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the microorganisms found in a given medium: soil, water, air, food products, human and animal bodies, and plants.
Natural substrates are usually inhabited by diverse microorganisms: bacteria, actinomycetes, yeasts, microscopic fungi, and algae. The number of microorganisms in a medium is determined by inoculating a batch (or volume unit) of the substance to be investigated on solid (or, using capillary technique, fluid) media. The number of colonies growing on a solid medium gives an idea of the number of microorganisms contained per g or milliliter (ml) of the sample of soil, water, or other medium to be investigated. The direct count method is also widely used. A preparation of the substance under study is stained, and the number of cells is counted under the microscope.
In the case of fluids, the liquid is poured through a membrane filter to determine the number of cells. It has been established by this method (based on the results of inoculations) that considerably more microorganisms inhabit water and soil than had previously been supposed. Depending on the degree of contamination, water contains from 5,000 to 100,000 cells per ml; in soil, the number usually reaches 2–3 billion per g. The skin, mucosae, stomach, intestine, and other organs serve as permanent habitats of normal microflora, which have no noticeable harmful effect on the body.
V. M. ZHDANOV