A patient from Kerala returns with a 4-day history of bloody diarrhoea, tenesmus, and fever. Stool microscopy shows RBCs, pus cells but no parasites. Stool culture on deoxycholate citrate agar (DCA) grows non-motile, lactose non-fermenting, H2S-negative colonies. Biochemical testing reveals no gas from glucose and no H2S production. The organism is:
- A Shigella dysenteriae ✓
- B Salmonella Typhi
- C Enteroinvasive E. coli (EIEC)
- D Campylobacter jejuni
Explanation
Shigella spp. are non-motile, non-lactose-fermenting, H2S-negative, and do NOT produce gas from glucose fermentation (an absolute distinguishing feature from Salmonella). S. dysenteriae is the most virulent Shigella species and produces Shiga toxin. Salmonella Typhi is non-lactose-fermenting but motile (H flagella), H2S positive (variable), and produces acid but no gas in most cases. EIEC mimics Shigella clinically but biochemically it IS E. coli (ferments lactose, produces gas). Campylobacter grows on Skirrow medium at 42°C under microaerophilic conditions, not on DCA. DCA is a selective medium that supports Salmonella and Shigella growth.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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Written and medically reviewed by the StethoPrep medical team.