Topical Antibacterial Drugs

Topical antibacterial drugs treat localised bacterial skin infection, decolonise carriers, and treat acne and rosacea. Use is limited by resistance and sensitisation, so they are given for short courses and for localised disease only.

Mechanism. Mupirocin inhibits isoleucyl tRNA synthetase; fusidic acid inhibits elongation factor G; retapamulin binds the 50S ribosome; ozenoxacin inhibits DNA gyrase and topoisomerase IV; clindamycin and erythromycin inhibit protein synthesis; metronidazole is antimicrobial and anti-inflammatory in rosacea; silver sulphadiazine releases silver ions; neomycin and bacitracin act on protein and cell wall synthesis respectively.

Pharmacokinetics. Negligible systemic absorption on intact skin; absorption rises with large wounds, burns and broken skin (silver sulphadiazine, neomycin).

• Mupirocin 2 percent ointment or cream: impetigo, secondary infected dermatoses, nasal MRSA decolonisation; resistance (mupA) emerges with repeated use.
• Fusidic acid 2 percent: impetigo and staphylococcal infection; widespread resistance, so limited to short courses and never combined long-term with a steroid.
• Retapamulin 1 percent, ozenoxacin 1 percent: impetigo, including MRSA in part; limited availability.
• Silver sulphadiazine 1 percent: burns.
• Metronidazole 0.75 to 1 percent: rosacea.
• Clindamycin 1 percent: acne (with benzoyl peroxide).
• Neomycin, bacitracin, polymyxin B combinations: minor wounds; frequent contact sensitisers.
• Antiseptics: chlorhexidine, povidone-iodine, hydrogen peroxide 1 percent cream (first-line for localised impetigo in some regions), honey dressings.

Regimens.

• Localised non-bullous impetigo: hydrogen peroxide 1 percent cream two to three times daily for 5 days, or topical mupirocin or fusidic acid two to three times daily for 5 to 7 days. Extensive, bullous or systemic impetigo: oral flucloxacillin or cloxacillin, or cefalexin; use clindamycin or doxycycline if MRSA is likely.
• Nasal MRSA decolonisation: mupirocin 2 percent ointment to each nostril twice daily for 5 days, usually with chlorhexidine body washes; repeated courses are avoided.
• Burns: silver sulphadiazine 1 percent once or twice daily on partial-thickness wounds in specialist settings.
• Rosacea: metronidazole 0.75 percent twice daily or 1 percent once daily for 8 to 12 weeks.
• Do not use topical antibiotics for uninfected wounds, eczema prophylaxis or leg ulcer colonisation.

Selection. Prefer antiseptics and the shortest courses; reserve mupirocin for MRSA decolonisation and fusidic acid for the first-line short course where local resistance is low.

Contraindications. Hypersensitivity; neomycin where the eardrum is perforated (ototoxicity); silver sulphadiazine in pregnancy near term, neonates, and G6PD deficiency (haemolysis).

Effects. Irritation, contact dermatitis (neomycin, bacitracin), resistance (mupirocin and fusidic acid), systemic nephrotoxicity or ototoxicity with neomycin on large areas, argyria and leucopenia with silver sulphadiazine.

Interactions. None clinically relevant at topical doses.

Monitoring. Review at 3 to 5 days; stop if no improvement and send swabs; repeat swabs after failed decolonisation.

Special groups. Pregnancy: mupirocin and hydrogen peroxide acceptable. Children: impetigo common, with household spread; hygiene advice. Region: impetigo and infected scabies are common in East African children and contribute to post-streptococcal glomerulonephritis, so adequate and complete treatment and treatment of the underlying scabies matter.