Surface and Membrane Binding Properties of the Lipopeptide Daptomycin

Abstract

Daptomycin, an antimicrobial lipopeptide used to treat infections caused by Gram-positive bacteria that are resistant to many conventional therapies, acts through calcium-mediated binding to and rapid depolarization of the target bacterial membrane. Convincing evidence has recently been reported suggesting that small daptomycin oligomers form at the membrane surface and that these complexes represent the active state of the drug. Daptomycin's activity is closely correlated with the presence of phosphatidylglycerol (PG) in the target membrane. Although there have no cases of clinical resistance to daptomycin reported, troubling signs are emerging indicating that changes in lipid composition of bacterial membranes cause decreased susceptibility to the drug. It is therefore of interest to gain a more profound understanding of the details of daptomycin's mechanism of activity at the membrane level and the possible causes of potential resistance and their relationship to lipid composition. In the current study, we report on our investigation into the surface and membrane binding properties of daptomycin. From the Gibb's adsorption isotherm, we estimate the molecular area of daptomycin at the air-aqueous interface. Using Langmuir monolayers as membrane models, we also report limiting surface pressures and kinetics for daptomycin insertion to lipid films comprised of pure PG or PG-phosphatidylcholine mixtures. Finally, we attempt to correlate daptomycin's binding behavior in monolayers to that in bilayers, in the form of unilamellar vesicles, by presenting results from isothermal titration experiments. The results represent, for the first time, thermodynamic binding parameters for daptomycin-membrane interactions.