SELECTIVE MEDIA: Selective media are culture media that promote the growth of certain type of bacteria while inhibiting the growth of the undesired organisms. Such culture media contain inhibitory substances such as dyes, salts and antibiotics which prevent the growth of undesired microorganisms by suppressing them so that only the desired microbes will grow. Selective media used in the microbiology laboratory for the culture of microbes include mannitol salt agar (which contain NaCl that inhibit some bacteria), MacConkey agar (which contain bile salts and crystal violet that inhibit the growth of Gram-positive bacteria), Sabouraud dextrose agar (which contain antibiotics that inhibit bacterial growth) and Tellurite media (which contain potassium tellurite that inhibit many bacteria excluding Corynebacterium diphtheria). Another example of a selective medium used for bacteriological investigation is the Lowenstein-Jensen media (that contains egg yolk) used for the isolation of Mycobacterium tuberculosis from mixed cultures or specimens. Sabouraud dextrose agar (SDA) and Salmonella-Shigella agar (SSA) are selective media used for the cultivation and isolation of fungi and enteric pathogens (e.g. Shigella and Salmonella) respectively. SDA is selective because it contains cycloheximide and chloramphenicol which inhibit the growth of saprophytic fungi and bacteria respectively while allowing only the fungi of interest to grow.
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