A 32-year-old woman from Delhi presents with acute bloody diarrhea, abdominal cramps, and fever for 3 days. Stool culture isolates a gram-negative rod that is oxidase-negative, non-motile, and ferments glucose without gas production. Which of the following is NOT a characteristic feature of Shigella species?

Explanation

## Distinguishing Shigella from Other Enterobacteria ### Key Biochemical Features of Shigella **Key Point:** Shigella is characteristically **lactose-negative** (or very slowly lactose-fermenting), which is a cardinal biochemical marker that separates it from E. coli and Salmonella. ### Pathogenic Mechanisms Shigella employs multiple virulence strategies: 1. **Enterotoxins (ShET1 and ShET2)** — secreted early, cause watery diarrhea and fluid loss before mucosal invasion 2. **Type III Secretion System (Mxi-Spa)** — injects effector proteins into host cells, enabling intracellular invasion and spread 3. **Mucosal invasion** — penetrates colonic epithelium, destroys crypts, causes bloody diarrhea and dysentery ### Epidemiology **High-Yield:** Shigella has an exceptionally **low infectious dose (10–100 organisms)**, making it highly transmissible in crowded settings and childcare facilities. This is why person-to-person spread is the dominant transmission route. ### Carbohydrate Fermentation Profile | Feature | Shigella | E. coli | Salmonella | |---------|----------|---------|------------| | **Lactose** | **Negative** (or very slow) | Positive | Negative | | **Glucose** | Positive (no gas) | Positive (gas) | Positive (gas) | | **Mannitol** | Variable (S. sonnei negative) | Positive | Positive | | **Motility** | Non-motile (no flagella) | Motile | Motile | **Warning:** The statement "ferments lactose" is the **incorrect** characterization — this is what makes option 2 the answer. ### Why This Matters Clinically The lactose-negative phenotype on MacConkey agar (colorless colonies) is a critical first-step identification clue. Combined with non-motility and oxidase-negativity, it narrows the differential to Shigella or Salmonella; serotyping and additional biochemical tests (e.g., H₂S production, ONPG test for β-galactosidase) confirm the organism. [cite:Textbook of Microbiology by Ananthanarayan & Paniker 10e Ch 32]