Shigella dysenteriae type 1 is distinguished from other Shigella species by the production of which toxin that causes systemic complications?

Explanation

## Shiga Toxin and Shigella dysenteriae Type 1 **Key Point:** Shiga toxin is the defining virulence factor of *Shigella dysenteriae* type 1 (Sd1) and is responsible for systemic complications including hemolytic uremic syndrome (HUS). ### Shiga Toxin: Structure and Mechanism **High-Yield:** Shiga toxin is an **AB toxin** (A-B enterotoxin) encoded by a chromosomal gene in Sd1 only. #### Mechanism of Action 1. **B subunit** binds to globotriaosylceramide (Gb3) receptors on endothelial cells and renal tubular epithelium 2. **A subunit** (catalytic domain) inactivates the 60S ribosomal subunit by depurinating a specific adenine in 28S rRNA 3. **Result:** Inhibition of protein synthesis → cell death → vascular injury and renal failure ### Systemic Complications of Shiga Toxin | Complication | Mechanism | Incidence | |--------------|-----------|----------| | **Hemolytic uremic syndrome (HUS)** | Endothelial damage in renal microvasculature | 5–10% of infections | | **Thrombotic thrombocytopenic purpura (TTP)** | Systemic endothelial injury | Rare in Sd1 | | **Acute kidney injury** | Glomerular capillary necrosis | Common in HUS | | **Microangiopathic hemolytic anemia** | RBC fragmentation in damaged vessels | Part of HUS triad | **Clinical Pearl:** *Shigella dysenteriae* type 1 causes the most severe form of bacillary dysentery with the highest mortality (10–15%) compared to other Shigella species (1–3%), primarily due to Shiga toxin-mediated systemic complications. **Mnemonic:** **Sd1 = Shiga toxin Dysentery type 1** — only Sd1 produces Shiga toxin; other Shigella species are non-toxigenic. ### Why Only Sd1? The Shiga toxin gene is located on the **chromosome** of *S. dysenteriae* type 1 but is absent in: - *S. flexneri* - *S. boydii* - *S. sonnei* These species cause dysentery through plasmid-encoded Ipa proteins alone, without systemic toxin effects.