Stool from a patient with suspected enteric fever is inoculated into selenite F broth and incubated for 12 hours, after which it is subcultured onto deoxycholate citrate agar. What is the purpose of the selenite F step?
- A It enriches Salmonella and Shigella by suppressing normal faecal coliforms ✓
- B It directly identifies Salmonella by biochemical reactions in the broth
- C It lyses red blood cells so that Salmonella can be seen microscopically
- D It neutralises antibiotics present in the stool specimen
Correct answer: A. It enriches Salmonella and Shigella by suppressing normal faecal coliforms
Explanation
Selenite F broth is an enrichment medium: sodium selenite inhibits the abundant commensal coliforms of stool while allowing Salmonella (and to some extent Shigella) to multiply, so that when the broth is subcultured after 6 to 18 hours the pathogen is present in relative excess. Tetrathionate broth serves the same purpose for salmonellae. Selenite F performs no identification, has no haemolytic action, and plays no role in neutralising drugs, which is why subculture on a differential medium still follows.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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