bacteria



bacteria defined in 1951 year

bacteria - bacteria (sghizomycophyta);
bacteria - Group (Division) of unicellular or multicellular, microscopic, procaryotic organisms, lacking chlorophyll (a few have bacteriochlorophyll). Have usually been classified with plants rather than with animals but are distinct from both. A third kingdom, Protista is sometimes constituted for bacteria and all other organisms of simple biological organization. In shape rod-like, more or less spiral, filamentous, occasionally forming a mycelium, or, in some forms, spherical, varying in breadth mostly from 0.5 to 2.0 micrometres; non-motile or motile by one or more flagella. Multiplication is by simple fission; other forms of asexual reproduction, by formation of aerially dispersed spores, flagellated swarmers, occur in some bacteria. Sexual reproduction has also been demonstrated in certain forms. Ubiquitous, occurring in large numbers in favourable habitats, e.g. a gram of soil may contain from a few thousand to several hundred million bacteria; a cubic centimetre of sour milk, many millions. Most bacteria are saprophytes or parasites. A few are autotrophic, either obtaining energy by oxidation processes or from light in the presence of bacteriochlorophyll. The activities of bacteria are of great importance. In the soil they are concerned in the decay of plant and animal tissues, making available food materials for higher plants, and certain bacteria are of particular significance in this respect in making available nitrogenous compounds. See also: Nitrogen Cycle. Others are sources of antibiotics, e.g. Streptomyces griseus, source of streptomycin. Bacteria also play an important part in the disposal of sewage. As agents of plant disease, bacteria are not so important as fungi, but they cause many serious diseases in animal and man, e.g. tuberculosis, diphtheria, typhoid, pneumonia.

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