A 28-year-old man from rural India presents with acute onset fever, abdominal pain, and diarrhea for 3 days. Stool culture grows a gram-negative rod that ferments lactose and produces gas. Among the Enterobacteriaceae, which is the most common cause of acute bacterial diarrhea in the Indian subcontinent?

Explanation

## Most Common Bacterial Diarrhea in India **Key Point:** Enterotoxigenic E. coli (ETEC) is the most common bacterial cause of acute diarrhea in the Indian subcontinent and globally, responsible for 30–40% of bacterial diarrhea cases in endemic areas. ### ETEC Pathogenesis 1. **Colonization** — Colonization factor antigen (CFA) binds to small bowel epithelium 2. **Toxin production** — Heat-labile toxin (LT) and/or heat-stable toxin (ST) 3. **Mechanism of diarrhea:** - LT activates adenylate cyclase → ↑ cAMP → secretory diarrhea - ST activates guanylate cyclase → ↑ cGMP → secretory diarrhea 4. **Result** — watery diarrhea without blood or inflammation ### Comparative Epidemiology of Diarrheal Pathogens in India | Organism | Frequency | Key Features | Transmission | |----------|-----------|--------------|---------------| | **ETEC** | 30–40% | Watery diarrhea, CFA+ toxin+, self-limited | Contaminated water/food | | Shigella flexneri | 10–15% | Dysentery (bloody), inflammatory, endemic | Fecal-oral, poor sanitation | | Vibrio cholerae | 2–5% | Severe watery diarrhea, epidemic potential | Contaminated water | | Campylobacter jejuni | 5–10% | Bloody diarrhea, fever, abdominal pain | Poultry, unpasteurized milk | **Clinical Pearl:** ETEC causes a self-limited, non-inflammatory watery diarrhea ("traveler's diarrhea") that resolves in 3–5 days without antibiotics. The presence of lactose fermentation and gas production in the stool culture is consistent with E. coli. **High-Yield:** ETEC is the leading cause of acute diarrhea in developing countries with poor sanitation. Shigella is the second most common bacterial cause but is associated with dysentery (bloody diarrhea) and inflammatory features. **Mnemonic:** **ETEC = Enterotoxin + Toxin-mediated secretory diarrhea** — think watery, non-bloody, self-limited. **Warning:** Do not confuse ETEC with Shigella — Shigella causes dysentery (bloody, inflammatory diarrhea), while ETEC causes watery, non-inflammatory diarrhea. The clinical presentation here emphasizes acute diarrhea without mention of blood or severe inflammation, favoring ETEC.