MacConkey agar plates represent which type of medium?

• A. Enriched
• B. Simple
• C. Selective and differential
• D. Reducing

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the purpose of MacConkey agar. It is designed to isolate and differentiate gram-negative bacteria, especially members of the Enterobacteriaceae family.
2. Step 2: Recognize that a selective medium contains components that inhibit the growth of certain microorganisms while allowing others to grow. MacConkey agar contains bile salts and crystal violet, which inhibit gram-positive bacteria, making it selective for gram-negative bacteria.
3. Step 3: Understand that a differential medium contains indicators that allow differentiation between types of bacteria based on their biological characteristics. MacConkey agar contains lactose and a pH indicator (neutral red) to differentiate lactose fermenters (which produce acid and turn the medium pink/red) from non-fermenters (which do not change the color).
4. Step 4: Compare MacConkey agar to other types of media: enriched media provide extra nutrients for fastidious organisms, simple media support general growth without selective or differential properties, and reducing media are used to grow anaerobes by removing oxygen.
5. Step 5: Conclude that MacConkey agar is both selective (for gram-negative bacteria) and differential (based on lactose fermentation), which matches the correct classification.