A 32-year-old man from Kolkata presents to the emergency department with acute onset of profuse, watery diarrhea and vomiting for 6 hours. He reports that the stool is colorless, odorless, and resembles "rice water." He is severely dehydrated with a blood pressure of 80/50 mmHg, heart rate 128/min, and poor skin turgor. Stool culture on TCBS agar shows oxidase-positive, comma-shaped gram-negative rods. Which toxin is primarily responsible for the pathophysiology of his diarrhea?

Explanation

## Pathophysiology of Cholera Diarrhea **Key Point:** Vibrio cholerae produces cholera toxin (CTX), an AB~5~ enterotoxin that is the primary virulence factor responsible for the characteristic rice-water diarrhea. ### Mechanism of Cholera Toxin 1. **Structure and Action:** CTX is an AB~5~ toxin with an A subunit (catalytic) and five B subunits (binding). 2. **Binding:** B subunits bind to GM~1~ ganglioside receptors on the intestinal epithelial cell surface. 3. **ADP-ribosylation:** The A subunit catalyzes ADP-ribosylation of the α-subunit of the Gs (stimulatory G) protein. 4. **cAMP Elevation:** This permanently activates adenylyl cyclase, causing massive intracellular cAMP accumulation. 5. **Ion Secretion:** Elevated cAMP activates protein kinase A (PKA), which phosphorylates the cystic fibrosis transmembrane conductance regulator (CFTR), leading to: - Increased Cl^−^ secretion into the intestinal lumen - Decreased Na^+^ absorption - Osmotic water loss → profuse, watery diarrhea **High-Yield:** The rice-water stool is pathognomonic for cholera and results from massive secretory diarrhea (up to 1 L/hour in severe cases), not mucosal invasion or inflammation. ### Clinical Correlation | Feature | Cholera | Other Bacterial Diarrheas | |---------|---------|---------------------------| | Stool appearance | Rice-water (colorless, odorless) | Mucoid, bloody, or purulent | | Mechanism | Secretory (toxin-mediated) | Inflammatory or invasive | | Mucosal damage | Minimal/none | Present | | Blood in stool | Absent | Often present | **Clinical Pearl:** The absence of blood, mucus, and fecal leukocytes in cholera stool distinguishes it from inflammatory diarrheas caused by Shigella, Campylobacter, or invasive E. coli. **Mnemonic:** **CTXGS** — Cholera Toxin → Gs protein → cAMP ↑ → Secretory diarrhea. [cite:Jawetz, Melnick & Adelberg's Medical Microbiology 28e Ch 19]