## Enterotoxigenic E. coli (ETEC) — Virulence Mechanisms ### Correct Statements (Options 0, 1, 2) **Key Point:** ETEC causes watery diarrhea via two main enterotoxins: | Toxin | Mechanism | Effect | |-------|-----------|--------| | **Heat-labile (LT)** | ADP-ribosylates Gs-α protein; activates adenylate cyclase | ↑ cAMP → ↑ Cl⁻ secretion | | **Heat-stable (ST)** | Directly activates guanylate cyclase (GC-C receptor) | ↑ cGMP → ↑ Cl⁻ secretion | **Clinical Pearl:** Both toxins cause secretory diarrhea by increasing intracellular cyclic nucleotides, leading to electrolyte and water loss into the intestinal lumen. **High-Yield:** Colonization factor antigen (CFA/fimbriae) is essential for ETEC pathogenesis — it mediates adherence to small intestinal epithelium before toxin secretion. Without CFA, the organism cannot establish infection. ### Why Option 3 is Wrong **Warning:** Shiga toxin (Stx) is produced by **Shiga-toxigenic E. coli (STEC)**, NOT ETEC. Shiga toxin is a Shiga toxin-producing E. coli virulence factor that causes hemorrhagic colitis and hemolytic uremic syndrome (HUS) — a completely different pathogenic mechanism from ETEC's enterotoxins. **Mnemonic:** **ETEC** = **E**nterotoxins (LT/ST) + **T**oxigenic; **STEC** = **S**higa toxin + **T**oxigenic. Do not confuse the two pathotypes. [cite:Jawetz, Melnick & Adelberg's Medical Microbiology 28e Ch 16] ## Summary ETEC virulence depends on: 1. **CFA** (adherence) 2. **LT toxin** (cAMP-mediated secretion) 3. **ST toxin** (cGMP-mediated secretion) Shiga toxin belongs to STEC, which causes bloody diarrhea and HUS, not the watery secretory diarrhea of ETEC.
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