Polymyxin B sulfate: Polymyxin B sulfate (PMB) is a strongly cationic cyclic polypeptide antibiotic isolated from fermentation of Bacillus polymyxa. PMB is a variable mixture of B1 and B2 (predominantly B1). Polymyxins combine with cell membranes and disrupt normal permeability to small molecules. Polymyxin B and the other polymyxin antibiotics act primarily by binding membrane phospholipids and disrupting the cytoplasmic membrane, inducing pore formation in bacterial walls (large enough to permit nucleotide leakage). Polymyxin B binds to the lipid-A portion of the lipopolysaccharide in the cell membrane of Gram-negative bacteria. The interaction involves ionic forces between amino groups in Polymyxin B and phosphate and carboxyl groups in the lipid A-Kdo region, with hydrophobic interactions between the respective acyl groups. Data suggest a stoichiometric binding of one LPS monomer to one polymyxin B molecule. “Polymyxin B has a bactericidal action on most Gram-negative bacilli (E. coli, e.g.) except proteus spp. ... not active against Neisseria species, ... most fungi and Gram-positive bacteria." Activity is inhibited by iron (II), Co (II), Mn (II) and magnesium ions. Polymyxin B may be incompatible with other microbial agents, including amphotericin, cephalothin sodium, cephasolin sodium, chloramphenicol sodium succinate and tetracycline hydrochloride; it is also incompatible with heparin sodium and prednisolone sodium phosphate