Which of the following is the most common selective and differential medium used for the isolation of Vibrio cholerae in clinical microbiology laboratories?

Question

Accepted Answer

B. Thiosulfate-citrate-bile salts-sucrose (TCBS) agar. ## Selective and Differential Media for Vibrio cholerae

**Key Point:** TCBS agar is the gold standard selective and differential medium for isolation of Vibrio cholerae from clinical specimens, particularly stool samples.

### Why TCBS Agar is Optimal

**High-Yield:** TCBS agar contains:
- **Thiosulfate and citrate** — inhibit gram-positive bacteria and coliforms
- **Bile salts** — inhibit gram-positive bacteria
- **Sucrose** — fermented by V. cholerae, producing yellow colonies (differential feature)
- **Tellurium compounds** — reduce to black/gray, enhancing colony appearance

### Colony Characteristics on TCBS

| Feature | Description |
|---------|-------------|
| **Colony color** | Flat, yellow, opaque (sucrose-fermenting) |
| **Size** | 2–3 mm after 18–24 hours |
| **Appearance** | Smooth, mucoid |
| **Selectivity** | Inhibits most gram-positive bacteria and many coliforms |

**Clinical Pearl:** TCBS is particularly useful in endemic areas (like India) where rapid, presumptive identification of V. cholerae is critical for outbreak response and public health surveillance.

### Why Other Options Are Suboptimal

- **Tellurite agar** — Used primarily for Corynebacterium diphtheriae, not Vibrio species
- **Mannitol salt agar** — Selective for Staphylococcus aureus (high salt concentration inhibits Vibrio)
- **MacConkey agar** — Non-selective for Vibrio; allows growth of many coliforms and makes Vibrio difficult to identify

**Mnemonic:** **TCBS = The Cholera Best Selection** — remember it as the first-line medium when cholera is suspected.

Answer

A. MacConkey agar

Answer

C. Mannitol salt agar

Answer

D. Tellurite agar

Which of the following is the most common selective and differential medium used for the isolation of Vibrio cholerae in clinical microbiology laboratories?

Explanation

Selective and Differential Media for Vibrio cholerae

Why TCBS Agar is Optimal

Colony Characteristics on TCBS

Why Other Options Are Suboptimal

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Feature	Description
Colony color	Flat, yellow, opaque (sucrose-fermenting)
Size	2–3 mm after 18–24 hours
Appearance	Smooth, mucoid
Selectivity	Inhibits most gram-positive bacteria and many coliforms