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The bactericidal mechanism of action of poly­ myxin B against susceptible bacteria has been sug­ gested to result from electrostatic interactions of the basic polypeptide antibiotic with acidic cell envelope phospholipids 1,2. This process causes the breakdown of the selective permeability barrier of the cyto­ plasmic membrane, and finally cell death. The identities of the polymyxin receptor molecules in sensitive bacteria, however, have never been deter­ mined with certainty. The problem is further com­ plicated by the fact that polymyxin exhibits a strikingly selective activity against Gram-negative bacteria3 which, in addition to the plasma mem­ brane, possess a second membrane structure, the outer membrane or plastic layer composed of pro­ teins, lipopolysaccharide and phospholipids4. A possible strategy to identify the polymyxin receptors consists in examining isolated, individual com­ ponents and substructures of the cell envelope ( = outer membrane, murein and inner membrane) for their ability to combine with polymyxin B in vitro under physiological conditions. We have pre­ viously reported2 that extraction of isolated cell envelopes from Salmonella typhimurium with chloroform-methanol reduced their binding capacity for radioactive polymyxin B by 70%. In the present communication, it is shown that phosphatidic acid and phosphatidyl glycerol, but not phosphatidylethanolamine, phosphatidylserine or cardiolipin (diphosphatidylglycerol), form stable complexes with polymyxin B which can be isolated by paperchromatography or extraction into chloroform. This is the first instance for a specific preference of polymyxin B for defined acidic phospholipids. The phospholipids occurring in S. typhimurium 5, phosphatidic acid (P A ), phosphatidylglycerol (PG), phosphatidylserine (PS), phosphatidylethanolamine (PE) and cardiolipin (CL), were purchased from Supelco Inc. (Bellefonte, Pa., U SA ). PE and PG were also purified from nutrient broth grown S. typhimurium G30 according to G. F. A m es5.

Action of Polymyxin Bon Bacterial Membranes II. Formation of Lipophilic Complexes with Phosphatidic Acid and Phosphatidylglycerol