MOLECULAR MECHANISMS OF POLYMYXIN-B MEMBRANE INTERACTIONS - DIRECT CORRELATION BETWEEN SURFACE-CHARGE DENSITY AND SELF-PROMOTED TRANSPORT

We have studied the interaction of the polycationic peptide antibiotic polymyxin B (PMB) with asymmetric planar bilayer membranes via electr ical measurements. The bilayers were of different compositions, includ ing those of the lipid matrices of the outer membranes of various spec ies of Gram-negative bacteria. One leaflet, representing the bacterial inner leaflet, consisted of a phospholipid mixture (PL; phosphatidyle thanolamine, -glycerol, and diphosphatidylglycerol in a molar ratio of 81:17:2). The other (outer) leaflet consisted either of lipopolysacch aride (LPS) from deep rough mutants of PMB-sensitive (Escherichia coli F515) or -resistant strains (Proteus mirabibis R45), glycosphingolipi d (GSL-1) from Sphingomonas paucimobilis IAM 12576, or phospholipids ( phosphatidylglycerol, diphytanoyl-phosphatidylcholine). In all membran e systems, the addition of PMB to the outer leaflet led to the inducti on of current fluctuations due to transient membrane lesions. The mini mal PMB concentration required for the induction of the lesions and th eir size correlated with the charge of the lipid molecules. In the mem brane system resembling the lipid matrix of a PMB-sensitive strain (F5 15 LPS/PL), the diameters of the lesions were large enough (d = 2.4 nm +/- 8%) to allow PMB molecules to permeate (self-promoted transport), but in all other systems they were too small. A comparison of these p henomena with membrane effects induced by detergents (dodecyltriphenyl phosphonium bromide, dodecyltrimethylammonium bromide, sodiumdodecylsu lfate) revealed a detergent-like mechanism of the PMB-membrane interac tion.