Phospholipid Fatty Acid Structure and the Activity of Membrane-Active Peptides

Abstract

Virtually all studies on the interaction of antimicrobial peptides with model bilayers have focused on phospholipid headgroup composition, using unsaturated phospholipids to model bacterial cell membranes. The use of unsaturated lipid ensures fluid-phase membranes at room temperature, but unsaturated lipids are rarely found in cell membranes of gram-(+) bacteria. In Staphylococcus aureus, for instance, the major membrane lipids are saturated iso- and anteiso-branched phospholipids with phase transition temperatures well below room temperature. We found previously that increasing the degree of lipid acyl chain unsaturation markedly reduces the susceptibility of vesicles to delta-lysin, a potent, membrane-active peptide. We thus postulated that the acyl chain structure of bacterial lipids would play a role in the sensitivity of bacteria to antimicrobial peptides. The idea is corroborated by the observation that in methicillin-resistant S. aureus (MRSA), the fraction of anteiso-branched acyl chains is elevated relative to susceptible strains, pointing to a relation between fatty acid structure and bacterial resistance. In the work presented here, we tested this hypothesis by synthesizing asymmetric, saturated phospholipids containing an iso- or anteiso-branched fatty acid in the sn-2 position. We then examined the effect of the acyl-chain structure on peptide activity by constructing lipid vesicles and measuring the kinetics of dye release induced by delta-lysin. The results were compared with those obtained from lipid vesicles composed of natural staphylococcal phospholipid extracts enriched in either iso- or anteiso-branched phospholipids. Acyl chain structure was shown to indeed have a dramatic effect on membrane stability and the susceptibility to membrane-active peptides. In general, vesicles constructed from bacterial lipid extracts were considerably more stable and more resistant to peptide attack than those made from synthetic, branched lipids.