The use of bacitracin as a growth promoter in animals produces no risk to human health

Zinc bacitracin is a mixture of high molecular weight polypeptides (bacitracin A, B and C and several minor components), first described in 1945 as a product of a Bacillus sp. (now recognized as Bacillus licheniformis), contaminating wounds. It has activity against some Gram-positive organisms, among which Staphylococcus aureus is somewhat less susceptible and Streptococcus pyogenes is highly susceptible. It is noteworthy that the high susceptibility of S. pyogenes, established by Maxted in 1953, still obtains and continues to be used as a diagnostic test for Lancefield Group A streptococci. Bacitracin acts bactericidally by binding to isoprenyl pyrophosphate, the lipid carrier that transfers N-acetylmuramyl-N-acetylglucosamyl-amino acid cell wall building blocks across the cytoplasmic membrane, a mechanism unlike that of any other commercially available antibiotic. Acquired resistance is known, for example in S. aureus, but is uncommon. Other organisms, such as enterococci, seem always to have varied in their susceptibility. There is no cross-resistance with other antibiotics. Upon introduction into clinical use in man, bacitracin was found to be nephrotoxic, and preparations for systemic use were soon withdrawn. However, it continued to be used topically, and is still used for the treatment of infected dermatoses and other skin infections, for infected wounds and the prevention of infection in dirty wounds, although to a diminished extent now other more effective agents have taken its place. The products containing bacitracin are ointments (including ophthalmic preparations) and various antibiotic sprays, in which it is mixed with such antibiotics as polymyxin and neomycin. The antibiotic sprays have rarely been favoured by microbiologists since they have been associated with both bacitracin and neomycin resistance, even though only the latter is of great clinical importance. The current lack of importance of bacitracin in human medicine is demonstrated by the fact that very little space is given to it in specialized (let alone general) textbooks. The seventh Edition of Antibiotic and Chemotherapy by O’Grady et al., the major British textbook on antibiotics, gives it only half a page or so among its 1000 pages, while Lorian’s compendious textbook Antibiotics in Laboratory Medicine mentions it virtually only in passing, with MIC information only on Neisseria meningitidis—one line in about 150 pages of tabulated susceptibility data. In contrast to its low level of use in humans, bacitracin has had an important role as a growth promoter in animal husbandry. Here, in addition to being effective in its primary role, it appears to have the additional advantage of suppressing necrotizing enteritis owing to Clostridium perfringens.