a fluid or solid substance that is used in the laboratory or under industrial conditions for growing bacteria, yeasts, and microscopic fungi, as well as algae, protozoans, viruses, and cultures of plant or animal cells. Synthetic nutrient mediums supply specified proportions of necessary organic and inorganic compounds providing for carbon, nitrogen, phosphorus, sulfur, potassium, sodium, and trace elements. Examples of complex organic nutrient mediums are beef and peptone broth, wort, and milk.

A fluid medium may be solidified by adding 2-percent agar or 10-percent gelatin. Solid mediums are also prepared with pieces of potato or carrot, grains of rice or millet, coagulated horse serum, and pieces of internal organs of animals. All nutrient mediums are first sterilized in an autoclave. It was formerly believed that certain pathogenic bacteria could grow only on nutrient mediums that contain such physiological substances as blood, serum, and ascitic fluid. However, studies have shown that the majority of pathogenic microbes can grow on synthetic nutrient mediums that contain glucose and ammonium sulfate as sources of carbon and nitrogen and that also contain the essential vitamins, amino acids, purine and pyrimidine bases, and trace elements. Certain microorganisms, for example, the causative agent of syphilis—Treponema pallidum —have not yet successfully been grown on synthetic mediums.

Mediums can be suitable for all microorganisms, or they can be specially selected to create more favorable conditions for the growth of only certain species. The latter type are called selective mediums. For growing anaerobic microorganisms, substances that decrease oxidation-reduction potential can be added.

Selective nutrient mediums of known composition are used to elucidate a number of physiological properties of microorganisms. For example, mediums that do not contain nitrogenous compounds are used to isolate nitrogen-fixing bacteria. The capacity of a microorganism to reduce nitrates is examined on a fluid medium that contains nitrates and an indicator, which changes color when the medium becomes alkaline, that is, when there is a shift in pH. Fluid mediums that contain various carbohydrates or alcohols, in addition to an indicator that changes color with acidification, are poured into test tubes in the bottoms of which are placed tiny inverted test tubes; when the medium is acidified by microorganisms, the indicator changes color and the gas formed accumulates in the small test tubes.

Growth on beef and peptone gelatin may be accompanied by liquefaction of the gelatin, which indicates biosynthesis of proteolytic enzymes. Inoculations on blood agar are used to examine the hemolytic properties of a microorganism (see HEMOLYSIS). The ability of microorganisms to hydrolyze starch is tested by inoculation on potato agar and subsequent treatment of colonies or streaks with solutions that contain iodine. A positive test is indicated by the disappearance of the dark blue color that starch gives in the presence of iodine. Special-purpose nutrient mediums are used to identify pathogenic species of microorganisms. (See CULTURE and TISSUE CULTURE.)