A nonlactose fermenter is isolated from a stool specimen. A biochemical multitest assay identifies the isolate as Shigella sp. Subsequently, the isolate demonstrates agglutination in Shigella group B antisera. Which of the following is most likely?

• A. The isolate is Shigella sonnei.
• B. The isolate is Shigella flexneri.
• C. The isolate should be reported as Shigella species.
• D. Antisera test should be repeated.

Answer: (B)

The key idea is identifying Shigella species by its O antigen serogroup. Shigella is categorized into serogroups based on the surface O antigen, and each serogroup maps to a species. Group B corresponds to Shigella flexneri. When your isolate, already identified as Shigella by biochemical testing, agglutinates with Shigella group B antisera, this pins the organism to Shigella flexneri. If it had reacted with a different group antisera, you’d be looking at a different species (for example, group D would indicate Shigella sonnei, group A Shigella dysenteriae, group C Shigella boydii). Because the serogroup reaction is specific, naming Shigella flexneri is more precise than reporting just Shigella species.