Stool from a 4-year-old with bloody diarrhoea is plated on XLD agar. After overnight incubation there are pink colonies with black centres. Which organism is identified?
- A Shigella dysenteriae
- B Salmonella typhimurium ✓
- C Escherichia coli O157:H7
- D Campylobacter jejuni
Correct answer: B. Salmonella typhimurium
Explanation
On xylose lysine deoxycholate agar, Salmonella produces red-pink colonies with black centres due to hydrogen sulphide production from thiosulphate reduction. Shigella forms pink colonies without black centres because it does not produce H2S, E. coli O157 ferments sorbitol slowly and appears as pale colonies, and Campylobacter requires selective Skirrow media with microaerophilic incubation at 42°C.
Reference: Mackie and McCartney Practical Medical Microbiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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