MacConkey agar plates represent which type of microbiological medium?

• A. An enriched medium
• B. A selective and differential medium
• C. A transport medium
• D. A general-purpose medium

Answer: (B)

Verified step by step guidance

1. Understand the purpose of MacConkey agar in microbiology: it is designed to isolate and differentiate certain types of bacteria, particularly Gram-negative enteric bacteria.
2. Recall that a selective medium contains components that inhibit the growth of some microorganisms while allowing others to grow. MacConkey agar contains bile salts and crystal violet, which inhibit Gram-positive bacteria.
3. Recognize that a differential medium contains indicators that allow differentiation between microorganisms based on their biological characteristics. MacConkey agar contains lactose and a pH indicator that differentiates lactose fermenters (which produce acid and change the color of the medium) from non-fermenters.
4. Compare the options: enriched media provide extra nutrients for fastidious organisms, transport media preserve specimens during transport, and general-purpose media support the growth of a wide variety of organisms without selectivity or differentiation.
5. Conclude that MacConkey agar is both selective (inhibits Gram-positive bacteria) and differential (distinguishes lactose fermenters from non-fermenters), making it a selective and differential medium.