On MacConkey agar, lactose fermenters produce what color colonies?

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Prepare for the NBME Microbiology Test. Study with flashcards and multiple-choice questions. Each query comes with hints and detailed explanations. Ace your exam!

Multiple Choice

On MacConkey agar, lactose fermenters produce what color colonies?

Explanation:
MacConkey agar differentiates organisms by lactose fermentation using a neutral red pH indicator. When lactose-fermenting bacteria grow, they acidify the medium, lowering the pH and causing the colonies to appear pink to red. Non-lactose fermenters don’t acidify the medium, so their colonies stay colorless or pale. Thus, lactose fermenters produce pink colonies.

MacConkey agar differentiates organisms by lactose fermentation using a neutral red pH indicator. When lactose-fermenting bacteria grow, they acidify the medium, lowering the pH and causing the colonies to appear pink to red. Non-lactose fermenters don’t acidify the medium, so their colonies stay colorless or pale. Thus, lactose fermenters produce pink colonies.

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