Colistin is a polypeptide antibiotic produced by certain strains of bacillus polymyxa var.colistinus. Colistin is an antibiotic that has a bactericidal action when given in normal doses. Colistin attaches itself to the bacterial cell wall, so disturbing the permeability of the cell wall and causing the plasma contents as purine and pyrimidines to escape from the cell. This results usually in lysis. Acts mainly against gram – negative bacteria.