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Selective and Differential Media  

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Ashley N. Rundles Microbiology-1p.m March 21, 2005 Selective and Differential 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 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...

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