Pasteurella multocida grows on sheep blood agar but not on MacConkey agar.

Pasteurella multocida grows on sheep blood agar but not on MacConkey agar.

• A. It grows on sheep blood agar but not on MacConkey agar.
• B. It grows on MacConkey agar but not on sheep blood agar.
• C. It grows on both sheep blood and MacConkey agar.
• D. It grows on neither sheep blood nor MacConkey agar.

Answer: (A)

The key idea is how different culture media select and differentiate bacteria. Sheep blood agar is an enriched, non-selective medium that supports the growth of many organisms, including Pasteurella multocida, which forms small, smooth colonies under these conditions. MacConkey agar, on the other hand, is selective for Gram-negative enteric bacteria and differentiates lactose fermenters from non-fermenters by a color change; it contains bile salts and crystal violet that inhibit Gram-positive bacteria and many fastidious species, and it favors organisms that can handle those conditions and/or lactose metabolism.

Pasteurella multocida is a Gram-negative coccobacillus that grows well on enriched blood-containing media but is not a lactose fermenter and often does not grow well on MacConkey agar due to its fastidious growth requirements and the selective components of MacConkey. So you’d observe growth on sheep blood agar but little to no growth on MacConkey agar, which is exactly what the statement describes.