inhibition. Spectrum of activity and susceptibility data Bacitracin is a narrow-spectrum antibiotic. It targets Gram-positive organisms, especially those that cause skin infections. The following represents susceptibility data for a few medically significant microorganisms. Staphylococcus aureus – ≤0.03 μg/mL – 700 μg/mL Staphylococcus epidermidis – 0.25 μg/mL – >16 μg/mL Streptococcus pyogenes – 0.5 μg/mL – >16 μg/mL Mechanism of action Bacitracin interferes with the dephosphorylation of C55-isoprenyl pyrophosphate, and a related molecule known as bactoprenol pyrophosphate; both of these lipids function as membrane carrier molecules that transport the building-blocks of the peptidoglycan bacterial cell wall outside of the inner membrane. Some have claimed that bacitracin is a protein disulfide isomerase inhibitor, but this is disputed by in vitro studies. History Bacitracin was isolated by Balbina Johnson, a bacteriologist at the Columbia University College of Physicians and Surgeons. Its name derives from the fact that a compound produced by a microbe in young Margaret Treacy's (1936–1994) leg injury showed antibacterial activity. The surname was misspelled and the name shortened to the more common spelling "Tracy." One strain isolated from tissue debrided from a compound fracture of the tibia was particularly active. We named this growth-antagonistic strain for the patient, "Tracy I." When cell-free filtrates of broth cultures of this bacillus proved to possess strong antibiotic activity and to be non-toxic, further study seemed warranted. We have called this active principle "Bacitracin. Bacitracin was approved by FDA in 1948. Synthesis Bacitracin is synthesised via nonribosomal peptide synthetases (NRPSs), which means that ribosomes are not directly involved in its synthesis. The three-enzyme operon is called BacABC, not to be confused with BacABCDE of bacilycin synthesis. Bacitracin is commercially manufactured by growing the bacteria Bacillus subtilis var Tracy I in a container of liquid growth medium. Over time, the bacteria synthesizes the antibiotic and secretes the antibiotic into the medium. The antibiotic is then extracted from the medium using chemical processes. Composition Bacitracin is composed of a mixture of related compounds with varying degrees of antibacterial activity. Notable fractions include bacitracin A, A1, B, B1, B2, C, D, E, F, G, and X. Bacitracin A has been found to have the most antibacterial activity. Bacitracin B1 and B2 have similar potencies and are approximately 90% as active as bacitracin A. Other bacitracin components including F and X do not appear to be extensively studied. References Polypeptide antibiotics Cyclic peptides