solution freed of all microorganisms by sterilization (usually in an autoclave, where it undergoes heating under pressure for a specific time) and containing the substances required for the growth of microorganisms such as bacteria, protozoans, algae, and fungi. The medium may be solidified by the addition of agar. Some media consist of complex ingredients such as extracts of plant or animal tissue (e.g., peptone, meat extract, yeast extract); others contain exact quantities of known inorganic salts and one or more organic compounds (synthetic or chemically defined media). Various types of living cells, or tissue cultures, also may be used as media. Many special-purpose media are used in microbiology. These include enriched media, which contain nutrients in addition to those found in the usual growth medium, and selective media, which contain substances that prevent the growth of certain organisms but not others.