Reduced Susceptibility to Host-Defense Cationic Peptides and Daptomycin Coemerge in Methicillin-Resistant Staphylococcus aureus From Daptomycin-Naive Bacteremic Patients

Background. We hypothesized that, for methicillin-resistant Staphylococcus aureus (MRSA), in vitro daptomycin susceptibility could be influenced by exposures to endogenous host defense peptides (HDPs) prior to clinical exposure to daptomycin. Methods. Two endovascular HDPs were used: thrombin-induced platelet microbicidal protein (tPMP) and human neutrophil defensin-1 (hNP-1) from neutrophils. Forty-seven unique MRSA isolates obtained from bacteremic patients in multicenter prospective clinical trials were studied. Clinical characteristics, microbiologic parameters, prior vancomycin therapy, and susceptibilities to tPMP, hNP-1, and daptomycin were compared using univariate and multivariate analyses. Results. All strains were daptomycin susceptible. Daptomycin minimum inhibitory concentrations (MICs) were inversely related to in vitro tPMP (but not hNP-1) killing. Strains with a daptomycin MIC of 1 mg/L exhibited significantly less killing by tPMP, compared with strains with an MIC of < 0.5 mg/L. Prior vancomycin therapy did not influence this relationship. Regression tree modeling confirmed that reduced tPMP-induced killing in vitro was the strongest predictor of higher daptomycin MICs within the daptomycin-susceptible range. Conclusions. Among daptomycin-susceptible MRSA isolates from patients who had never received daptomycin, higher daptomycin MICs tracked with increased resistance to killing by platelet-derived but not neutrophil-derived HDPs. These findings support the notion that endogenous exposure of MRSA to specific HDPs may play a role in selecting strains with an intrinsically higher daptomycin MIC phenotype.