Topical Antibiotics

Topical Antibiotics: Basics & Spectrum - Bug Busters 101

• Definition: Antimicrobials applied directly to skin/mucous membranes.
• Goal: Treat/prevent superficial bacterial infections (e.g., impetigo, folliculitis).
• Advantages:
  • ↑ High local drug concentrations at infection site.
  • ↓ Minimal systemic absorption, reducing systemic side effects.
• Spectrum Overview:
  • Gram-positive cocci: S. aureus, Strep. pyogenes (e.g., Mupirocin, Fusidic Acid).
  • Gram-negative rods: Pseudomonas aeruginosa (e.g., Polymyxin B, Gentamicin).
  • Broad-spectrum: Cover multiple types (e.g., Neomycin, Silver Sulfadiazine).
• ⚠️ Considerations: Risk of allergic contact dermatitis; bacterial resistance with overuse.

⭐ Topical antibiotics achieve high local concentrations with minimal systemic absorption, reducing systemic side effects but increasing risk of contact sensitization.

Gram-Positive Coverage: Mupirocin, Fusidic Acid - Golden Staph Slayers

Key topical agents targeting Gram-positive bacteria, especially Staphylococcus aureus.

• Apply Mupirocin 2% ointment/cream.

⭐ Mupirocin is highly effective for nasal decolonization of MRSA and treatment of impetigo; acts by inhibiting isoleucyl tRNA synthetase.

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Acne & Broad Spectrum Agents - Pimple Patrol & More

• Acne Vulgaris Agents:
  • Clindamycin: 1% gel/lotion. Targets Cutibacterium acnes.
    • Combine with benzoyl peroxide (BPO) to ↓ resistance.
  • Erythromycin: 2% gel/solution. Alternative for clindamycin resistance.
  • Nadifloxacin: 1% cream. Fluoroquinolone for mild-moderate acne.
• Broad Spectrum Topical Combinations:
  • Neomycin + Polymyxin B + Bacitracin (Triple antibiotic):
    • Gram (+ve) & Gram (-ve) coverage. For minor skin infections, cuts.
    • 📌 Mnemonic: "Neo Poly Bac" - New Police Back-up.
• Burns & Specific Infections:
  • Silver Sulfadiazine (SSD): 1% cream.
    • Burn wound prophylaxis; targets Pseudomonas aeruginosa.
    • ⚠️ Caution: Sulfa allergy, G6PD deficiency, neonates.
    • Forms pseudoeschar.

⭐ Topical Clindamycin resistance in Cutibacterium acnes is a growing concern; it is often combined with benzoyl peroxide to mitigate this and enhance efficacy.

Adverse Effects & Resistance - Cautionary Tales

• Key AEs:
  • Contact dermatitis: Especially neomycin (notorious sensitizer!), bacitracin.
  • Photosensitivity: e.g., sulfonamides.
• Systemic AEs (if absorbed, e.g., large areas, broken skin):
  • Neomycin: Ototoxicity, nephrotoxicity. ⚠️
  • Polymyxin B: Neurotoxicity, nephrotoxicity.
• Antibiotic Stewardship:
  • Crucial to ↓ resistance.
  • Use short duration, narrow spectrum if possible.
  • Avoid for viral/fungal infections or sterile inflammation.
• Pregnancy/Lactation:
  • Generally considered safer: Erythromycin, clindamycin.
  • Mupirocin: Use if benefit outweighs risk.
  • Avoid/Caution: Tetracyclines (systemic absorption risk), Metronidazole (1st trimester). Sulfonamides (near term - kernicterus risk).

⭐ Neomycin is a common component of triple antibiotic ointments but is a notorious sensitizer causing allergic contact dermatitis.

High‑Yield Points - ⚡ Biggest Takeaways

• Mupirocin inhibits isoleucyl tRNA synthetase, effective for impetigo and MRSA decolonization.
• Fusidic acid inhibits protein synthesis (EF-G), excellent for staphylococcal skin infections.
• Topical clindamycin is a mainstay for acne vulgaris; watch for rare pseudomembranous colitis.
• Topical metronidazole is first-line for papulopustular rosacea and effective for perioral dermatitis.
• Neomycin in triple antibiotics carries a high risk of allergic contact dermatitis.
• Silver sulfadiazine is used for burn wound infections; monitor for leukopenia.