A 28-year-old woman from Delhi presents with acute onset bloody diarrhea, abdominal cramps, and fever (38.5°C) for 3 days. She reports attending a wedding reception 2 days prior where she consumed street food. On examination, she has mild dehydration and diffuse abdominal tenderness. Stool microscopy shows numerous RBCs, WBCs, and gram-negative bacilli. Culture on MacConkey agar shows non-lactose fermenting colonies. The organism is non-motile and produces acid from glucose but not from lactose. Which is the most likely causative organism?

Explanation

## Clinical Presentation & Epidemiology **Key Point:** Shigella causes acute bacillary dysentery with bloody mucoid stools, fever, and abdominal cramps — the classic presentation in this case. **Clinical Pearl:** Shigella is transmitted via the fecal-oral route; contaminated food at social gatherings is a classic epidemiological link. The 2-day incubation period fits Shigella perfectly. ## Microbiological Identification | Feature | Shigella | Salmonella | EIEC | V. cholerae | |---------|----------|-----------|------|-------------| | **Lactose fermentation** | Non-fermenting | Non-fermenting | Variable | Non-fermenting | | **Motility** | Non-motile | Motile (peritrichous) | Motile | Motile | | **Glucose fermentation** | Acid only | Acid + gas | Acid + gas | Non-fermenting | | **H~2~S production** | Negative | Positive | Negative | Negative | | **Stool appearance** | Bloody mucoid | Rarely bloody | Bloody | Watery (rice-water) | **High-Yield:** The combination of **non-motility + non-lactose fermentation + acid from glucose only** is pathognomonic for Shigella. ## Pathogenesis 1. Invasion of colonic epithelium via M cells 2. Intracellular multiplication and cell-to-cell spread (actin-based motility) 3. Mucosal ulceration → bloody diarrhea 4. Shiga toxin production (especially S. dysenteriae) → HUS in severe cases **Mnemonic: SHIGELLA = Sheds Hemolysin, Invades gut, Gram-negative, Enteric, Lacks motility, Lactose-negative, Acid-only fermentation** ## Why Shigella sonnei? - Most common Shigella species in India and urban areas - Causes milder disease than S. dysenteriae - Consistent with acute dysentery in an immunocompetent adult [cite:Park 26e Ch 32]