microbes, an extensive group of predominantly unicellular organisms that are distinguishable only under a microscope and are organized more simply than plants and animals. Bacteria, mycoplasms, actinomycetes, yeasts, microscopic fungi, and algae are classified as microorganisms; the category sometimes also includes protozoans and viruses.

Microorganisms are divided into prokaryotes and eukaryotes. In prokaryotes the primitive nucleus contains a single chromosome, lacks a membrane, and divides by the forming of a constriction and the subsequent separation of daughter cells. The cytoplasm lacks mitochondria and, in most cases, chromatophores. Eukaryotes resemble the cells of higher plants and animals. The nucleus has a membrane and contains a set of chromosomes; in many there is a normal sexual cycle, and the cells contain endoplasmic reticulum and mitochondria (in the photosynthetic cells, chloroplasts). The bacteria, mycoplasms, actinomycetes, and blue-green algae are prokaryotes; the yeasts, microscopic fungi, and algae are eukaryotes. The science of microorganisms is called microbiology.

Microbial morphology and life cycles are extremely diverse. For example, the majority are unicellular, but many mold fungi have a multicellular mycelium. Microorganisms do not as a rule contain chlorophyll, but purple and green photoautotrophic bacteria and microscopic algae contain photosynthetic pigments (bacteriochlorophylls and chlorophyll). Bacteria reproduce by division, yeasts and mycobacteria by budding, and mold fungi by cell division and the formation of conidia and spores. Bacteria are descended from a variety of taxonomic groups: actinomycetes are related to fungi; some filamentous bacteria, to blue-green algae; and spirochetes, to protozoans.

Microorganisms are divided into pathogenic (disease-producing) and nonpathogenic types. The causative agents of the majority of infections are bacteria (considerably less often, yeasts, mold fungi, and actinomycetes).

Microscopic fungi that form white, green, or black downy patches (colonies) on food products have been known to man earlier than have yeasts or bacteria. The study of yeasts and bacteria by microscope was complicated by the fact that they were grown in liquid nutrient media, which made it difficult to obtain pure cultures. The introduction of solid nutrient media opened the possibility of growing isolated colonies of given species of bacteria or yeasts and thereby of studying their various properties. Methods have been developed for describing and defining the taxonomic positions of microorganisms.

Microorganisms are widespread in nature. A single gram of soil or silt may contain 2–3 billion microorganisms. It is estimated that modern microbiology knows of no more than 10 percent of the species of microorganisms existing in nature. New genera and species are described every year; from the 1940’s through the 1960’s, the number of species of actinomycetes studied increased from 35 to 350.

Microorganisms have adapted to the most varied ecological conditions during the course of evolution. For example, thermophilic bacteria are known that reproduce at 65°-75°C, but there are psychrophilic microorganisms that grow at—6°C. Halophilic microorganisms reproduce in a medium that contains as much as 25 percent NaCl. The bacteria in the water used to cool atomic reactors can tolerate irradiation of 3–4 million rads. There are osmophilic yeasts that can live in honey and jam, acidophilic bacteria that reproduce in acid media at pH 1.0, and barotolerant bacteria that survive pressures of several hundred atmospheres. The unusual resistance of microorganisms to various environmental factors allows them to occupy the extreme boundaries of the biosphere. They are found in the ocean bottom at depths of 11 km, on the surface of glaciers and snow in the arctic and antarctic and high in the mountains, in desert soil, and in the atmosphere at altitudes of 20 km.

Thanks to progress in microbial biochemistry and to the development of microbial and molecular genetics, it has become clear that many of the processes of biosynthesis and energy metabolism (electron transport, tricarboxylic acid cycle, nucleic acid synthesis, protein synthesis) occur in the same way in microorganisms as in the cells of higher plants and animals. Thus, identical processes are the basis of the growth, development, and reproduction of both higher and lower forms of life. In addition, however, microorganisms are characterized by specific enzyme systems and biochemical reactions that are not observed in other organisms; this is the basis for the capacity of microorganisms to decompose cellulose, lignin, chitin, petroleum hydrocarbons, keratin, and wax.

Microorganisms have extremely diverse ways of obtaining energy. Chemoautotrophs obtain it from the oxidation of inorganic substances. Photoautotrophic bacteria use light energy in the part of the spectrum that is not accessible to higher plants. Some microorganisms are capable of assimilating molecular nitrogen (nitrogen-fixing microorganisms), synthesizing proteins from the most varied sources of carbon, and manufacturing numerous biologically active substances (antibiotics, enzymes, vitamins, growth stimulants, toxins). The use of microorganisms in agriculture and industry is based on these specific features of their metabolism.