Microbiology Lab Practical Practice Exam

On MacConkey agar, what happens to colonies that ferment lactose?

They form colorless colonies They turn blue They appear pink or red on the agar They disappear from the plate

Correct Answer:

They appear pink or red on the agar

Explanation:

MacConkey agar is both selective for Gram-negative bacteria and differential for lactose fermentation using a pH indicator. When bacteria ferment lactose, acids are produced and lower the local pH, causing the neutral red indicator to turn pink/red. So lactose-fermenting colonies appear pink or red on the agar (often with a pink halo). Non-lactose fermenters do not acidify the medium enough, so their colonies stay colorless or pale. The other outcomes—colorless colonies, blue coloration, or colonies disappearing—do not reflect lactose fermentation on this medium.

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