Which bacteria are lactose fermenters on MacConkey agar?

• A. Staphylococcus aureus
• B. Streptococcus pneumoniae
• C. Pseudomonas aeruginosa
• D. Escherichia coli

Answer: (D)

MacConkey agar is selective for Gram-negative bacteria and differential for lactose fermentation. It uses bile salts to inhibit Gram-positives and a pH indicator that turns pink when lactose is fermented, signaling acid production. Escherichia coli reliably ferments lactose, so it lowers the pH and forms pink (or red) colonies on MacConkey. The other organisms listed are either Gram-positive and inhibited by the medium (so they don’t grow well) or are non-lactose fermenters among Gram-negatives (producing colorless or pale colonies). Therefore, this organism is the lactose fermenter on MacConkey agar.