A selective medium is a substance (usually agar-based) which grows a specific type of microbe. Using different nutrient ratios, chemical compounds (e.g. methylene blue) and incubation times, speciation of bacteria can be performed before to use other methods for the presuntive identification of bacteria or fungus.

Examples

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• Eosin-methylen blue agar (EMB) that contains methylene blue which is toxic for Gram-positive bacteria allowing only the growth (thus selecting) of Gram negative bacteria.
• YM (Yeast and Mold) which has a low pH, deterring bacterial growth.
• Blood agar (used in strep tests), which contains beef heart blood which becomes transparent in the presence of hemolytic Streptococcus.
• MacConkey agar for Gram-negative bacteria.

Selective genes

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These bacterial transgenes interact with selective growth media:

• GAL transgene:
  • + growth on galactose
  • - no growth on glucose
• URA3 transgene:
  • + growth on uracil-deficient medium
  • - no growth on FOA medium

See also

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• Growth medium

Microbiology