Daptomycin Strongly Affects the Phase Behavior of Model Lipid Bilayers

Daptomycin (DAP) is a calcium-dependent cyclic lipopeptide with great affinity for negatively charged phospholipids bearing the phosphatidylglycerol (PG) headgroup and has been used since 2003 as a last resort antibiotic in the treatment of severe infections caused by Gram-positive bacteria. The first step of its mechanism of action involves the interaction with the bacterial membrane, which not only represents a physical barrier but also accommodates transmembrane proteins, such as receptors, transporters, and enzymes, whose activity is crucial for the survival of bacteria. This results in a less efficient development of resistance strategies by pathogens compared to common antibiotics that activate or inhibit biochemical pathways connected to specific target proteins. Although already on the market, the molecular mechanism of action of DAP is still a controversial subject of investigation and it is most likely the result of a combination of distinct effects. Understanding how DAP targets the membrane of pathogens could be of great help in finding its analogues that could better avoid the development of resistance. Here, exploiting fluorescence microscopy and atomic force microscopy (AFM), we demonstrated that DAP affects the thermodynamic behavior of lipid mixtures containing PG moieties. Regardless of whether the PG lipids are in the liquid or solid phase, DAP preferably interacts with this headgroup and is able to penetrate more deeply into the lipid bilayer in the regions where this headgroup is present. In particular, considering the results of an AFM/spectroscopy investigation, DAP appears to produce a stiffening effect of the domains where PG lipids are mainly in the fluid phase, whereas it causes fluidification of the domains where PG lipids are in the solid phase.