Stool culture from a 5-year-old with acute bloody diarrhoea grows a lactose non-fermenting organism on MacConkey agar forming colourless colonies. It is non-motile, does not produce H2S, and does not decarboxylate lysine. It ferments mannitol. Which Shigella species is most likely?

• A Shigella dysenteriae type 1
• B Shigella flexneri
• C Shigella boydii
• D Shigella sonnei ✓

Explanation

Shigella sonnei ferments lactose late (late lactose fermenter) and also ferments mannitol; it is the only Shigella species that produces beta-galactosidase (ONPG positive). All Shigella species share the property of being non-motile, H2S-negative, and lysine decarboxylase-negative (distinguishing them from Salmonella). S. dysenteriae type 1 (Shiga's bacillus) does NOT ferment mannitol, which is the key biochemical test distinguishing it from other Shigella. S. sonnei is increasingly common in developed countries and typically causes milder disease than S. dysenteriae.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.