Question

The specific medium for isolation of Shigella spp. is

• A. Cefsulodin-Irgasan-Novobiocin (CIN) agar
• B. De Man, Rogosa and Sharpe agar
• C. Desoxycholate citrate agar
• D. Cycloheximide and amphotericin B agar

Hint:

Non-lactose fermenting pathogens like Shigella form pale, colorless colonies on selective media containing lactose and neutral red, such as DCA and MacConkey agar.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Isolating specific enteric pathogens from food or dairy samples requires selective and differential media.
These media suppress background microflora while allowing the target genus to grow.

Step 2: Detailed Explanation:
Desoxycholate Citrate Agar (DCA) is a selective and differential medium formulated for the isolation of Salmonella and Shigella species.
The formulation contains sodium desoxycholate and sodium citrate, which inhibit Gram-positive bacteria and restrict the growth of most coliforms.
Lactose is included as the fermentable carbohydrate, with neutral red serving as the pH indicator.
Because Shigella species are non-lactose fermenters, they do not produce acid and grow as pale, translucent, colorless colonies.
This differentiates them from lactose-fermenting coliforms, which produce pink-red colonies.
Reviewing the other options:
Cefsulodin-Irgasan-Novobiocin (CIN) agar is a highly selective medium designed specifically for the isolation of Yersinia enterocolitica.
De Man, Rogosa, and Sharpe (MRS) agar is an enriched, non-selective medium used to cultivate lactic acid bacteria, especially Lactobacillus spp.
Cycloheximide and amphotericin B agar are selective media containing antifungal agents, used to suppress yeasts and molds during bacterial isolation.

Step 3: Final Answer:
Therefore, Desoxycholate Citrate Agar is the specific medium for the isolation of Shigella spp.