Abstract
Clinical treatment options for daptomycin (DAP)-resistant (DAP-R), methicillin-resistant Staphylococcus aureus (MRSA) infections are relatively limited. Current therapeutic strategies often take advantage of potential synergistic activity of DAP plus β-lactams; however, the mechanisms underlying their combinatorial efficacy are likely complex and remain incompletely understood. We recently showed that in vitro β-lactam passaging can resensitize DAP-R strains to a DAP-susceptible (DAP-S) phenotype. To further investigate the implications of selected β-lactam pretreatments on DAP plus β-lactam combination efficacy, we utilized DAP-R strain D712. We studied six such combinations, featuring β-lactams with a broad range of penicillin-binding protein-targeting profiles (PBP-1 to -4), using DAP-R strain D712. Of note, preconditioning with each β-lactam antibiotic (sequential exposures), followed by DAP exposure, yielded significantly enhanced in vitro activity compared to either DAP treatment alone or simultaneous exposures to both antibiotics. To explore the underpinnings of these outcomes, proteomic analyses were performed, with or without β-lactam preconditioning. Relative proteomic quantitation comparing β-lactam pretreatments (versus untreated controls) identified differential modulation of several well-known metabolic, cellular, and biosynthetic processes, i.e., the autolytic and riboflavin biosynthetic pathways. Moreover, these differential proteomic readouts with β-lactam preconditioning were not PBP target specific. Taken together, these studies suggest that the cellular response to β-lactam preconditioning in DAP-R MRSA leads to distinct and complex changes in the proteome that appear to resensitize such strains to DAP-mediated killing.