Colistin is a polymyxin E bactericidal polypeptide antibiotic. It produces its antibacterial effect by interacting strongly with phospholipids in bacterial cell membrane & radically disturbed their permeability & function. Colistin is active against G -ve bacteria such as Pseudomonas , E. coli ,Klebsiella , Aerobacter , Enterobacter , Salmonella , Shigella & Haemophilus.