A 32-year-old man from Kolkata presents to the emergency department with acute-onset watery diarrhea for 6 hours. He reports 8–10 bowel movements since morning, all non-bloody. Vital signs: BP 88/54 mmHg, HR 118/min, RR 24/min, temperature 37.2°C. Examination reveals severe dehydration, sunken eyes, and loss of skin turgor. Stool microscopy shows no RBCs or WBCs. Stool culture on thiosulfate-citrate-bile-salt-sucrose (TCBS) agar shows oxidase-positive, Gram-negative curved rods. What is the most likely causative organism?

Explanation

## Identification of Vibrio cholerae ### Clinical Presentation **Key Point:** The classic presentation of cholera is acute-onset, painless, watery diarrhea ("rice-water stools") with rapid dehydration and electrolyte loss in a patient from an endemic area (Kolkata, India). ### Microbiological Features **High-Yield:** Vibrio cholerae is identified by: 1. **Morphology:** Gram-negative, curved (comma-shaped) rods 2. **Oxidase test:** Positive (purple color within 10 seconds) 3. **Culture medium:** Grows on TCBS agar as flat, mucoid, translucent colonies 4. **Stool findings:** Absence of RBCs/WBCs (non-inflammatory diarrhea) ### Pathophysiology **Clinical Pearl:** V. cholerae produces cholera toxin (CTX), an A–B enterotoxin that: - Activates adenylate cyclase via ADP-ribosylation of Gs protein - Increases intracellular cAMP → secretory diarrhea - Results in massive fluid loss (up to 1 L/hour) without mucosal inflammation ### Epidemiology **Key Point:** Cholera is endemic in the Ganges Delta (West Bengal, Bangladesh). Transmission is fecal-oral via contaminated water. The 7th pandemic (El Tor biotype) is currently circulating globally. ### Diagnostic Confirmation | Feature | V. cholerae | V. parahaemolyticus | E. coli (EIEC) | Shigella | |---------|------------|-------------------|----------------|----------| | **Morphology** | Curved rod | Curved rod | Straight rod | Straight rod | | **Oxidase** | Positive | Positive | Negative | Negative | | **TCBS growth** | Yes | Yes | No | No | | **Stool WBCs** | Absent | Present | Present | Present | | **Toxin** | Enterotoxin | Cytotoxin | Invasive | Invasive | | **Osmotic pressure** | Secretory | Osmotic | Osmotic | Osmotic | **Mnemonic: TCBS = Thiosulfate-Citrate-Bile-Sucrose** — selective medium for Vibrio species; V. cholerae ferments sucrose (yellow colonies), V. parahaemolyticus does not (blue-green colonies). ### Management Implications **Key Point:** Oral rehydration solution (ORS) is the mainstay of treatment; antibiotics (tetracycline, fluoroquinolones) reduce duration and stool output by ~50%.