a group of polypeptide antibiotics, or acylcy-clopeptides, produced by certain strains of bacilli, primarily Bacillus polymyxa. The molecules of most polymyxins contain residues of threonine, lucine, α,γ-diaminobutyric acid, and 6-methyloctanoic acid.

Polymyxins are active only against gram-negative bacteria such as Pseudomonas aeruginosa, dysentery causative agent, Escherichia coli, Salmonellae, and Klebsiellae. The antimicrobial mechanism in polymyxins is associated with damage to the membrane of the bacterial cell. Polymyxins are differentiated by their nature and by the intensity of their side effects, which are chiefly neurotoxic and nephrotoxic and which restrict their use. Polymyxins B, M, and E (colistin) are used in medical practice.