Pasteurella species typically does not grow on MacConkey agar.

Pasteurella species typically does not grow on MacConkey agar.

• A. It does not grow on MacConkey agar
• B. It grows as lactose fermenter on MacConkey
• C. It grows only anaerobically
• D. It grows on MacConkey agar under high salt

Answer: (A)

MacConkey agar is used to isolate Gram-negative enteric bacteria and to differentiate lactose fermenters from nonfermenters. Pasteurella species are Gram-negative coccobacilli that are non-enteric and do not ferment lactose. The medium’s bile salts and crystal violet inhibit many non-enteric organisms, and Pasteurella doesn’t thrive on this selective/differential environment. Therefore, it typically does not form colonies on MacConkey agar, which is why the statement is correct. If it did grow, it would not be showing lactose fermentation characteristics, unlike true lactose fermenters which would appear pink due to lactose metabolism. The other options describe growth or requirements that don’t match Pasteurella’s usual behavior on this medium.

MacConkey agar is used to isolate Gram-negative enteric bacteria and to differentiate lactose fermenters from nonfermenters. Pasteurella species are Gram-negative coccobacilli that are non-enteric and do not ferment lactose. The medium’s bile salts and crystal violet inhibit many non-enteric organisms, and Pasteurella doesn’t thrive on this selective/differential environment. Therefore, it typically does not form colonies on MacConkey agar, which is why the statement is correct. If it did grow, it would not be showing lactose fermentation characteristics, unlike true lactose fermenters which would appear pink due to lactose metabolism. The other options describe growth or requirements that don’t match Pasteurella’s usual behavior on this medium.