Which characteristic of MacConkey agar makes it both selective and differential in microbiological studies?

• A. It supports the growth of all bacteria and differentiates based on glucose fermentation.
• B. It inhibits the growth of Gram-positive bacteria and distinguishes lactose fermenters from non-fermenters.
• C. It contains blood to detect hemolysis and selects for spore-forming bacteria.
• D. It is used exclusively for fungal isolation and differentiates based on colony morphology.

Answer: (B)

Verified step by step guidance

1. Understand that MacConkey agar is designed to be both selective and differential, meaning it allows certain bacteria to grow while inhibiting others, and it also differentiates bacteria based on a specific metabolic property.
2. Identify the selective component: MacConkey agar contains bile salts and crystal violet, which inhibit the growth of Gram-positive bacteria, thus selecting for Gram-negative bacteria.
3. Identify the differential component: MacConkey agar contains lactose and a pH indicator (neutral red). Bacteria that ferment lactose produce acid, lowering the pH and causing the colonies to turn pink/red, while non-lactose fermenters produce colorless or pale colonies.
4. Combine these two characteristics to understand that MacConkey agar selects for Gram-negative bacteria and differentiates them based on their ability to ferment lactose.
5. Use this knowledge to evaluate the answer choices and select the one that correctly describes both the selective and differential properties of MacConkey agar.