Question

Which one of the following statements is correct about various microbial culture media?

• A. Mannitol salt agar is an enriched and differential media.
• B. Selective components in MacConkey (MAC) agar are eosin Y and methylene blue which inhibits the growth of gram positive bacteria.
• C. Blood agar is a differential media which is differentiated on the basis of bacterial ability to produce hemolysins.
• D. Bile salts and crystal violet present in the EMB agar media inhibits gram positive bacteria growth and hence helps to differentiate between gram positive and gram negative.

Hint:

Remember: Blood agar is differential, not selective. MacConkey and Mannitol salt agar are both selective and differential, but not enriched.

Answer 1

The Correct Option is C

Step 1: Analyze the function of each culture medium.
Mannitol salt agar is selective and differential, not enriched. It selects for staphylococci and differentiates based on mannitol fermentation.
MacConkey (MAC) agar contains bile salts and crystal violet—not eosin Y or methylene blue. It inhibits gram-positive bacteria and differentiates lactose fermenters.
Blood agar is a differential medium that differentiates bacteria based on their hemolytic properties (alpha, beta, gamma hemolysis) due to production of hemolysins.
EMB agar (Eosin Methylene Blue agar) does contain eosin Y and methylene blue—not bile salts or crystal violet. 
Step 2: Conclude the correct statement. 
Only statement (3) correctly describes blood agar as a differential medium based on hemolysin production.