Which characteristic makes blood agar a differential medium in microbiology?

• A. It selects for Gram-negative bacteria only.
• B. It contains antibiotics to inhibit non-pathogenic bacteria.
• C. It distinguishes bacteria based on their ability to hemolyze red blood cells.
• D. It provides a nutrient-rich environment for all bacteria to grow equally.

Answer: (C)

Verified step by step guidance

1. Understand the definition of a differential medium: it allows the growth of multiple types of bacteria but differentiates them based on specific biochemical characteristics.
2. Recall that blood agar contains red blood cells, which can be broken down (hemolyzed) by certain bacteria producing hemolysins.
3. Recognize that the ability to hemolyze red blood cells creates visible changes on the agar, such as clear zones (beta-hemolysis), greenish zones (alpha-hemolysis), or no change (gamma-hemolysis).
4. Note that blood agar does not select for Gram-negative bacteria exclusively, nor does it contain antibiotics to inhibit non-pathogenic bacteria, so it is not a selective medium.
5. Conclude that blood agar is differential because it distinguishes bacteria based on their hemolytic activity, which is the ability to break down red blood cells.