Branched phospholipids render lipid vesicles more susceptible to membrane-active peptides

Iso- and anteiso-branched lipids are abundant in the cytoplasmic membranes of bacteria. Their function is assumed to be similar to that of unsaturated lipids in other organisms - to maintain the membrane in a fluid state. However, the presence of terminally branched membrane lipids is likely to impact other membrane properties as well. For instance, lipid acyl chain structure has been shown to influence the activity of antimicrobial peptides. Moreover, the development of resistance to antimicrobial agents in Staphylococcus aureus is accompanied by a shift in the fatty acid composition toward a higher fraction of anteiso-branched lipids. Little is known about how branched lipids and the location of the branch point affect the activity of membrane-active peptides. We hypothesized that bilayers containing lipids with low phase transition temperatures would tend to exclude peptides and be less susceptible to peptide-induced perturbation than those made from higher temperature melting lipids. To test this hypothesis, we synthesized a series of asymmetric phospholipids that only differ in the type of fatty acid esterified at the sn-2 position of the lipid glycerol backbone. We tested the influence of acyl chain structure on peptide activity by measuring the kinetics of release from dye-encapsulated lipid vesicles made from these synthetic lipids. The results were compared to those obtained using vesicles made from S. aureus and Staphylococcus sciuri membrane lipid extracts. Anteiso-branched phospholipids, which melt at very low temperatures, produced lipid vesicles that were only slightly less susceptible to peptide-induced dye release than those made from the iso-branched isomer. However, liposomes made from bacterial phospholipid extracts were generally much more resistant to peptide-induced perturbation than those made from any of the synthetic lipids. The results suggest that the increase in the fraction of anteiso-branched fatty acids in antibiotic-resistant strains of S. aureus is unlikely to be the sole factor responsible for the observed increased antibiotic resistance. This article is part of a Special Issue entitled: Antimicrobial peptides edited by Karl Lohner and Kai Hilpert.