During an outbreak investigation in a coastal village in West Bengal, two Vibrio species are isolated from stool samples. One isolate is from a patient with profuse watery diarrhea and severe dehydration; the other from a patient with acute dysentery and fever. Which feature best discriminates the isolate from the watery diarrhea case as V. cholerae rather than V. parahaemolyticus?

Explanation

## Biochemical & Phenotypic Discrimination of V. cholerae **Key Point:** The string test (positive in V. cholerae, negative in V. parahaemolyticus) combined with Voges-Proskauer positivity is a classic laboratory discriminator that identifies V. cholerae with high specificity. **High-Yield:** The string test is a rapid, inexpensive screening test used in resource-limited settings (common in India) to presumptively identify V. cholerae from stool isolates. ### String Test Mechanism When a V. cholerae colony is emulsified in 0.5% sodium deoxycholate solution, the bacterial cell wall is lysed, releasing viscous capsular material that creates a characteristic mucoid "string" when a loop is drawn through the suspension. This is due to the hyaluronic acid capsule present in V. cholerae but absent in V. parahaemolyticus. ### Biochemical Comparison Table | Test | V. cholerae | V. parahaemolyticus | | --- | --- | --- | | **String test** | **Positive (mucoid string)** | **Negative** | | **Voges-Proskauer** | **Positive** | **Negative** | | **Methyl Red** | Negative | Positive | | **Glucose fermentation** | Positive (acid) | Positive (acid) | | **Lactose fermentation** | Positive | Negative | | **Sucrose fermentation** | Positive | Negative | | **Hemolysis** | Non-hemolytic | β-hemolytic | | **Motility** | Motile (polar flagellum) | Motile (polar flagellum) | **Clinical Pearl:** The combination of watery diarrhea (not dysentery) + string test positive + VP positive is virtually diagnostic of V. cholerae in an endemic area. V. parahaemolyticus typically causes bloody diarrhea (dysentery) and is string test negative. **Mnemonic:** **String + VP = Cholerae** — if both tests are positive, V. cholerae is the answer. ### Why This Combination Is Tested 1. **Rapid identification:** String test takes <2 minutes; no need to wait for culture confirmation 2. **High specificity:** VP positive + string positive is >95% specific for V. cholerae 3. **Practical in India:** These tests are performed in district and primary health centers without advanced equipment 4. **Epidemiological relevance:** Rapid identification allows public health response in endemic areas [cite:Jawetz, Melnick & Adelberg's Medical Microbiology 28e Ch 22; Park 26e Ch 5]