A 28-year-old woman from Delhi presents with acute onset bloody diarrhea, abdominal cramping, and tenesmus for 3 days. She reports no fever. Stool microscopy shows numerous RBCs and WBCs with mucus. Stool culture on HE agar yields non-motile, non-lactose-fermenting gram-negative rods that are oxidase-negative and produce acid from glucose but not gas. The isolate is Shigella flexneri. Which virulence factor is MOST directly responsible for the bloody diarrhea and mucosal invasion seen in this patient?

Explanation

## Pathogenesis of Shigella Infection **Key Point:** Shigella causes dysentery through direct mucosal invasion and intracellular multiplication, not through toxin-mediated secretory diarrhea. ### Virulence Factors in Shigella | Virulence Factor | Mechanism | Clinical Manifestation | |---|---|---| | **Type III Secretion System (T3SS)** | Injects Ipa proteins directly into epithelial cells; enables bacterial entry and intracellular survival | Mucosal invasion, bloody diarrhea, dysentery | | **Invasion Plasmid Antigens (IpaA, IpaB, IpaC, IpaD)** | Disrupt actin cytoskeleton; facilitate bacterial internalization | Epithelial cell invasion and spread | | **LPS endotoxin** | Systemic inflammatory response; contributes to systemic symptoms | Fever, systemic toxicity (secondary) | | **Enterotoxins (ShET1, ShET2)** | Cause secretory diarrhea in some strains | Watery diarrhea (minor role) | | **Flagella** | Absent in virulent strains; lost during pathogenesis | Non-motile phenotype | ### Why T3SS and Ipa Proteins Are the Answer 1. **Direct mucosal invasion:** The T3SS allows Shigella to inject Ipa proteins into host epithelial cells, causing rearrangement of the actin cytoskeleton and bacterial entry. 2. **Intracellular multiplication:** Once inside, bacteria multiply within the cytoplasm and spread to adjacent cells via actin-based propulsion. 3. **Bloody diarrhea:** This invasive mechanism causes mucosal ulceration, bleeding, and the characteristic bloody mucoid stools with RBCs and WBCs. 4. **Absence of fever:** The patient has no fever, which is typical of Shigella dysentery (unlike invasive Salmonella), indicating the pathology is primarily local mucosal invasion rather than systemic toxemia. **High-Yield:** The T3SS is the hallmark of Shigella virulence. It is chromosomally encoded (unlike the plasmid-encoded T3SS in Salmonella) and is essential for the invasive phenotype. **Clinical Pearl:** Shigella flexneri and S. sonnei cause bloody dysentery with tenesmus and mucus, whereas S. dysenteriae type 1 produces the potent Shiga toxin (causing hemolytic uremic syndrome). All rely on T3SS for initial invasion. **Mnemonic:** **IPAAA** — Invasion Plasmid Antigens (IpaA, IpaB, IpaC, IpaD) are the key to understanding Shigella's invasive strategy.