Lipid Extracting Effect of Daptomycin Correlated to its Action on Bacterial Cell Membranes

Abstract

Daptomycin is the first approved member of a new structural class of antibiotics, the cyclic lipopeptides. The peptide interacts with the lipid matrix of cell membranes, inducing membrane permeability to ions, but its molecular mechanism has been a puzzle. Unlike the ubiquitous membrane-acting host-defense antimicrobial peptides, daptomycin does not induce pores in the cell membranes. Thus how it affects the membrane permeability to ions is unclear. We studied its interaction with giant unilamellar vesicles (GUVs) and discovered a lipid-extracting phenomenon that correlates with the direct action of daptomycin on bacteria membranes observed in a recent fluorescence-microscopic study. Lipid extraction occurred only when the GUV lipid composition contained phosphatidylglycerol (PG) and in the presence of Ca2+ ions. Lipid extraction did not happen when Ca2+ was replaced by other divalent ions such as Mg2+ or PG was substituted by cardiolipin, another main component in bacterial membrane. With daptomycin, DOPG and Ca2+, we found Ca2+ permeating into the GUV, while a content dye, Texas red dextran, did not leak out. This result suggests that daptomycin and Ca2+ do not form pores in the membrane of DOPG-contained GUV, but cause leakage of ions. Furthermore, the lipid extraction effect occurred only when the peptide-lipid ratio exceeded a threshold value. This threshold value explains the order of the magnitude of the minimum inhibitory concentration of daptomycin. Previously tea catechin, Epigallocatechin gallate (EGCg) and human islet amyloid polypeptide (hIAPP) have also been noted for their lipid extracting effects and their bactericidal or cytotoxic activities. The correlated membrane activities of EGCg, hIAPP and daptomycin suggest a causal relationship between the lipid extracting effect and membrane permeability to ions, which represents a so-far unrecognized antibacterial mechanism.