Shigella culture media: Which of the following selective/differential media are routinely used for isolating Shigella from stool specimens?

Introduction / Context:
Recovering Shigella from fecal samples requires selective and differential media that suppress normal flora while highlighting pathogen-specific reactions. Multiple media are commonly employed in parallel to maximize diagnostic yield.

Given Data / Assumptions:

• Candidate media include HE, SS, and XLD agar.
• Objective is primary isolation and presumptive differentiation.
• Laboratory practice often uses more than one medium per specimen.

Concept / Approach:
HE agar incorporates bile salts, bromothymol blue, and indicators to differentiate Shigella (typically green to blue-green colonies without H2S) from Salmonella (often black-centered due to H2S). SS agar is highly selective; Shigella forms colorless colonies, whereas Salmonella often produces H2S (black centers). XLD agar is less inhibitory to Shigella and differentiates based on xylose fermentation, lysine decarboxylation, and H2S production. Using these media together increases the likelihood of recovering Shigella while distinguishing from other enterics.

Step-by-Step Solution:
List standard selective media for enteric pathogens. Associate Shigella colony morphology on each medium. Recognize complementary selectivity and differential systems across the three media. Select the inclusive option indicating all are used.

Verification / Alternative check:
Diagnostic algorithms and manuals recommend plating on multiple enteric selective agars, including HE, SS, and XLD, for optimal recovery of Shigella and Salmonella.

Why Other Options Are Wrong:

• Single-medium choices limit recovery; laboratories typically use several media.
• MacConkey only: Useful but not as selective for Shigella as HE/SS/XLD.

Common Pitfalls:
Relying on one medium or misinterpreting H2S production; Shigella are typically H2S-negative across these media.

Final Answer:
All of these.