Colistimethate is a methanesulfonate of polymyxin antibacterial colistin. Colistimethate is a nonactive prodrug. In aqueous solutions, colistimethate is hydrolyzed and forms a complex mixture of partially sulfomethylated derivatives and colistin. The antimicrobial activity of colistin is similar to that of polymyxin B and is restricted to gram-negative bacteria, including P aeruginosa, Acinetobacter species, Enterobacter-Klebsiella tribe, Escherichia coli, Salmonella and Shigella species, Citrobacter species, Yersinia pseudotuberculosis, Morganella morganii and Haemophilus influenzae. Colistin has also been shown to possess considerable in vitro activity against Stenotrophomonas maltophilia. Colistin and polymyxin B, however, do not have activity against Proteus, Providencia, Serratia species, Pseudomonas mallei, Burkholderia cepacia, Brucella species, most gram-positive bacteria, gram-negative cocci, anaerobes, fungi and parasites. Parenteral or nebulized colistimethate is indicated for the treatment of acute or chronic infections due to sensitive strains of certain gram-negative bacilli. It is particularly indicated when the infection is caused by sensitive strains of Pseudomonas aeruginosa.