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 **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.