bacteria (redirected from Bactera)

bacteria [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, Monera. Bacteria fall into one of two groups, Archaebacteria (ancient forms thought to have evolved separately from other bacteria) and Eubacteria. A recently proposed system classifies the Archaebacteria, or archaea, and the Eubacteria 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 Leeuwenhoek 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 Pasteur and Robert Koch.

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 vents 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.

Characteristics

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 stain). 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.

Reproduction

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 eukaryote). 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 recombination, 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 bacteriophage (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.

Nutrition

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 photosynthesis and chemosynthesis (see autotroph). The photosynthetic bacteria include the green and purple bacteria and the cyanobacteria. Many of the thermophilic archaebacteria are chemosynthetic autotrophs.

Beneficial Bacteria

Harmless and beneficial bacteria far outnumber harmful varieties. 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 cycle), 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 mutations, or rare variations.

Pathogenic Bacteria

Bacterial parasites that cause disease are called pathogens. Among bacterial plant diseases are leaf spot, fire blight, and wilts; animal diseases caused by bacteria include tuberculosis, cholera, syphilis, typhoid fever, and tetanus. Some bacteria attack the tissues directly; others produce poisonous substances called toxins. Natural defense against harmful bacteria is provided by antibodies (see immunity). Certain bacterial diseases, e.g., tetanus, can be prevented by injection of antitoxin or of serum containing antibodies against specific bacterial antigens; immunity to some can be induced by vaccination; and certain specific bacterial parasites are killed by antibiotics

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 coli has caused serious illness and death in victims of food poisoning. See also drug resistance.

Bibliography

See P. Singleton, Introduction to Bacteria (1992); W. Biddle, A Field Guide to Germs (1995).

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bacteria

Group of microscopic, single-celled organisms that inhabit virtually all environments, including soil, water, organic matter, and the bodies of multicellular animals. Bacteria are distinguished in part by their morphological and genetic features; for instance, they may have spherical, rodlike, or spiral shapes. They also can be divided into two main groups, gram-positive or gram-negative, based on the structure of their cell wall and their reaction to the gram stain. Many bacteria swim by means of flagella (see flagellum). The DNA of most bacteria is found in a single circular chromosome and is distributed throughout the cytoplasm rather than contained within a membrane-enclosed nucleus. Though some bacteria can cause food poisoning and infectious diseases in humans, most are harmless and many are beneficial. They are used in various industrial processes, especially in the food industry (e.g., the production of yogurt, cheeses, and pickles). Bacteria are genetically distinct from the archaea. As prokaryotic organisms (having no membrane-bound nucleus), they are also distinct from eukaryotes. See also budding bacteria, coliform bacteria, cyanobacteria, denitrifying bacteria, nitrifying bacteria, sheathed bacteria, sulfur bacteria.

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bacteria [bak′tir·ē·ə]

(microbiology)

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

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Bacteria

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.