Topical antibiotics

Definition and Therapeutic Context

Topical antibiotics are pharmaceutical agents applied directly to the skin or mucosal surfaces to treat or prevent localized bacterial infections. They offer site-specific antimicrobial activity while minimizing systemic exposure and associated side effects. These agents are integral to dermatologic, surgical, and wound care management, particularly in superficial skin infections, minor wounds, abrasions, and burns.

They are not recommended for deep or systemic infections, as penetration is limited. Inappropriate or prolonged use can contribute to bacterial resistance, delayed wound healing, and hypersensitivity reactions. Clinical efficacy depends on the causative organisms, depth of infection, and the integrity of the skin barrier.

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Mechanisms of Action

Topical antibiotics act via one or more of the following mechanisms:

• Inhibition of bacterial protein synthesis (e.g., mupirocin, fusidic acid)

• Disruption of bacterial cell wall synthesis (e.g., bacitracin)

• Interference with bacterial nucleic acid metabolism (e.g., metronidazole)

• Membrane disruption or permeability alteration (e.g., polymyxin B, gramicidin)

These actions are bacteriostatic or bactericidal depending on the agent and concentration.

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Indications for Use

Topical antibiotics are indicated for:

• Impetigo (localized)

• Infected cuts, abrasions, and minor burns

• Secondary bacterial infections of dermatitis

• Furuncles, folliculitis, and carbuncles (adjunctive)

• Nasal decolonization of methicillin-resistant Staphylococcus aureus (MRSA)

• Acne vulgaris (specific agents like clindamycin and erythromycin)

• Periorificial infections (e.g., angular cheilitis when secondarily infected)

• Surgical site infection prophylaxis (select cases)

• Infected eczema or diaper rash (limited scope)

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Main Classes and Commonly Used Topical Antibiotics

1. Mupirocin

   • Mechanism: Inhibits bacterial isoleucyl-tRNA synthetase → halts protein synthesis.

   • Spectrum: Gram-positive cocci, especially Staphylococcus aureus (including MRSA).

   • Indications: Impetigo, folliculitis, MRSA nasal decolonization (2% nasal ointment).

   • Brands: Bactroban, Centany.

   • Application: 2–3 times daily for up to 10 days.

   • Formulations: Ointment, cream, nasal ointment.

   • Notes: Resistance emerging; avoid prolonged use.

2. Fusidic Acid

   • Mechanism: Inhibits elongation factor G (EF-G), blocking protein synthesis.

   • Spectrum: Primarily Gram-positive organisms including Staph aureus.

   • Indications: Impetigo, infected dermatitis, conjunctivitis (as eye drops).

   • Brands: Fucidin, Fucibet (with betamethasone).

   • Formulations: Cream, ointment, eye drops.

   • Warning: Resistance can develop rapidly with monotherapy.

3. Clindamycin (topical)

   • Mechanism: Inhibits 50S ribosomal subunit → blocks protein synthesis.

   • Spectrum: Anaerobes, some Gram-positive cocci.

   • Indications: Inflammatory acne vulgaris.

   • Brands: Clindagel, Dalacin T.

   • Formulations: Solution, gel, foam, lotion.

   • Cautions: Resistance risk; combine with benzoyl peroxide to reduce.

4. Erythromycin (topical)

   • Mechanism: Macrolide class; inhibits 50S subunit.

   • Indications: Mild to moderate acne vulgaris.

   • Brands: Eryacne, Akne-mycin.

   • Note: Increasing bacterial resistance reduces effectiveness.

5. Metronidazole

   • Mechanism: Disrupts DNA synthesis under anaerobic conditions.

   • Spectrum: Anaerobic bacteria, Propionibacterium acnes, and some protozoa.

   • Indications: Rosacea (anti-inflammatory more than antibacterial), perioral dermatitis.

   • Brands: Metrogel, Rozex.

   • Formulations: Cream, gel, lotion.

   • Usage: Usually once or twice daily.

6. Neomycin

   • Mechanism: Aminoglycoside antibiotic; disrupts 30S ribosomal subunit.

   • Spectrum: Broad, mainly Gram-negative.

   • Indications: Minor skin infections; often combined with bacitracin/polymyxin.

   • Warnings: High risk of allergic contact dermatitis and ototoxicity if absorbed.

7. Bacitracin

   • Mechanism: Inhibits peptidoglycan synthesis in bacterial cell wall.

   • Spectrum: Gram-positive bacteria.

   • Common use: In combination topical ointments.

   • Cautions: Hypersensitivity reactions are possible.

8. Polymyxin B

   • Mechanism: Disrupts bacterial cell membrane integrity.

   • Spectrum: Gram-negative bacteria including Pseudomonas aeruginosa.

   • Usage: Often in combination (e.g., with neomycin and bacitracin).

   • Formulations: Triple antibiotic ointments (Neosporin).

9. Gramicidin

   • Mechanism: Forms ion channels in bacterial membranes, leading to leakage.

   • Spectrum: Mainly Gram-positive organisms.

   • Use: Combined formulations (e.g., with polymyxin B).

10. Retapamulin (US only)

    • Mechanism: Inhibits protein synthesis by binding to 50S ribosome.

    • Indications: Impetigo due to S. pyogenes or S. aureus.

    • Brand: Altabax.

    • Note: Not available in Europe.

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Combination Products

Many formulations include multiple antibiotics or are combined with corticosteroids or antifungals:

• Neomycin + Bacitracin + Polymyxin B (Triple antibiotic)

  • Brand: Neosporin

• Fusidic acid + Betamethasone (Fucibet)

• Clindamycin + Benzoyl Peroxide (Duac)

• Mupirocin + Calcipotriol (under investigation for psoriatic lesions)

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Adverse Effects

Topical antibiotics are generally well tolerated. However, adverse effects may include:

1. Local Reactions:

   • Burning, stinging

   • Erythema

   • Dryness or peeling (especially with acne products)

2. Contact Dermatitis:

   • Common with neomycin

   • Also reported with bacitracin and polymyxin B

3. Photosensitivity:

   • Rare, mostly reported with erythromycin

4. Bacterial Resistance:

   • Misuse or overuse contributes to local resistance

   • Combine with non-antibiotic agents (e.g., benzoyl peroxide) to reduce resistance in acne therapy

5. Systemic Absorption:

   • Uncommon unless applied to broken skin, large areas, or under occlusion

   • Risk of ototoxicity (e.g., neomycin) if applied to open wounds near auditory canal

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Contraindications and Precautions

• Known allergy to the active drug or formulation components

• Avoid prolonged use on large areas

• Avoid in deep or systemic infections

• Monitor for delayed wound healing or secondary fungal overgrowth with chronic use

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Drug Interactions

Topical agents have low systemic absorption but potential interactions exist:

• Topical clindamycin: Avoid concurrent oral clindamycin to reduce systemic toxicity.

• Erythromycin: Combined use with clindamycin topically may induce cross-resistance.

• Metronidazole: Alcohol interaction unlikely but avoid in systemic use.

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Special Populations

• Pediatrics: Generally safe when used on small areas; avoid in neonates and infants for large surface application.

• Pregnancy: Most topical antibiotics (e.g., mupirocin, clindamycin) are considered safe (Category B in the U.S.).

• Geriatrics: No dose adjustment, but thin skin increases absorption risk.

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Dosing and Duration

• Apply 2–3 times daily, or as per label, for 5–14 days depending on indication.

• For acne and rosacea treatments (e.g., clindamycin, metronidazole), long-term use (up to 12 weeks) may be indicated with clinical monitoring.

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Comparison with Other Topical Agents

Class	Action	Indications	Examples
Topical antibiotics	Antibacterial	Impetigo, minor wounds	Mupirocin, fusidic acid
Topical antifungals	Antifungal	Tinea, candidiasis	Clotrimazole, terbinafine
Topical antivirals	Antiviral	HSV infections	Acyclovir
Topical corticosteroids	Anti-inflammatory	Eczema, dermatitis	Hydrocortisone, betamethasone

Representative Brand Names

Generic Name	Brand Name(s)	Formulation Type	Typical Use
Mupirocin	Bactroban, Centany	Ointment, nasal ointment	Impetigo, MRSA decolonization
Fusidic Acid	Fucidin, Fucibet	Cream, eye drops	Infected eczema
Clindamycin	Clindagel, Dalacin T	Gel, solution	Acne
Erythromycin	Eryacne, Akne-mycin	Gel, solution	Acne
Metronidazole	Metrogel, Rozex	Gel, cream	Rosacea
Neomycin Combo	Neosporin	Ointment	Minor skin infections