Shigella biochemistry — Which Shigella species is typically negative for fermentation of mannitol, helping in laboratory differentiation?

Introduction / Context:
Shigella species are biochemically similar but can be differentiated by certain sugar fermentations. Mannitol fermentation is a classic discriminator in clinical laboratories for presumptive species-level identification.

Given Data / Assumptions:

• Shigella genus includes S. dysenteriae, S. flexneri, S. boydii, and S. sonnei.
• Most Shigella ferment mannitol; one species is classically mannitol-negative.
• We assume standard interpretations on differential media.

Concept / Approach:
S. dysenteriae is typically mannitol non-fermenting, while S. flexneri, S. boydii, and S. sonnei generally ferment mannitol. This property, along with other biochemical and serologic differences, aids rapid differentiation pending serotyping.

Step-by-Step Solution:

Verification / Alternative check:
On appropriate media, mannitol fermentation results in indicator color change for most species except S. dysenteriae; serogrouping confirms identity.

Why Other Options Are Wrong:

• S. flexneri, S. boydii, S. sonnei: generally mannitol-positive, not matching the question’s criterion.

Common Pitfalls:
Relying solely on a single biochemical test; occasional strain variation exists, so confirm with serology or molecular assays.

Final Answer:
S dysenteriae