope, the presence in the bacterial cell of strands of DNA forming a nucleus without a membrane (the so-called nuclear region) has been established. The structure of the nucleus is different in different kinds of bacteria. Thus in the “higher,” more highly organized bacteria (Myxobacterales, Hyphomy-crobiales) the nuclei of stained specimens can be easily observed under an optical microscope. In many bacteria the cytoplasm is thicker near the edges of the cell and forms polar grains which stain easily. Bacterial cells contain storage material—fatty inclusions, glycogen, metachromatin, and granules, and also vacuoles containing liquids and gases. In contrast to fungi, bacteria do not contain mitochondria, which testifies to the more primitive structure of bacteria.

Many bacteria are motile. Usually such bacteria have long flagella composed of contractable protein. The bacterial cell is able to move owing to the undulating and spiral movements of its flagella. Types with one flagellum at the end of the cell are called monotrichins, cells with a bundle of flagella at one end are called lophotrichins, and bacteria whose whole surface is covered with flagella are called perit-richins. Myxobacteria, which are also motile, do not have flagella. They move by swelling up with mucus secreted into the surrounding medium by other cells (reaction mode of locomotion).

LIFE CYCLE. The change in the morphology of bacterial cells over time gives a representation of their life cycle. Thus many aerobic and anaerobic bacteria form oval and round shining spores. Such types of bacteria are called spore-forming bacteria (or bacilli). If the spores are large and are located in the center of the cell, the rod takes on a spindlelike form; in other types of bacteria the spore is located at the tip of the rod, and in this case the latter assumes the form of a staff or a drumstick. The spores are very small in most spore-forming bacteria, and therefore the formation of spores does not alter the rodlike shape of the bacteria. Later on the remains of the vegetative cell disintegrate and the spore is released. Only one spore forms in each cell; therefore the formation of spores cannot be seen as a form of reproduction. Spores are extremely resistant to the effects of high temperature and toxic materials. If they are in a favorable nutrient medium, the spores germinate and turn into young rodlike vegetative cells.

The cycle of bacterial development can vary. Thus, mycobacteria reproduce both by division and by budding. In the case of myxobacteria the vegetative cells contract and form round or oval microcysts, which later are able to germinate. The microcysts, held together by mucus, form spherical, funguslike, or corallike bodies which are green, pink, or other colors. In the course of their growth process, bacteria can produce filtrable forms, which pass through filters and, upon further culturing, yield bacteria similar or identical to those from which they originated.

MUTATION. The physiological and morphological characteristics of bacteria can be subjected to change. Bacteria can ’lose their motility or their capacity to form pigments or produce spores, their aptitude for synthesizing various organic compounds can be strengthened or weakened, the form and structure of their colonies can be changed on a solid nutrient medium, and so on. These changes can take place spontaneously—that is, without any external influence. But a significantly larger number of altered cells appears as a result of the application of mutagens (ultraviolet rays, ionizing radiation, ethylenimine, and other chemical materials). Each characteristic of the bacteria is related to the DNA— that is, is controlled by a corresponding gene. Thanks to advances in the genetics of microorganisms, the location of many genes on the DNA strands has been established. By separating the DNA from mutant cells and adding to it a culture of another strain, it is possible to produce inherited mutations, called transformations (as a result of the penetration of the DNA inside the cells). With the help of mutagens, mutants with valuable practical applications can be obtained, including mutants that produce a large quantity of various antibiotics, amino acids, vitamins, and other biologically active substances. With the help of bacterial mutants, the path of biosynthesis of various organic compounds has been plotted. Bacteria may be forced to adapt to new conditions of existence by gradually changing their environment. In this manner, strains of bacteria which are resistant to various toxins or which can grow under extreme conditions of temperature or environmental surroundings are obtained; this is the origin of strains of pathogenic bacteria which are resistant to certain antibiotics.

TAXONOMY. In order to ascertain the taxonomic status of bacteria it is necessary to specify their size, the morphology of the cells, the rate of growth of a pure culture in various nutrient mediums, and the form, color, and nature of the surface of the colonies growing in a dense medium. One must also establish the degree of liquefaction of gelatin by bacteria, the ability of the bacteria to ferment milk, to burn various carbohydrates, to reduce nitrates, to form ammonia, hydrogen sulfide, and indol out of the disintegration products of proteins and such. Having these characteristics of the isolated culture, it is possible to determine its taxonomic status. Bacteria are subdivided into three classes.

The first class, Eubacteria, includes those bacteria having a thick cell wall and not forming fruiting bodies. In this class the following orders are distinguished: (1) Eubacteriales— unicellular cocci, nonbranching rods, and spirally coiled strains, all spore-forming and nonspore-forming bacteria, photosynthetic bacteria, spirochetes, and others; (2)Tricho-bacteriales—multicellular, threadlike bacteria with transverse partitions; (3)Ferribacteriales—unicellular, nonthread-like, autotrophic iron bacteria; and (4) Thiobacteriales— unicellular, autotrophic sulfur bacteria.

The second class, Myxobacteria, includes bacteria with a fine cell wall and a reactive type of locomotion, which produce microcysts and spores of various kinds.

The third class, Hyphomicrobiales, contains bacteria forming long threads with buds at the ends; when the buds detach they are capable of movement. Current taxonomy distinguishes large groups of related organisms on the basis of evolutionary (phylogenetic) data. Thus, for example, branching mycobacteria are not placed in a separate group but are classified with the actinomycetes.

Physiology, GROWTH, REPRODUCTION, AND DEVELOPMENT. After the division of a bacterial cell, each of the two daughter bacterial cells begins to grow and reaches the size of the mother cell. In this instance, one speaks of the growth of an individual cell. The reproduction of cells composing a population leads to an increase in the total number of cells, and in this case one speaks of the growth of a culture. When a culture grows in a liquid nutrient medium, the medium becomes cloudy; the greater the number of cells in the culture, the cloudier the medium becomes. It is possible to determine the rate of growth by counting the cells in 1 milliliter of culture with the help of a microscope or by determining the degree of cloudiness of the nutrient medium by means of a nephelometer. By determining the number of cells at various points in the growth of a culture, it is possible to obtain a growth curve displaying several phases—at first the cells do not multiply, then they begin to divide and the rate of reproduction increases steadily; later they enter a phase characterized by a constant rate of cell division; and finally the rate of division decreases and a dying off of cells occurs. In order to obtain the maximum number of cells, bacteria are grown under conditions of so-called circulating culture; this means that a certain volume of the culture is allowed to flow out of the vessel in which the bacteria are reproducing, and at the same time an equal amount of sterile nutrient medium is added to the vessel. When bacteria reproduce under stationary instead of circulating conditions, a change in the nutrient medium takes place and is accompanied by an accumulation of the waste products of the bacteria in the medium; as a result of this the physiological characteristics of the bacteria undergo a change. For example, young Clostridium acetobutylkum are not capable of making acetone; this ability is only attained in an older culture. If spore-forming bacteria are grown under conditions of a circulating culture, they will divide but will not bear spores. When bacteria are grown in a dense nutrient medium they form clusters of cells of various sizes, forms and colors; these are called colonies.

NUTRITION. Bacterial cells are composed of the same life-supporting elements and trace elements as those which make up the cells of higher plants and animals. Among these are carbon, nitrogen, oxygen, hydrogen, sulfur, phosphorus, potassium, magnesium, calcium, chlorine, and iron. In addition to protein, carbohydrates, and fats, bacteria also contain RNA and a large quantity of DNA. All of these materials can be synthesized only from matter contained in the surrounding environment. As a rule, only soluble materials can pass through the semipermeable cell wall and the cytoplasmic membrane into the bacterial cell. Hydrolytic enzymes which migrate outside the bacterial cells break down more complex materials (for example, starch and cellulose) with the formation of soluble products (for example, monosaccharides), which can be absorbed by the bacteria. Bacteria can use proteins, amino acids, ammonium salts, and nitrates as sources of nitrogen. Different types of bacteria are capable of utilizing different sources of nitrogen. Formerly it was thought that certain pathogenic (infectious) and sour milk bacteria could grow only in nutrient mediums containing proteins, but later it became evident that such bacteria could use ammonium salts as a source of nitrogen. There are many kinds of bacteria in the various taxonomic groups which are capable not only of absorbing nitrogen from the various nitrogen-containing materials but also of fixating nitrogen from the atmosphere. These nitrogen-fixating microorganisms include Azotohacter, mycobacteria, purple photo-synthetic bacteria, and also tuberous bacteria. The salts phosphorus, sulfur, chlorine, potassium, iron, sodium, and calcium provide mineral nourishment for bacteria; many types of bacteria also require trace elements (molybdenum, manganese, copper, boron, vanadium, and others). In order to reproduce, bacteria also require various factors in the growth of microorganisms, including vitamins of the B group, pantothenic acid, folic acid, and others. Bacteria which are capable of synthesizing these materials are called auxoautotrophs. These include Pseudomonas and many other nonspore-forming bacteria. When growing bacteria which are not capable of synthesizing these growth factors, the factors must be added to the nutrient medium. Such bacteria are called auxoheterotrophs; they include various bacteria which ferment milk. Carbon sources for bacteria include carbohydrates, alcohols, organic acids, lignin, chi-tin, hydrocarbons, and fats. The ability to absorb carbon from these different sources varies for different bacteria; these differences are used in the classification of bacteria. Bacteria which absorb carbon from organic compounds are called heterotrophic, and bacteria which obtain carbon from carbon dioxide gas in the atmosphere are called autotrophic. Bacteria which use the energy of the sun’s rays for the fixation of carbon dioxide are called photoautotrophs. The group of bacteria obtaining energy as a result of the oxidation of such inorganic materials as ammonia, nitrates, sulfur, or hydrogen, and which are capable of obtaining carbon dioxide because of the energy released during the oxidation of the aforementioned inorganic compounds, are called chemoautotrophs. The process of assimilation of carbon dioxide—chemosynthesis—was discovered by the prominent Russian microbiologist S. N. Vinogradskii.

RESPIRATION AND METABOLISM. The synthesis of materials that make up the bacterial cell, the cell’s movement, and other processes are all accompanied by an expenditure of energy. The majority of bacteria obtain energy via the oxidation of organic materials, chemoautotrophic bacteria obtain energy as a result of the oxidation of inorganic compounds, and photosynthetic bacteria use the energy of the sun’s rays. Bacteria which are capable of growth only in the presence of oxygen are called aerobic bacteria; those which can grow in the absence of oxygen are called anaerobic bacteria. Aerobic respiration involves the oxidation of organic compounds with the release of carbon dioxide gas. If the oxidation is not complete, intermediate products accumulate in the medium. Such processes are called oxidizing fermentation (for example, acetic acid fermentation). The breakdown of organic materials under anaerobic conditions with the release of energy is called fermentation. The burning of carbohydrates by various bacteria can produce lactic or butyric acid, ethyl, propyl, or butyl alcohol, acetone, and other materials. Many biochemical processes (glycolysis, electron transport, the Krebs cycle, the synthesis of amino acids, proteins, nucleic acids, and so on) take place in bacteria in almost the same fashion as in plant and animal cells. The specific characteristics of metabolism in bacteria are high biochemical activity; the ability to oxidize inorganic sulfur, nitrogen (ammonia), and other compounds; the ability to synthesize proteins using phenol, methane, and other hydrocarbons as raw materials; and the ability to oxidize hydrogen, to fix nitrogen from the atmosphere, to synthesize enzymes which break down cellulose or lignin, and to produce methane from carbon dioxide and hydrogen. These processes have extremely valuable practical applications.

Ecology. DISTRIBUTION. Bacteria are cosmopolites. Identical types of bacteria can be found on all continents—that is, almost everywhere. The quantity of bacteria in the soil, water, or other mediums is determined directly by counting the cells in a stained sample or by inoculating various nutrient mediums with the bacteria. One gram of soil contains hundreds of thousands or millions of bacteria; 1 milliliter of water contains tens or hundreds of cells. Ecological conditions exert a great influence on bacterial organisms— cultivated soil not only contains more bacteria than, for example, the soil of the desert; the two also differ in the specific types of their microorganisms. Contemporary microbiology has not identified more than one-tenth of the bacteria existing in nature. The application of methods of capillary and electron microscope examination of soil has permitted the identification of many new types of bacteria. Bacteria are found in the most varied ecological conditions, and in the course of their evolution they have adapted to these conditions. This accounts for the appearance of thermophilic bacteria, which live in the water of hot springs and in warm piles of turf or manure; psychrophilic forms, which live in low temperatures in the water of the polar seas; halophilic bacteria, capable of reproduction in an environment containing up to 20 percent salt; acidophilic and basophilic bacteria, which can grow in an extremely acidic or extremely alkaline environment; and so on. The wide distribution in nature of specific sources of carbon and nitrogen has led in the course of evolution to physiological convergence—that is, to the ability of representatives of different taxonomic groups of bacteria to obtain a particular life-supporting element from the same source. Thus, bacteria that fixate nitrogen from the atmosphere can belong to different classes, orders, and families; many bacteria which have the ability to utilize cellulose are far apart in taxonomic classification. Bacteria can enter into antagonistic relationships as well as symbiotic relationships with other microorganisms, plants, and animals. Certain bacteria produce pigments, antibiotics, or organic acids which interfere with the life functions of other bacteria such as fungi, algae, unicellular bacteria, and certain cells of multicellular animals. Bacterial viruses—bacteriophages—penetrate inside the bacterial cell and, reproducing there, bring about the destruction and lysis of the microorganism. In a symbiotic relationship—that is, one founded on reciprocal advantage—one species of bacteria can use the waste products of the other species which, if they accumulated in the culture medium, would impede the growth of the latter. The symbiote in turn can secrete into the medium additional growth factors which are absolutely necessary to the other species. Bacteria living in the intestines of animals or of man and feeding on the contents of the intestines produce enzymes necessary for digestion and many other substances which are extremely important for the life of the host (irreplaceable amino acids, various vitamins, and so on).

Role in the circulation of matter in nature. By breaking down plant and animal remains, microorganisms participate in the circulation of all chemical elements which make up the living cell. Thus, the carbon dioxide in the atmosphere fixated in the processes of photosynthesis and chemosynthesis provides a source of carbon for higher plants and chemoautotrophic bacteria. The living mass of plants and animals is broken down by microorganisms capable of utilizing cellulose, pentoses, starch, lignin, pectin substances, and in the last analysis, even carbon dioxide and water. The role of microorganisms—including bacteria—in the nitrogen cycle is equally important. Animals feeding on vegetables synthesize protein and other nitrogen-containing products for their bodies using the plant protein. During the decomposition of animal and plant protein, putrefactive bacteria produce ammonia which is oxidized by nitrifying bacteria into nitrites and then into nitrates. Both ammonium salts and nitrates serve as a source of nitrogen for the higher plants which use these substances in order to synthesize their own protein. The ability of bacteria to break down substances also makes possible the circulation of other life-supporting elements. By breaking down the organic compounds of phosphorus (nucleic acids and such) bacteria enrich reservoirs and the soil with mineral compounds of phosphorus. The breakdown of organic compounds of sulfur also takes place under the influence of bacteria. Sulfur bacteria can oxidize hydrogen sulfide, sulfur, or certain other sulfur compounds, including sulfuric acid; other bacteria are capable of reducing sulfates, with the production of hydrogen sulfide. Bacteria are responsible for the oxidation of iron and manganese, the accumulation of calcium salts, the oxidation of methane and hydrogen, the erosion of rock by products of their life functions, and so on. All this enables us to consider bacteria a powerful geological force.

Practical significance. Bacteria provide the favorite tools for the solution of general problems in genetics, biochemistry, biophysics, astrobiology, and other fields. Bacterial cultures are used for the quantitative determination of amino acids, vitamins, and antibiotics. The fertility of soil is to a significant degree dependent on the activity of bacteria which break down vegetable and animal remains and produce compounds which can be consumed by agricultural plants. Aside from synthesizing the living materials of the cell, bacteria accumulate large quantities of organic compounds in the soil. The upper layers of one hectare of cultivated soil contain several tons of bacterial cells. Nitrogen-fixating bacteria living in the soil enrich the soil with nitrogen. An exceptionally important role is played by tuberous bacteria, which fixate gaseous nitrogen. The inoculation of seeds of leguminous plants with nitrogene, a preparation containing cells of tuberous bacteria, increases the yield of plants and the accumulation of nitrogen in the soil. Flax, hemp, kenaf, and other natural fibers are raised with the help of bacteria which break down pectin substances. Various species of bacteria are used in making sour milk products, butter, and cheese from milk.

The microbiological industry produces lactic acid, acetone, ethyl, butylic and other alcohols, the blood substitute dextran, diacetyl, antibiotics (gramicidin and others), vitamins, amino acids, and other substances from starch-containing or other raw material by means of the proper species of bacteria. Bacteria are especially widely used to obtain enzyme preparations (amylase, protease, and so on). Sauerkraut, silage, and pickles are kept from spoiling by the reproduction of bacteria which produce lactic acid from carbohydrates, since an acidic reaction prevents the growth of putrefactive bacteria. Bacteria which oxidize sulfur are used in the bacterial lixiviation of copper and many other metals from the ores which contain them. By placing bacteria which are capable of consuming gaseous hydrocarbons in vessels and then burying these vessels in the soil, it is possible to determine whether there is oil or natural gas in a given location by examining the growth of the bacteria.

It is necessary to wage a serious struggle against many kinds of bacteria in order to prevent them from spoiling or destroying grain, vegetables, fruits, all food products, various kinds of raw materials, fabrics, and manufactured items such as textiles, cardboard, rope, fishnets, and cable insulation. Many human diseases are caused by pathogenic bacteria. These diseases include various epidemic illnesses (cholera, typhoid, paratyphoid, plague, diphtheria, tularemia, and brucellosis) and also tuberculosis, blood infection (sepsis), leprosy, and syphilis. In animals, bacteria cause glanders, anthrax, tuberculosis, and other diseases. Many diseases in both cultivated and wild plants are caused by so-called phytopathogenic bacteria. The struggle against pathogenic bacteria is based on asepsis and antisepsis and on the use of bacteriostatic and bactericidal substances.


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Isachenko, B. L. Izbrannye trudy, vols. 1–2. Moscow-Leningrad, 1951.
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A. A. IMSHENETSKII (2–1620–2]


Extremely small, relatively simple prokaryotic microorganisms traditionally classified with the fungi as Schizomycetes.
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