Bacitracin kills bacteria by blocking a critical recycling step in cell wall construction. Without a functioning cell wall, bacteria lose structural integrity and die. This is why bacitracin works well on skin wounds: it targets the building process that most common skin bacteria depend on for survival.
The Cell Wall Target
Bacteria surround themselves with a rigid wall made of a mesh-like material called peptidoglycan. Building this wall requires a lipid carrier molecule embedded in the bacterial membrane that acts like a shuttle. This carrier picks up wall-building blocks inside the cell, ferries them to the exterior surface, drops them off to be woven into the growing wall, and then cycles back to grab more.
After each delivery, the carrier needs to be “reset” through a chemical reaction that removes a phosphate group. Bacitracin latches onto the carrier before this reset can happen, effectively locking it in its used state. With the carrier stuck, no new building blocks reach the wall. Construction stalls, the wall weakens, and the bacterium dies. This target is unique to bacterial cells, which is why bacitracin doesn’t harm human tissue at the concentrations found in topical ointments.
Which Bacteria It Kills
Bacitracin is most effective against gram-positive bacteria, the type responsible for the majority of skin infections. This includes species of Staphylococcus, Streptococcus, Corynebacterium, and Clostridium. A few gram-negative organisms, such as Neisseria species, are also susceptible, but most gram-negative bacteria are naturally resistant. That limited spectrum is one reason bacitracin is often paired with other antibiotics in combination products.
How Bacteria Resist It
Some bacteria have evolved defenses against bacitracin. The most well-studied mechanism involves a molecular pump powered by the cell’s energy supply. Certain Enterococcus and Bacillus species carry genes that code for this pump, which actively pushes bacitracin out of the cell before it can bind its target. A second strategy is simpler: some bacteria ramp up production of the lipid carrier itself, flooding the membrane with so many copies that bacitracin can’t sequester them all. A third mechanism involves enzymes that essentially perform the carrier’s reset step through an alternative chemical pathway, bypassing the block entirely.
Triple Antibiotic Ointment and Why It Exists
Over-the-counter triple antibiotic ointment combines bacitracin with two other antibiotics, each attacking bacteria through a different mechanism. Neomycin disrupts the machinery bacteria use to build proteins, while polymyxin B punches holes in the outer membrane of gram-negative bacteria using a detergent-like action. Together, the three cover a broader range of bacteria than any single ingredient would alone. The standard bacitracin ointment you’ll find on store shelves contains 500 units per gram, whether it’s sold alone or as part of a combination product.
Bacitracin vs. Plain Petroleum Jelly
A randomized trial published in JAMA compared bacitracin ointment to plain white petroleum jelly in 922 patients with a total of 1,249 surgical wounds. The overall infection rate was low in both groups: 0.9% with bacitracin versus 2.0% with petroleum jelly, a difference that was not statistically significant. Healing looked identical at day 1, day 7, and day 28. However, four patients using bacitracin (0.9%) developed allergic contact dermatitis, while none in the petroleum jelly group did.
This matters because bacitracin is one of the more common causes of allergic reactions among topical antibiotics, with some studies reporting contact allergy rates as high as 13% depending on the population tested. If your wound is clean and not at high risk for infection, petroleum jelly provides the same moist healing environment without the allergy risk.
Why It’s Only Used on the Skin
Bacitracin is almost exclusively a topical antibiotic because it causes significant kidney damage when absorbed into the bloodstream. Studies in humans and animals found that systemic bacitracin leads to protein in the urine, a drop in the kidneys’ filtration rate, and reduced blood flow through renal tissue. The damage occurs in the kidney’s tubules, the tiny structures responsible for filtering waste and reabsorbing nutrients. The good news is that this damage reverses once bacitracin is stopped, but the toxicity is severe enough that internal use is essentially off the table when safer alternatives exist.
When applied to intact or lightly broken skin, very little bacitracin reaches the bloodstream, which keeps kidney exposure negligible. An ophthalmic formulation also exists for eye infections, where the drug stays localized on the surface of the eye rather than entering systemic circulation.
Where the Name Comes From
Bacitracin was discovered in 1943. Researchers isolated it from bacteria growing in the leg wound of a seven-year-old girl named Margaret Tracy. The name is a blend of “Bacillus,” the genus of the bacteria that produce it, and “Tracy,” the patient’s surname. It’s produced naturally by two species, Bacillus licheniformis and Bacillus subtilis, as a chemical weapon against competing microbes.