The Staphylococcus aureus Two-Component Regulatory System, GraRS, Senses and Confers Resistance to Selected Cationic Antimicrobial Peptides

Abstract

ABSTRACTThe two-component regulatory system, GraRS, appears to be involved in staphylococcal responses to cationic antimicrobial peptides (CAPs). However, the mechanism(s) by which GraRS is induced, regulated, and modulated remain undefined. In this study, we used two well-characterized MRSA strains (Mu50 and COL) and their respective mutants ofgraRandvraG(encoding the ABC transporter-dependent efflux pump immediately downstream ofgraRS), and show that (i) the expression of two key determinants of net positive surface charge (mprFanddlt) is dependent on the cotranscription of bothgraRandvraG, (ii) reduced expression ofmprFanddltingraRmutants was phenotypically associated with reduced surface-positive charge, (iii) this net reduction in surface-positive charge ingraRandvraGmutants, in turn, correlated with enhanced killing by a range of CAPs of diverse structure and origin, including those from mammalian platelets (tPMPs) and neutrophils (hNP-1) and from bacteria (polymyxin B), and (iv) the synthesis and translocation of membrane lysyl-phosphatidylglycerol (anmprF-dependent function) was substantially lower ingraRandvraGmutants than in parental strains. Importantly, the inducibility ofmprFanddlttranscription via thegraRS-vraFGpathway was selective, with induction by sublethal exposure to the CAPs, RP-1 (platelets), and polymyxin B, but not by other cationic molecules (hNP-1, vancomycin, gentamicin, or calcium-daptomycin). AlthoughgraRregulates expression ofvraG, the expression ofgraRwas codependent on an intact downstreamvraGlocus. Collectively, these data support an important role of thegraRSandvraFGloci in the sensing of and response to specific CAPs involved in innate host defenses.