In a clinical microbiology laboratory, which is the most common enrichment medium used for the isolation of Salmonella from fecal samples in suspected enteric fever cases?

Explanation

## Enrichment Media for Salmonella from Fecal Samples **Key Point:** Selenite F broth is the **most commonly used** enrichment medium for the isolation of Salmonella (and Shigella) from fecal specimens in routine clinical microbiology practice, including suspected enteric fever cases. ### Mechanism of Selectivity 1. **Sodium Selenite**: Inhibits gram-positive organisms and most coliforms (Enterobacteriaceae), allowing preferential growth of Salmonella 2. **pH ~7.0**: Optimal for Salmonella multiplication 3. **Incubation**: 12–18 hours at 35–37°C; prolonged incubation reduces selectivity 4. **Subculture**: Onto selective/differential media such as XLD agar, DCA, or SS agar ### Comparison of Enrichment Media for Salmonella | Enrichment Medium | Primary Use | Selectivity | Clinical Application | |-------------------|------------|-------------|----------------------| | **Selenite F broth** | Salmonella & Shigella from feces | High (inhibits coliforms & gram-positive) | **Most common in routine practice** | | Tetrathionate broth | Salmonella from feces | High (inhibits coliforms & gram-positive) | Used more for environmental/food samples; less preferred for Shigella | | Enriched thioglycollate | Anaerobes & fastidious organisms | Low (non-selective) | Not suitable for Salmonella | | BHI broth | General enrichment | Non-selective | Not suitable for selective isolation | **High-Yield (Mackie & McCartney / Ananthanarayan & Paniker):** Selenite F broth is the standard enrichment medium recommended for isolation of Salmonella and Shigella from fecal samples in clinical laboratories. Tetrathionate broth is an alternative but is more commonly referenced in food/environmental microbiology and is inhibitory to Shigella. **Clinical Pearl:** In India, where enteric fever (typhoid) is endemic, fecal culture using Selenite F enrichment followed by plating on XLD or DCA agar is the standard protocol for case confirmation and carrier detection (Ananthanarayan & Paniker's Textbook of Microbiology, 10th ed.). **Mnemonic:** **Selenite F = Salmonella & Shigella from Feces** ### Recommended Protocol 1. Inoculate fecal sample into Selenite F broth (1:10 dilution) 2. Incubate 12–18 hours at 37°C 3. Subculture onto XLD agar or DCA 4. Identify black/pink colonies as presumptive Salmonella 5. Confirm with biochemical tests and serology (Widal / Vi agglutination)