Which statement about MacConkey agar is true?

• A. It is selective for Gram-positive bacteria.
• B. It differentiates lactose non-fermenters as pink colonies.
• C. It differentiates lactose fermenters among Gram-negative bacteria showing pink colonies.
• D. It is used to test for hydrogen sulfide production.

Answer: (C)

MacConkey agar is a selective and differential medium used to distinguish lactose fermenters among Gram-negative enteric bacteria. The bile salts and crystal violet inhibit most Gram-positive organisms, so Gram-positives don’t grow well. The medium also contains lactose and the pH indicator neutral red; when lactose is fermented, acid is produced, lowering the pH and turning the colonies pink. Therefore, lactose-fermenting Gram-negative bacteria appear pink, while non-fermenters do not acidify the medium and remain colorless or pale. Hydrogen sulfide production is not detected on this medium, as that feature is assessed with other media. So the statement that lactose-fermenting Gram-negative bacteria form pink colonies on MacConkey agar is correct.