MacConkey agar

MacConkey Agar is a selective and differential culture medium for bacteria. It is designed to selectively isolate Gram-negative and enteric (normally found in the intestinal tract) bacilli and differentiate them based on lactose fermentation.

Composition

MacConkey Agar contains lactose, bile salts, neutral red, crystal violet, and agar. The bile salts and crystal violet inhibit the growth of Gram-positive organisms which makes MacConkey Agar selective. Neutral red is a pH indicator that turns red at a pH below 6.8 and is colorless at any pH greater than 6.8. Lactose provides a source of fermentable carbohydrate, allowing for differentiation.

Uses

MacConkey Agar is used for the isolation of Gram-negative enteric bacteria. It is used in the differentiation of lactose fermenting from lactose non-fermenting gram-negative bacteria. It is used in the detection and/or verification of E.coli in water, food, and dairy products.

Interpretation

On MacConkey Agar, colonies of lactose fermenting bacteria are brick-red and may be surrounded by a zone of precipitated bile. The red color is due to production of acid from lactose, absorption of neutral red and a subsequent color change of the pH indicator from colorless to red. Non-lactose fermenting bacteria form transparent, colorless colonies.

History

MacConkey Agar was developed in 20th century by Alfred Theodore MacConkey, an English bacteriologist. It was the first formulated solid differential media.

See also

• Agar plate
• Bacteria
• Microbiology

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  Proteus on MacConkey agar

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  Lactose fermentation

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  MacConkey agar with lactose fermenting and non-lactose fermenting colonies