## Vibrio cholerae Virulence Mechanisms ### Correct Statements (Options 0, 1, 2) **Key Point:** Cholera toxin is an A-B enterotoxin (Option 0) that enters intestinal epithelial cells and ADP-ribosylates the α-subunit of the stimulatory G protein (Gs), locking it in the GTP-bound active state. This constitutively activates adenylate cyclase, raising intracellular cAMP and triggering secretory diarrhea. **High-Yield:** The toxin-coregulated pilus (TCP, also called Vibrio pathogenicity island 1 or VPI-1) is the primary adhesin for colonization. Its expression is tightly regulated by the ToxR/ToxS two-component regulatory system (Option 1). Without TCP, the organism cannot establish infection. **Clinical Pearl:** Vibrio cholerae secretes mucinase and other proteases that degrade the mucus layer (Option 2), reducing the physical barrier and allowing direct contact with epithelial cells for adhesion and toxin delivery. ### Why Option 3 is Incorrect **Warning:** Cholera toxin does NOT directly damage tight junctions or cause protein leakage through epithelial damage. Instead, it causes **secretory diarrhea** by: 1. Increasing cAMP in enterocytes 2. Opening crypt cell chloride channels 3. Driving net fluid secretion into the intestinal lumen 4. Leaving the epithelium structurally intact (hence "rice-water stools" are isotonic, not hemorrhagic) The diarrhea is purely secretory, not exudative. Histology shows normal or mildly inflamed mucosa with no epithelial loss or tight junction disruption. ### Summary Table | Virulence Factor | Mechanism | Effect | |---|---|---| | Cholera Toxin (CT) | A-B toxin; ADP-ribosylates Gs protein | ↑ cAMP → secretory diarrhea | | TCP | Adhesin; ToxR-regulated | Colonization & biofilm | | Mucinase | Proteolytic enzyme | Mucus degradation | | Tight Junction Damage | **NOT a mechanism of CT** | **Not applicable** | [cite:Jawetz, Melnick & Adelberg's Medical Microbiology Ch 24]
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