Which statement correctly describes Escherichia coli?

• A. It ferments lactose and appears pink on MacConkey agar.
• B. It is a Gram-positive diplococcus.
• C. It is oxidase-positive and forms grape-like colonies.
• D. It is non-motile and lacks fimbriae.

Answer: (A)

MacConkey agar differentiates lactose fermenters among Gram-negative enteric bacteria by using a pH indicator that turns pink in the acidic conditions produced when lactose is fermented. Escherichia coli is a Gram-negative rod that rapidly ferments lactose, so its colonies appear pink on MacConkey agar. It is oxidase negative and typically motile with fimbriae, not non-motile and lacking fimbriae, and it is not a Gram-positive diplococcus or oxidase-positive. That combination of lactose fermentation with pink colonies on MacConkey is the best description.