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Principle
- MacConkey agar is composed of peptone, protease peptone, S-sorbitol, bile salt, sodium chloride, neutral red, crystal violet, and agar. Peptone and protease peptone provide nitrogen and other nutrients necessary for the growth of microorganisms.
- D-sorbitol is a carbon source and plays an important role in the selection of sorbitol-fermenting microbes. Bile salt and crystal violet are selective agents, that inhibit the growth of gram-positive organisms. Neutral red is a pH indicator dye. Agar is the solidifying agent.
- When lactose is fermented, acid is produced which results in bile salt precipitation. This medium is formulated by Rappaport and Henigh (1952), particularly in isolation of enteropathogenic Escherichia coli 0157:H7, which ferments lactose but does not ferment sorbitol. Hence produce colorless colonies.
- Whereas the lactose fermenting colonies of coliform bacteria are pink in color and may be surrounded by a zone of bile precipitation.
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