Bacitracin
Antimicrobial / TopicalAlso known as: Bacitracin Zinc
Mechanism
One of the most common over-the-counter antibiotics found in medicine cabinets worldwide. A cyclic peptide from Bacillus subtilis that blocks bacterial cell wall synthesis. Used as a topical ointment for wound care, cuts, and scrapes. Like gramicidin, it's too toxic for injection (causes severe kidney damage), but is safe and effective topically.
Technical detail
Cyclic dodecapeptide (12 amino acids including unusual thiazoline ring from Ile-Cys condensation) produced by Bacillus subtilis var. Tracy (named after patient Margaret Tracy, 1943). Mechanism: inhibits bacterial cell wall synthesis by binding C55-isoprenyl pyrophosphate (undecaprenyl pyrophosphate, UPP), preventing its dephosphorylation back to C55-P by UPP phosphatase. This blocks the lipid carrier recycling essential for transporting peptidoglycan precursors across the membrane. Metal-dependent: requires Zn2+ for UPP binding (hence bacitracin zinc formulation). Active against gram-positives (Staphylococcus, Streptococcus), some gram-negatives (Neisseria, Haemophilus). Nephrotoxic (proximal tubular necrosis) — restricts use to topical. Annual US OTC sales in the hundreds of millions.
Effects
ANTIMICROBIAL MECHANISM: Cyclic polypeptide antibiotic produced by Bacillus subtilis (originally from a wound infection in a girl named Margaret Tracey — hence "bacitracin"). Inhibits cell wall synthesis by binding to C55-isoprenyl pyrophosphate (bactoprenol), the lipid carrier that transports peptidoglycan precursors across the cell membrane. By sequestering bactoprenol, bacitracin prevents the recycling of this essential carrier, halting peptidoglycan synthesis. This mechanism is distinct from beta-lactams (which target PBPs) and vancomycin (which targets D-Ala-D-Ala). Requires a divalent metal ion (zinc) for binding activity. SPECTRUM: Gram-positive organisms: Staphylococcus aureus (including many MRSA strains topically), Streptococcus pyogenes, Streptococcus pneumoniae, Corynebacterium diphtheriae, Clostridium spp. Limited Gram-negative activity (Neisseria, Haemophilus — topical relevance only). MIC BREAKPOINTS: No CLSI/EUCAST systemic breakpoints (not used systemically). Topical concentrations vastly exceed MICs for susceptible organisms. Typical topical formulation: 500 IU/g — achieves local concentrations 100-1000x above MIC. RESISTANCE: Bacitracin resistance via BcrABC ATP-binding cassette (ABC) transporter — exports bacitracin out of the cell. Undecaprenol kinase mutations reducing bactoprenol phosphorylation. Resistance is relatively uncommon in clinical Gram-positive pathogens despite decades of OTC use (possibly because topical concentrations are overwhelmingly above MIC). PHARMACOKINETICS: Topical — minimal systemic absorption through intact skin (<2%). Significant absorption through damaged/denuded skin, wounds, and mucous membranes. SYSTEMIC USE IS ESSENTIALLY OBSOLETE due to severe nephrotoxicity (causes proximal tubular necrosis). Oral bacitracin is poorly absorbed and has been used for C. difficile colitis (largely replaced by vancomycin and fidaxomicin). IM bacitracin — historical use only, abandoned due to nephrotoxicity. CLINICAL APPLICATIONS: OTC topical antibiotic for minor wounds, cuts, abrasions, and burns. Available as single agent or in combination (Polysporin = bacitracin + polymyxin B; Neosporin = bacitracin + polymyxin B + neomycin). Ophthalmic ointment for bacterial conjunctivitis and prophylaxis. Surgical irrigation (some centers). Bacitracin susceptibility test used as laboratory surrogate for group A Streptococcus identification (GAS is bacitracin-susceptible, other beta-hemolytic strep are resistant). TOXICITY (Topical): Very safe. Contact dermatitis/allergic sensitization in 5-10% of patients with prolonged use (delayed-type hypersensitivity). Anaphylaxis has been reported but is extremely rare (case reports, primarily with wound irrigation/peritoneal use). Cross-reactivity with neomycin allergy is possible but not certain.
Practitioner Guide
CLINICAL PEARLS — WOUND CARE AND DERMATOLOGY PERSPECTIVE: THE OTC DEBATE: For decades, triple antibiotic ointment (Neosporin: bacitracin + neomycin + polymyxin B) was the gold standard for minor wound care. Current evidence suggests: (1) White petrolatum alone is equivalent to topical antibiotics for preventing infection in clean minor wounds (RCTs). (2) Neomycin is the most common contact allergen among antibiotics (~10-15% sensitization rate). (3) Bacitracin alone or bacitracin + polymyxin B (Polysporin) is preferred over Neosporin if antibiotic ointment is desired (avoids neomycin). (4) Many wound care specialists now recommend petrolatum-based ointments over antibiotic ointments. WHEN TOPICAL BACITRACIN IS APPROPRIATE: Minor wounds (cuts, abrasions, small burns) in patients who prefer antibiotic coverage. Impetigo (streptococcal/staphylococcal) — topical bacitracin is second-line after mupirocin. Nasal decolonization for MRSA (bacitracin ointment applied intranasally — alternative to mupirocin, especially with mupirocin-resistant MRSA). Post-surgical wound prophylaxis (applied at wound edges, though evidence is mixed). OPHTHALMIC USE: Bacitracin ophthalmic ointment is first-line for bacterial conjunctivitis prophylaxis in neonates (alternative to erythromycin ointment). Used post-operatively after eye surgery. Safe for periocular application. CONTACT DERMATITIS: 5-10% of patients develop delayed-type hypersensitivity with repeated/prolonged topical use. Presents as worsening redness, itching, vesiculation at the application site — often mistaken for wound infection (leading to MORE antibiotic application, worsening the reaction). Key clinical pearl: if a wound is getting worse despite topical antibiotic, consider contact allergy BEFORE escalating antibiotics. Patch testing confirms (bacitracin is part of the standard TRUE test panel). SURGICAL IRRIGATION: Some surgeons add bacitracin to irrigation solutions (50,000 IU per liter of saline) for wound irrigation, joint arthroplasty irrigation, or spinal surgery irrigation. Evidence is mixed. Risk of rare anaphylaxis with large-volume wound irrigation (case reports). APPLICATION: Clean wound first. Apply thin layer to wound surface 1-3x daily. Cover with sterile dressing or bandage. No occlusive dressing required. Duration: 5-7 days or until wound is epithelialized. STORAGE: OTC ointments — room temperature, stable for years. Ophthalmic — room temperature, discard after manufacturer-specified period. Bacitracin is stable in anhydrous vehicles (ointments) but degrades in aqueous solutions.
Research Summary
TIER 1 (Gold Standard): Dire et al., 1995 — RCT comparing bacitracin ointment vs. petrolatum vs. no treatment for minor lacerations: bacitracin reduced infection rate but petrolatum was equivalent (Annals of Emergency Medicine, PMID: 7741334). Smack et al., 1996 — RCT of white petrolatum vs. bacitracin for wound healing after Mohs surgery: equivalent outcomes, bacitracin had higher allergic contact dermatitis rate (Journal of the American Academy of Dermatology, PMID: 8698902). FDA OTC monograph for topical antibiotics. TIER 2 (Strong): Sood et al., 2012 — rising prevalence of bacitracin contact allergy (Dermatitis, comprehensive review). Bacitracin mechanism: Stone & Strominger, 1971 — binding to C55-isoprenyl pyrophosphate (PNAS, classic biochemistry paper). DrugBank DB00626. AAD (American Academy of Dermatology) wound care guidelines. TIER 3 (Moderate): Decades of OTC clinical use (billions of applications) with excellent safety record. MRSA nasal decolonization protocols using bacitracin (multiple institution protocols). Surgical irrigation practices (variable by institution and specialty). Pediatric wound care guidelines. Poison control data confirming minimal toxicity from accidental ingestion of topical bacitracin. International data: bacitracin is a global OTC product with extensive post-marketing data. KEY FINDINGS: (1) Petrolatum may be equivalent to topical antibiotics for clean minor wounds. (2) Contact dermatitis is the main adverse effect of prolonged use. (3) Systemic bacitracin is obsolete due to nephrotoxicity. (4) The bacitracin susceptibility test remains useful in clinical microbiology labs. (5) Nasal MRSA decolonization is an underappreciated application. GAPS: Modern RCTs comparing bacitracin vs. petrolatum vs. newer wound care products (honey, silver). Resistance surveillance in community Gram-positive pathogens. Optimal duration of topical application. Whether chronic OTC bacitracin use drives resistance. ACTIVE TRIALS: Limited — well-established OTC product. Some ongoing for MRSA decolonization protocols.