On MacConkey agar, lactose-fermenting Enterobacteriaceae typically display which result?

• A. Pink colonies
• B. Colorless colonies
• C. Green colonies
• D. Black-centered colonies

Answer: (A)

Lactose fermentation on MacConkey agar lowers the local pH, and the neutral red indicator in the medium turns pink in acidic conditions. Enterobacteriaceae that ferment lactose produce enough acid to make their colonies appear pink or red, often with a pink halo. This differential color change is what identifies them as lactose fermenters on this medium.

Colorless colonies would indicate non‑lactose fermenters, since they don’t acidify the medium enough to change the indicator color. Green colonies are not the typical appearance on MacConkey for these bacteria; that’s more characteristic of other media like certain eosin methylene blue tests. Black-centered colonies would suggest hydrogen sulfide production on media that show sulfide reactions, which MacConkey does not typically display.