Laboratory Media


Types of Media Used in the Laboratory

The media used in the laboratory have to be chosen to suit the nutritional requirements of the species of organism to be grown. Isolation from a mixture can sometimes be facilitated by the use of media designed for a special purpose.

Growth Media: Media designed specifically to allow for the growth of microorganisms.

Differential Media: These are media in which some metabolic activity of an organism can be detected by inspection of the growth of the organism on the medium. This is often accomplished by observing changes in the color of a pH indicator. Examples include Triple Sugar Iron agar, Simmon's citrate agar, urea agar, carbohydrate broth tubes, amino acid decarboxylase or dihydrolase tubes, MIO medium (for motility, indole, and ornithine decarboxylase), and MacConkey agar.  Note: Some media can be both selective and differential.

Selective Media: In the broadest sense, all media are selective, in that there is no universal medium on which all species of bacteria can grow. This term, however, is generally restricted to situations where an ingredient is added which allows the growth of a particular organism, while inhibiting to a considerable extent the growth of other organisms which might be found in the same environment. Inhibitors such as dyes in low concentration, bile salts, high NaCl concentration and other substances such as phenylethyl alcohol are often used. Examples include PEA agar (phenylethyl alcohol) which inhibits the growth of gram-negative enteric bacilli and facilitates the isolation of gram-positive organisms such as staphylococci in aerobic cultures. In anaerobic culture, it is additionally selective for certain gram-negative anaerobic bacilli such as Bacteroides spp.. MacConkey agar, containing bile salts and dyes, inhibits gram-positive organisms and Thayer-Martin agar, containing small quantities of the antimicrobial agents vancomycin, colistin, and nystatin, inhibits the common microbiota of the genital area, while selecting for Neisseria spp.  Note: Some media can be both selective and differential.

Test Media: Media designed specifically to determine biochemical or metabolic traits of a bacterial isolate.



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