A stool culture from a patient with bloody diarrhoea grows small, non-lactose-fermenting, H₂S-negative colonies on MacConkey and Salmonella-Shigella (SS) agar. Triple sugar iron (TSI) agar shows alkaline slant/acid butt with NO gas and NO H₂S. Motility test is negative. Serotyping reveals Group D antigen. Which organism does this most likely represent?
- A Shigella sonnei (Group D Shigella) ✓
- B Salmonella Typhi
- C Shigella dysenteriae type 1 (Shiga bacillus)
- D Campylobacter jejuni
Explanation
Shigella species are non-motile, non-lactose fermenters, and H₂S-negative on TSI (alkaline slant/acid butt without gas in most species — S. flexneri may produce trace gas). The Lancefield grouping of Shigella follows Kauffmann-White grouping: Group A = S. dysenteriae; Group B = S. flexneri; Group C = S. boydii; Group D = S. sonnei. S. dysenteriae produces Shiga toxin and causes the most severe disease, but it is Group A serotypically. S. sonnei is Group D, is the mildest species, and notably is the only Shigella with a ONPG-positive (beta-galactosidase) reaction and late lactose fermentation after 24 hours. Salmonella Typhi produces H₂S and is motile. Campylobacter is a curved gram-negative rod requiring microaerophilic culture.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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