## Most Common Secondary Bacterial Pathogen in Atopic Dermatitis **Key Point:** Staphylococcus aureus is the most frequent bacterial colonizer and pathogen in atopic dermatitis, found in 70–90% of infected lesions. ### Pathophysiology of S. aureus Colonization in AD 1. **Impaired skin barrier function** — defective filaggrin and ceramide synthesis allow bacterial penetration 2. **Reduced antimicrobial peptides** — decreased cathelicidin and β-defensins in AD skin 3. **Altered immune response** — Th2-dominant inflammation fails to mount effective antibacterial response 4. **Increased adhesion molecules** — S. aureus binds to fibronectin and collagen in damaged skin **High-Yield:** S. aureus produces superantigens (toxic shock syndrome toxin, enterotoxins) that amplify Th2 inflammation and perpetuate the eczematous cycle. ### Clinical Significance | Feature | S. aureus in AD | Other Organisms | |---------|-----------------|------------------| | Prevalence | 70–90% of infected lesions | <10% each | | Virulence factors | Superantigens, exotoxins | Limited | | Treatment impact | Anti-staphylococcal therapy improves AD | Rarely needed | | Resistance pattern | Increasing MRSA prevalence | Uncommon | **Clinical Pearl:** Even colonization without clinical infection can worsen AD through superantigen-mediated immune activation. Topical antibiotics (mupirocin) or systemic anti-staphylococcal agents (cephalosporins, fluoroquinolones) are indicated when secondary infection is suspected. **Warning:** Do not assume all bacterial growth on culture represents pathogenic infection — S. aureus colonization is normal in AD and does not always require treatment unless signs of infection (oozing, crusting, pustules) are present. [cite:Robbins 10e Ch 25]
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