Polymyxin B sulfate is a polypeptide antibiotic and is composed of polymyxins B1, B2, and B3 with fractions B1 and B2 comprising the majority of the mixture. Polymyxin B targets and alters permeability lipopolysaccharide (LPS) of gram negative bacteria leading to lysing of the cell. Polymyxin B only needs to interact with LPS, it is not required to enter the cell. Polymyxin B is routinely used as a selection agent in several types of isolation media.