Topical Antimicrobials

Topical Antibacterials - Skin Savers

• Mupirocin:
  • MOA: Inhibits bacterial isoleucyl-tRNA synthetase.
  • Spectrum: Gram(+) cocci, including MRSA.
  • Uses: Impetigo, folliculitis, eradication of nasal MRSA.
• Fusidic Acid:
  • MOA: Inhibits protein synthesis (blocks EF-G). Steroid-like structure.
  • Spectrum: Potent against Staphylococci (including some MRSA).
  • Uses: Staphylococcal skin infections (impetigo, infected eczema). Good skin penetration.
• Retapamulin:
  • MOA: Pleuromutilin; inhibits protein synthesis at 50S ribosomal subunit (P site).
  • Spectrum: S. aureus, S. pyogenes.
  • Uses: Impetigo.
• Neomycin, Polymyxin B, Bacitracin (Triple Antibiotic Ointment):
  • MOA: Neomycin (30S inhibitor), Polymyxin B (damages cell membrane), Bacitracin (inhibits cell wall synthesis).
  • Spectrum: Broad (Gram +ve & -ve).
  • Uses: Minor skin trauma. ⚠️ High risk of allergic contact dermatitis (esp. Neomycin).
• Clindamycin (Topical):
  • MOA: Lincosamide, binds 50S ribosome.
  • Uses: Acne vulgaris, bacterial vaginosis.
• Erythromycin (Topical):
  • MOA: Macrolide, binds 50S ribosome.
  • Uses: Acne vulgaris. Widespread resistance.
• Metronidazole (Topical):
  • MOA: Disrupts bacterial DNA synthesis.
  • Uses: Rosacea, bacterial vaginosis, infected ulcers.
• Silver Sulfadiazine:
  • MOA: Silver ions (antimicrobial) + sulfadiazine (folate synthesis inhibitor).
  • Uses: Prevention/treatment of burn wound infections. Can cause transient leukopenia.

⭐ Mupirocin is a key agent for decolonizing MRSA from the nares, often used pre-operatively to reduce surgical site infections (SSIs).

Topical Antifungals - Fungus Fighters

• Azoles: Inhibit fungal P450 enzyme (14-α-demethylase) → ↓ergosterol synthesis. Broad spectrum.
  • Imidazoles: Clotrimazole, Miconazole, Ketoconazole (also for seborrheic dermatitis, Pityriasis versicolor).
  • Triazoles (newer): Luliconazole, Efinaconazole (onychomycosis).
• Allylamines: Inhibit squalene epoxidase → squalene accumulation (toxic) & ↓ergosterol. Fungicidal.
  • Terbinafine: Excellent for dermatophytes (tinea pedis, cruris, corporis).
  • Naftifine, Butenafine.
• Polyenes: Bind to ergosterol in fungal cell membrane → form pores → leakage.
  • Nystatin: Candida infections (oral thrush, vaginal, intertrigo). Not absorbed.
  • Amphotericin B (topical): Cutaneous/mucocutaneous candidiasis.
• Other Antifungals:
  • Ciclopirox Olamine: Unique MoA (chelates trivalent cations); broad spectrum. Nail lacquer.
  • Tolnaftate: Distorts hyphae; dermatophytes only (tinea).
  • Amorolfine: Inhibits ergosterol synthesis (different enzymes than azoles). Nail lacquer.

⭐ Terbinafine typically requires shorter treatment duration (e.g., 1-2 weeks for tinea corporis/cruris) compared to many azoles for similar dermatophyte infections.

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Topical Antivirals & Antiparasitics - Virus & Vermin Vanquishers

Antivirals:

• Acyclovir (5% cr/oint): Herpes simplex, VZV. Guanosine analog → inhibits DNA pol. Apply 5-6x/day.
• Penciclovir (1% cream): Herpes labialis. Like acyclovir. Apply q2h awake.
• Imiquimod (5% cream): HPV warts, actinic keratosis, sBCC. TLR7 agonist. Apply 3x/wk up to 16 wks (warts).
• Podophyllotoxin (0.5% sol/gel): HPV warts. Antimitotic. BID x3d, 4d off. ⚠️ Teratogenic.

Antiparasitics:

• Permethrin (5% cream): Scabies, pediculosis. Neurotoxin (Na+ ch. blocker).
  • Scabies: Apply neck-down, wash 8-14h. Repeat 1 wk if needed.
  • Lice: Apply, wash 10 min.

  ⭐ Permethrin 5% cream: DoC for scabies; single overnight (8-14h) application often curative.

• Lindane (GBHC 1%): Scabies, pediculosis (2nd line). CNS stimulant. Wash 8-12h (scabies). ⚠️ Neurotoxic (kids).
• Ivermectin (0.5%-1%): Pediculosis capitis, rosacea (Demodex). Glu-Cl- ch. modulator. Lice: 1 app, wash 10 min.
• Crotamiton (10%): Scabies (↓effective), antipruritic.
• Benzyl Benzoate (25%): Scabies. Irritant. Apply 24h.
• Malathion (0.5%): Pediculosis. Organophosphate. Flammable. Apply 8-12h.

High‑Yield Points - ⚡ Biggest Takeaways

• Mupirocin: Targets isoleucyl-tRNA synthetase; effective for MRSA (impetigo, nasal carriage).
• Retapamulin: Binds 50S ribosomal subunit (L3 protein); treats impetigo (S. aureus, S. pyogenes).
• Fusidic acid: Steroidal antibiotic inhibiting protein synthesis (EF-G); for Staphylococcal skin infections.
• Triple antibiotic (Neomycin, Bacitracin, Polymyxin B): Neomycin is a common cause of contact dermatitis.
• Topical Clindamycin: Used for acne vulgaris; systemic absorption carries a rare risk of pseudomembranous colitis.
• Silver sulfadiazine: For burn wound prophylaxis; combines silver ions and sulfadiazine (folate inhibitor).