MacConkey

MacConkey Agar is a selective and differential culture medium for bacteria designed to selectively isolate Gram-negative and enteric (normally found in the intestinal tract) bacilli and differentiate them based on lactose fermentation. The crystal violet and bile salts inhibit the growth of Gram-positive organisms which allows for the selection and isolation of gram-negative bacteria. Enteric bacteria that have the ability to ferment lactose can be detected using the carbohydrate lactose, and the pH indicator neutral red.

Composition

MacConkey Agar contains the following ingredients:

• Peptone: This is a nitrogen source in the medium.
• Lactose: This is a fermentable carbohydrate.
• Bile salts: These inhibit the growth of Gram-positive bacteria.
• Neutral red: This is a pH indicator.
• Crystal violet: This inhibits Gram-positive bacteria.
• Agar: This is a solidifying agent.

Uses

MacConkey Agar is used for the isolation of gram-negative enteric bacteria and the differentiation between lactose fermenters and lactose non-fermenters. It is also used for the isolation of E.coli and Klebsiella.

Interpretation

Colonies of lactose fermenters are red or pink, and are surrounded by a zone of precipitated bile. The presence of acid end-products from lactose fermentation causes the pH of the agar to drop, changing the color of the medium from neutral red to bright pink. Non-lactose fermenters, on the other hand, produce colorless or transparent colonies against the light red or pink background of the agar.

Limitations

MacConkey Agar should not be used for the isolation of fastidious organisms since they may not grow or will grow poorly on this medium. It is also not recommended for direct inoculation with clinical specimens.

See also

• Eosin methylene blue
• Hektoen enteric agar
• XLD agar
• Salmonella Shigella agar

MacConkey Resources
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