A 35-year-old man from Mumbai presents with acute onset watery diarrhea (8–10 stools/day), abdominal cramps, and mild fever (38.2°C) for 3 days. He reports consuming street food 2 days before symptom onset. Stool microscopy shows no blood or fecal leukocytes. Stool culture grows a gram-negative, lactose-fermenting rod that is positive for heat-labile (LT) enterotoxin. The organism is susceptible to trimethoprim-sulfamethoxazole. What is the most likely organism and pathotype?

Explanation

## Organism and Pathotype Identification **Key Point:** A lactose-fermenting gram-negative rod producing heat-labile enterotoxin (LT) in a patient with acute watery diarrhea and NO fecal leukocytes is enterotoxigenic E. coli (ETEC). ### Pathogenic Mechanism of ETEC **High-Yield:** ETEC produces two types of enterotoxins: 1. **Heat-labile (LT) toxin:** ADP-ribosylates G~s~ protein → ↑cAMP → secretory diarrhea (watery, no inflammation). 2. **Heat-stable (ST) toxin:** Activates guanylate cyclase → ↑cGMP → secretory diarrhea. Both toxins cause **secretory diarrhea** (no mucosal invasion, no blood, no leukocytes in stool). ### Comparison of Diarrheagenic E. coli Pathotypes | Pathotype | Virulence Factor | Stool Findings | Clinical Presentation | Epidemiology | |-----------|------------------|-----------------|----------------------|---------------| | **ETEC** | LT and/or ST toxins | **No leukocytes, no blood** | Watery diarrhea, cramps, fever | Traveler's diarrhea; street food | | **EIEC** | Invasion genes (inv, ipa) | **Leukocytes + blood** | Dysentery-like (bloody diarrhea) | Rare in developed countries | | **STEC** | Shiga toxin (Stx1/Stx2) | **Leukocytes + blood** | Hemorrhagic colitis → HUS | Beef, unpasteurized milk | | **EAEC** | Adherence + biofilm | **Leukocytes** | Persistent watery diarrhea | Immunocompromised, children | | **EPEC** | A/E lesions | **Leukocytes** | Infantile diarrhea (watery) | Children <2 years | ### Clinical Pearl **Mnemonic: ETEC = Traveler's diarrhea** - **E** = Enterotoxigenic - **T** = Toxin-mediated (LT/ST) - **E** = Epidemic (in travelers) - **C** = Cholera-like (secretory, watery) The absence of fecal leukocytes and blood is the critical finding that excludes invasive pathotypes (EIEC, STEC) and points to a toxin-mediated mechanism (ETEC). ### Epidemiological Context The history of street food consumption 2 days before symptom onset (incubation 1–3 days) is classic for traveler's diarrhea caused by ETEC. ETEC is transmitted via contaminated food/water and is the most common bacterial cause of traveler's diarrhea in developing countries, including India. ### Antibiotic Susceptibility Trimethoprim-sulfamethoxazole (TMP-SMX) susceptibility is typical for community-acquired ETEC. Most ETEC strains are susceptible to fluoroquinolones and TMP-SMX; resistance to ampicillin is variable.