A 32-year-old woman from rural Maharashtra presents with acute bloody diarrhea, tenesmus, and abdominal cramps for 3 days. Stool culture grows a gram-negative rod that is non-motile, non-lactose fermenting, and produces acid on glucose and mannitol. Which of the following is NOT a characteristic feature of Shigella?

Explanation

## Identifying the Incorrect Feature **Key Point:** Shigella is characteristically a **non-lactose fermenter**, which is one of its defining biochemical properties that distinguishes it from E. coli and other coliforms. ### Correct Features of Shigella (Options 0, 1, 2) | Feature | Details | |---------|----------| | **Enterotoxin production** | Shigella dysenteriae type 1 produces Shiga toxin; other species produce enterotoxins causing secretory diarrhea in small bowel | | **Invasive mechanism** | Invades colonic epithelium via M cells; causes mucosal ulceration and bloody diarrhea (dysentery) | | **Type III secretion system** | Encodes virulence factors on a large plasmid (~220 kb); essential for intracellular invasion and spread | ### Why Option 3 is Wrong **High-Yield:** Shigella is a **non-lactose fermenter** — this is a cardinal biochemical feature used in laboratory identification. It ferments glucose and mannitol but NOT lactose, which is the exact opposite of the statement. **Clinical Pearl:** The non-lactose fermenting property is why Shigella appears as **colorless colonies on MacConkey agar** (unlike E. coli, which produces pink/red colonies). This is a key differential point in stool culture interpretation. ### Pathogenesis Summary 1. Invasion of colonic M cells via type III secretion system 2. Intracellular multiplication and cell-to-cell spread 3. Mucosal destruction → bloody diarrhea and tenesmus 4. Enterotoxin production → additional secretory component **Mnemonic:** **SHIGELLA = SHy from Lactose** (doesn't ferment it) — helps recall the non-lactose fermenting property. [cite:Textbook of Microbiology Ananthanarayan 10e Ch Enterobacteriaceae]