In a microbiology laboratory, which is the most common enrichment broth used for the primary isolation of Salmonella species from clinical specimens?

Question

Accepted Answer

B. Selenite F broth. ## Enrichment Media for Salmonella Isolation

**Key Point:** Selenite F broth is the most widely used enrichment medium for Salmonella species in clinical microbiology, particularly for stool and urine samples.

### Selenite F Broth: Composition and Function

**High-Yield:** Selenite F broth contains:
- **Sodium metabisulfite** — selective agent that inhibits most coliforms and gram-positive bacteria
- **Lactose** — fermented by many coliforms but NOT by Salmonella (differential feature)
- **Peptone and beef extract** — nutritive components

### Mechanism of Selectivity

1. Sodium metabisulfite inhibits:
   - Most Escherichia coli strains
   - Proteus species (partially)
   - Gram-positive bacteria
   - Enterococcus species

2. Lactose non-fermentation by Salmonella allows preferential growth

3. Incubation at 35–37°C for 18–24 hours enriches Salmonella while suppressing competitors

**Clinical Pearl:** Selenite F is particularly valuable in endemic areas (including India) where typhoid and non-typhoidal salmonellosis are common. A positive enrichment culture is then subcultured onto selective-differential agar (e.g., HE agar, XLD agar) for identification.

### Comparison of Enrichment Media

| Medium | Primary Use | Selectivity | Notes |
|--------|-------------|-------------|-------|
| **Selenite F broth** | Salmonella enrichment | High (metabisulfite) | Gold standard for stool/urine |
| **Peptone water** | General enrichment | None (non-selective) | Used for vibrios, not Salmonella |
| **Brain-heart infusion (BHI)** | General enrichment | None | Supports most bacteria; not selective |
| **Nutrient broth** | General growth | None | Non-selective; poor for Salmonella |

**Mnemonic:** **Selenite = Selective for Salmonella** — the sulfite ion suppresses coliforms while Salmonella thrives.

Answer

A. Nutrient broth

Answer

C. Brain-heart infusion broth

Answer

D. Peptone water

In a microbiology laboratory, which is the most common enrichment broth used for the primary isolation of Salmonella species from clinical specimens?

Explanation

Enrichment Media for Salmonella Isolation

Selenite F Broth: Composition and Function

Mechanism of Selectivity

Comparison of Enrichment Media

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Medium	Primary Use	Selectivity	Notes
Selenite F broth	Salmonella enrichment	High (metabisulfite)	Gold standard for stool/urine
Peptone water	General enrichment	None (non-selective)	Used for vibrios, not Salmonella
Brain-heart infusion (BHI)	General enrichment	None	Supports most bacteria; not selective
Nutrient broth	General growth	None	Non-selective; poor for Salmonella