A 4-year-old boy presents with fluid-filled blisters on the neck and axillae that rupture easily, leaving a thin, varnish-like crust. Gram stain confirms Gram-positive cocci. What is the drug of choice for bullous impetigo?

Explanation

## Drug of Choice for Bullous Impetigo **Key Point:** Bullous impetigo (~30% of impetigo cases) is caused **exclusively** by *Staphylococcus aureus* (phage group II strains producing exfoliative toxins). Despite the different clinical presentation, the antibiotic choice remains **antistaphylococcal penicillin** — cloxacillin is first-line. ### Pathophysiology of Bullous Impetigo Bullous impetigo results from toxin-mediated intraepidermal acantholysis: - **Exfoliative toxins** (ETA/ETB) produced by *S. aureus* disrupt **desmoglein-1** (superficial epidermis) - Creates **subcorneal blistering** (unlike pemphigus vulgaris, which is autoimmune) - Blisters rupture easily → thin, varnish-like crust (characteristic finding) ### Treatment Comparison: Bullous vs. Non-Bullous | Feature | Non-Bullous | Bullous | | --- | --- | --- | | **Causative organism** | S. aureus + S. pyogenes | S. aureus only | | **First-line drug** | Cloxacillin | Cloxacillin | | **Route** | Oral (localized) or IV | Oral (if mild) or IV (if severe) | | **Duration** | 7–10 days | 7–10 days | | **Systemic toxicity risk** | Low | Moderate (risk of staphylococcal scalded skin syndrome in infants) | **High-Yield:** The **drug choice is identical** for both bullous and non-bullous impetigo because both are staphylococcal infections. The difference lies in **severity assessment** — bullous impetigo may warrant IV therapy if extensive or in infants <5 years. ### Why Cloxacillin, Not Amoxicillin-Clavulanate? **Clinical Pearl:** While amoxicillin-clavulanate covers β-lactamase-producing S. aureus, cloxacillin is **preferred** because: 1. **Inherent resistance** to β-lactamase (no need for clavulanate) 2. **Better tissue penetration** into skin and soft tissue 3. **Lower cost** in India 4. **Established efficacy** in bullous impetigo trials Amoxicillin-clavulanate is a reasonable alternative but is not first-line. **Warning:** Do NOT use erythromycin or TMP-SMX as monotherapy — resistance is high and they lack the superior skin penetration of β-lactamase-resistant penicillins. **Mnemonic:** **BISS** = **B**-lactamase-resistant penicillin for **I**mpetigo (both **S**ub-types) and **S**taphylococcal skin infections. [cite:Fitzpatrick's Dermatology 9e Ch 190; Park 26e Ch 6]