It is primarily used topically where it has very low toxicity. Polymyxin B Polymyxin B is a narrow spectrum antibiotic derived from a bacterium Bacillus polymyxa , an aerobic spore-forming rod. It is a basic peptide with a molecular weight of approximately 1000. Prior to the development of superior drugs with less toxicity, it was used to treat systemic infections caused by gram-negative bacteria, e.g., Pseudomonas aeruginosa , Salmonella , and Campylobacter . It acts by being a cationic surface active agent, due to its lipophilic and lipophobic ends, that disturbs the integrity of the cell membrane.
Effective concentrations are cidal regardless of the growth phase of the organism.
Because of its severe systemic toxicity which includes nephrotoxicity, neuromuscular blockade, and severe pain upon injection, it is used only topically. In this application it is very safe.