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

The 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 ofgraR andvraG (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 (mprF anddlt ) is dependent on the cotranscription of bothgraR andvraG , (ii) reduced expression ofmprF anddlt ingraR mutants was phenotypically associated with reduced surface-positive charge, (iii) this net reduction in surface-positive charge ingraR andvraG mutants, 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 ingraR andvraG mutants than in parental strains. Importantly, the inducibility ofmprF anddlt transcription via thegraRS-vraFG pathway 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). AlthoughgraR regulates expression ofvraG , the expression ofgraR was codependent on an intact downstreamvraG locus. Collectively, these data support an important role of thegraRS andvraFG loci in the sensing of and response to specific CAPs involved in innate host defenses.