A 42-year-old man presents with acute diarrhea, abdominal cramps, and bloody stools for 3 days. He recently consumed undercooked ground beef at a restaurant. On examination, he has mild fever (38.1°C) and diffuse abdominal tenderness. Stool culture grows a gram-negative bacillus that is sorbitol-fermenting, motile, and produces Shiga toxin. Which of the following is the most likely causative organism?

Explanation

## Diagnosis: Shiga Toxin-Producing E. coli O157:H7 (STEC) ### Clinical Presentation The patient presents with the classic triad of **hemorrhagic colitis**: - Bloody diarrhea (hemolytic diarrhea) - Severe abdominal cramps - Fever (though often low-grade or absent in STEC) **Key Point:** The consumption of undercooked ground beef is a major epidemiological clue — STEC O157:H7 is the leading cause of foodborne hemolytic uremic syndrome (HUS) in developed countries, with ground beef being the most common vehicle. ### Microbiological Features | Feature | STEC O157:H7 | EIEC | ETEC | EAEC | |---------|-------------|------|------|------| | **Motility** | Motile | Non-motile | Motile | Motile | | **Fermentation** | Sorbitol-fermenting (most strains) | Sorbitol-fermenting | Sorbitol-fermenting | Sorbitol-fermenting | | **Toxin** | Shiga toxin (Stx1, Stx2) | Invasin (invasion) | Enterotoxins (LT, ST) | Adhesins (AAF) | | **Diarrhea Type** | **Bloody (hemorrhagic)** | Dysentery-like (invasive) | Watery | Persistent watery | | **Incubation** | 1–8 days | 1–3 days | 1–3 days | 8–18 hours | **High-Yield:** STEC O157:H7 is **sorbitol-fermenting** (unlike the sorbitol-non-fermenting O157:H7 strains that are rare). The production of **Shiga toxin** is the defining pathogenic feature. ### Pathogenesis of STEC ```mermaid flowchart TD A[STEC O157:H7 ingestion]:::outcome --> B[Adherence via intimin<br/>to intestinal epithelium]:::action B --> C[Shiga toxin production<br/>Stx1 or Stx2]:::action C --> D[Toxin enters epithelial cells<br/>via Shiga toxin receptor]:::action D --> E[Inhibition of protein synthesis<br/>Ribosomal 60S subunit]:::action E --> F[Epithelial cell death<br/>& mucosal ulceration]:::outcome F --> G[Bloody diarrhea<br/>& inflammatory response]:::outcome C --> H[Systemic absorption<br/>of toxin]:::action H --> I[Endothelial damage<br/>in kidneys & brain]:::action I --> J[Hemolytic Uremic Syndrome<br/>HUS]:::urgent ``` **Clinical Pearl:** Approximately 5–10% of STEC infections progress to **hemolytic uremic syndrome (HUS)**, characterized by the triad of: 1. Microangiopathic hemolytic anemia (MAHA) 2. Thrombocytopenia 3. Acute kidney injury (AKI) **Warning:** Antibiotic use (especially fluoroquinolones) in STEC infection may **increase the risk of HUS** by promoting toxin release from dying bacteria. Supportive care and fluid management are the mainstays of treatment. ### Why STEC O157:H7 Is the Answer - **Shiga toxin production** is pathognomonic for STEC - **Sorbitol-fermenting** phenotype (sorbitol-MacConkey agar shows pink colonies) - **Motile** (distinguishes from non-motile EIEC) - **Bloody diarrhea** is the hallmark presentation - **Undercooked beef** is the classic epidemiological link [cite:Harrison 21e Ch 157]