Which toxin produced by Staphylococcus aureus is responsible for the characteristic bullae formation in bullous impetigo?

Explanation

## Toxins in Staphylococcal Skin Infections **Key Point:** Bullous impetigo is caused by exfoliative toxins (exfoliatin A and B), which are serine proteases that cleave desmoglein-1 in the granular layer of the epidermis, leading to intraepidermal acantholysis and blister formation. ### Mechanism of Bullae Formation Exfoliative toxins target **desmoglein-1** (a desmosomal adhesion protein) at the level of the **stratum granulosum**, creating a plane of cleavage that results in: - Rapid blister formation - Clear, flaccid bullae filled with serum - Erosions after rupture (not crusted lesions as in non-bullous impetigo) ### Distinction: Bullous vs Non-Bullous Impetigo | Feature | Bullous Impetigo | Non-Bullous Impetigo | |---------|------------------|----------------------| | **Causative organism** | *S. aureus* (exfoliative toxin+) | *S. aureus* or *S. pyogenes* | | **Toxin involved** | Exfoliative toxin (serine protease) | Hyaluronidase, lipase, proteases | | **Lesion type** | Flaccid bullae | Pustules → honey-crusted erosions | | **Plane of split** | Intraepidermal (stratum granulosum) | Subcorneal or superficial | | **Frequency** | ~10% of impetigo cases | ~90% of impetigo cases | **High-Yield:** Exfoliative toxins are the defining virulence factor of bullous impetigo; they are **not** present in non-bullous forms, which explains the clinical morphology difference. **Clinical Pearl:** Bullous impetigo typically presents in infants and young children with flaccid bullae on intact or erythematous skin, often on the diaper area or extremities. The bullae rupture easily, leaving erosions.