MacConkey agar:

MacConkey agar is a selective, differential plating medium which contains:
  • Bile salts to inhibit (but not completely block) the growth of non-enteric bacteria
  • Lactose as the sole carbon source
  • Neutral red indicator dye
Organisms which are able to ferment lactose produce acid as a result. This turns the neutral red indicator in the agar around the colony dark pink, like this:

Organisms which are not able to ferment lactose do not produce acid. The neutral red indicator in the agar around the colony remains colourless producing pale colonies, like this:

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