Shigella isolation media: Why is Xylose Lysine Deoxycholate (XLD) agar generally preferred over Deoxycholate Citrate Agar (DCA) for recovering Shigella (especially S. dysenteriae and S. flexneri)?

Introduction / Context:
Selective media are used to isolate enteric pathogens from stool by suppressing normal flora while allowing target organisms to grow. Choosing the right medium for Shigella is essential because some formulations over-inhibit certain Shigella species, causing false-negative cultures.

Given Data / Assumptions:

• Comparison is between XLD agar and DCA.
• Target organisms: Shigella dysenteriae and Shigella flexneri.
• Goal is successful recovery (growth with readable colony morphology).

Concept / Approach:
XLD is selective and differential, using xylose fermentation, lysine decarboxylation, and hydrogen sulfide indicators to distinguish pathogens. Importantly, XLD is formulated to be less inhibitory to Shigella than many older deoxycholate-based media. DCA can excessively suppress some Shigella strains, reducing recovery from mixed fecal flora. Therefore, XLD often yields higher isolation rates for Shigella while still suppressing much of the competing microbiota.

Step-by-Step Solution:
Define the isolation challenge: recover Shigella from heavily contaminated stool. Compare inhibitory profiles: XLD is milder toward Shigella than DCA. Note that XLD remains selective and provides differential indicators (H2S, fermentation). Select the reason emphasizing lower inhibition of Shigella by XLD.

Verification / Alternative check:
Laboratory recovery studies and many diagnostic algorithms recommend XLD or Hektoen enteric agar for improved Shigella isolation versus older DCA formulations.

Why Other Options Are Wrong:

• High salt concentration: Not the key property of XLD.
• Specifically kills Salmonella: False; XLD also recovers Salmonella with characteristic H2S-positive colonies.
• General statements (selective/differential): True but not the specific reason for superiority over DCA.

Common Pitfalls:
Overlooking the balance between selectivity and recovery; excessive inhibition may eliminate target pathogens along with commensals.

Final Answer:
Because XLD is less inhibitory to S. dysenteriae and S. flexneri than DCA.