Colistimethate sodium is a cyclic polypeptide antibiotic derived from Bacillus polymyxa var. colistinus and belongs to the polymyxin group. The polymyxin antibiotics are cationic surface-active agents that work by damaging the cell membrane. The resulting physiological effects are lethal to the bacterium. Polymyxins are selective for Gramnegative bacteria that have a hydrophobic outer membrane.