Selective And Differential Media

In health clinics and hospitals, it is necessary to detect microorganisms that are associated with disease. Selective and differential media are therefore used. Selective media are made to encourage the growth of some bacteria while inhibiting others. An example of this is bismuth sulfite agar. Bismuth sulfite agar is used to isolate Salmonella typhi from fecal matter. Salmonella typhi is a gramnegative bacterium that causes salmonella. Differential media make it easy to distinguish colonies of desired organisms from nondesirable colonies growing on the same plate. Pure cultures of microorganisms have identifiable reactions with different media. An example is blood agar. Blood agar is a dark red/brown medium that contains red blood cells used to identify bacterial species that destroy red blood cells. An example of this type of bacterium is streptococcus pyogenes, the agent that causes strep throat.

MacConkey agar is both selective and differential. MacConkey agar contains bile salts and crystal violet, which inhibit the growth of gram-positive bacteria, and lactose, in which gram-negative bacteria can grow.

Enrichment cultures are usually liquids and provide nutrients and environmental conditions that provide for the growth of certain microorganisms, but not others.

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