Question

The culture media that is used to differentiate lactose fermenters and non-lactose fermenters is

• A. Bile Salt Sugar
• B. Thayer-Martin agar
• C. Sabouraud Dextrose Agar
• D. Mac-Conkey agar

Hint:

Mac-Conkey agar is used to differentiate lactose fermenters (pink colonies) from non-fermenters (colorless colonies), and it is particularly useful for isolating gram-negative enteric bacteria.

Answer 1

The Correct Option is D

Mac-Conkey agar is a selective and differential culture medium used to isolate gram-negative bacteria and differentiate lactose fermenters from non-lactose fermenters. The key feature of Mac-Conkey agar is the inclusion of lactose and a pH indicator (neutral red). 
Lactose fermenters, such as Escherichia coli, produce acid as a byproduct of lactose fermentation. This lowers the pH of the medium, causing the colonies to appear pink or red. 
Non-lactose fermenters, such as Salmonella or Shigella, do not ferment lactose and form colonies that are colorless or pale, due to the absence of acid production.

Thus, Mac-Conkey agar is ideal for differentiating between these two groups of organisms.