Mutated response regulator graR is responsible for phenotypic conversion of Staphylococcus aureus from heterogeneous vancomycin-intermediate resistance to vancomycin-intermediate resistance

Multistep genetic alteration is required for methicillin-resistant Staphylococcus aureus (MRSA) to achieve the level of vancomycin resistance of vancomycin-intermediate S. aureus (VISA). In the progression of vancomycin resistance, strains with heterogeneous vancomycin resistance, designated hetero-VISA, are observed. In studying the whole-genome sequencing of the representative hetero-VISA strain Mu3 and comparing it with that of closely related MRSA strains Mu50 (VISA) and N315 (vancomycin-susceptible S. aureus [VSSA]), we identified a mutation in the response regulator of the graSR two-component regulatory system. Introduction of mutated graR, designated graR*, but not intact graR, designated graRn, could convert the hetero-VISA phenotype of Mu3 into a VISA phenotype which was comparable to that of Mu50. The same procedure did not appreciably increase the vancomycin resistance of VSSA strain N315, indicating that graR* expression was effective only in the physiological milieu of hetero-VISA cell to achieve a VISA phenotype. Interestingly, the overexpression of graR* increased the daptomycin MICs in both Mu3 and N315 and decreased the oxacillin MIC in N315.