Question

The selective medium used for the isolation of Bacillus cereus from milk and other foods is

• A. Eosine Methylene blue agar
• B. Mannitol Egg Yolk Polymyxin agar
• C. MRS agar
• D. Neomycin blood agar

Hint:

On MYP agar, Bacillus cereus colonies appear pink-red with a surrounding zone of precipitation, indicating that the organism is mannitol-negative and lecithinase-positive.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Isolating specific spore-forming pathogens like Bacillus cereus from milk requires selective media that inhibit background microflora while providing differential indicators.

Step 2: Detailed Explanation:
Mannitol Egg Yolk Polymyxin (MYP) agar is the standard selective and differential medium for Bacillus cereus.
The selective agent, Polymyxin B, inhibits Gram-negative bacteria while allowing Bacillus cereus to grow.
The medium differentiates colonies using two reactions:
Mannitol fermentation: Bacillus cereus does not ferment mannitol, forming pink-red colonies, while mannitol-fermenting organisms form yellow colonies.
Lecithinase activity: Bacillus cereus produces lecithinase, which hydrolyzes lecithin in the egg yolk. This forms a white precipitate zone around the pink-red colonies.
Other options:
Eosine Methylene Blue (EMB) agar selects for Gram-negative enteric rods.
MRS agar is used to cultivate lactic acid bacteria.
Neomycin blood agar selects for anaerobic pyogenic cocci.

Step 3: Final Answer:
Thus, Mannitol Egg Yolk Polymyxin agar is the selective medium for isolating Bacillus cereus.