Vibrio cholerae requires which specific growth factor or condition for optimal isolation and culture on selective media?

Explanation

## Vibrio cholerae: Culture Requirements and Selective Media **Key Point:** Vibrio cholerae is a halophilic (salt-loving), alkaliphilic bacterium with unique growth requirements that are exploited for selective isolation. ### Optimal Growth Conditions | Parameter | Requirement | Rationale | |-----------|-------------|----------| | **pH** | 8.5–9.0 (alkaline) | Most Gram-negative pathogens prefer neutral pH; alkalinity suppresses competing flora | | **Salt concentration** | 1–3% NaCl (high) | V. cholerae is halophilic; high salt inhibits most other bacteria | | **Temperature** | 35–37°C | Optimal for pathogenic strains | | **Oxygen** | Aerobic | Facultative anaerobe but grows better aerobically | | **Atmosphere** | Aerobic with 5–10% CO~2~ (optional) | Enhanced growth with CO~2~ | ### Selective Media for Isolation **High-Yield:** The combination of **alkaline pH + high salt** is the basis for selective media used in cholera diagnosis: 1. **Thiosulfate-Citrate-Bile Salts-Sucrose (TCBS) agar** - pH 8.5 (alkaline) - Contains bile salts (inhibit Gram-positive bacteria) - Selective for Vibrio species - V. cholerae forms **yellow colonies** (sucrose fermentation) 2. **Alkaline Peptone Water (APW)** - pH 8.5 - Enrichment medium (not selective) - Used as an enrichment step before plating on TCBS - Incubate 6–8 hours; V. cholerae floats to the surface (motile) 3. **Monsur's medium** - pH 8.5 - Contains tellurite (produces dark colonies with V. cholerae) - Highly selective for V. cholerae **Clinical Pearl:** In field settings or resource-limited laboratories, **Alkaline Peptone Water (APW) enrichment followed by TCBS plating** is the gold standard for cholera diagnosis. The alkaline pH is the critical selective factor. **Mnemonic:** **TCBS = Thio-Citrate-Bile-Sucrose** — remember it's **alkaline** and **salty**, and V. cholerae turns **yellow** on it.