Polymyxins are antibiotics, with a general structure of a cyclic peptie with a long hydrophobic tail. They disrupt the structure of the bacterial cell membrane by interacting with its phospholipids. They are produced by the Gram-positive bacterium Bacillus polymyxa and are selectively toxic for Gram-negative bacteria due to their specificity for lipopolysaccharide in the outer membrane of the cell wall of these organisms.