walK and clpP Mutations Confer Reduced Vancomycin Susceptibility in Staphylococcus aureus

Vancomycin-intermediateStaphylococcus aureus (VISA) is generated from vancomycin-susceptibleStaphylococcus aureus by multiple spontaneous mutations. We previously reported that sequential acquisition of mutations in the two-component regulatory systemsvraSR andgraRS was responsible for the VISA phenotype of strain Mu50. Here we report on the identification of a novel set of regulator mutations, a deletion mutation in two-component regulatory systemwalRK (synonyms,vicRK andyycFG ), and a truncating mutation in a proteolytic regulatory gene,clpP , responsible for the raised vancomycin resistance in a laboratory-derived VISA strain, LR5P1-V3. The contributory effect of the two mutations to vancomycin resistance was confirmed by introducing thewalK andclpP mutations into the vancomycin-susceptible parent strain N315LR5P1 by a gene replacement procedure. The vancomycin MIC of N315LR5P1 was raised from 1 to 2 mg/liter by the introduction of thewalK orclpP mutation, but it was raised to 4 mg/liter by the introduction of both thewalK andclpP mutations. The vancomycin MIC value of the double mutant was equivalent to that of strain LR5P1-V3. Like VISA clinical strains, LR5P1-V3 and the double mutant strain LR5P1walK *clpP * exhibited a thickened cell wall, slow growth, and decreased autolytic activity. Transcriptional profiles of the mutants with gene replacements demonstrated that introduction of both thewalK andclpP mutations could alter expression of dozens or hundreds of genes, including those involved in cell envelope and cellular processes, intermediary metabolism, and information pathway. A mutation prevalence study performed on 39 worldwide clinical VISA strains showed that 61.5, 7.7, 10.3, and 20.5% of VISA strains harbored mutations inwalRK ,clpP ,graRS , andvraSR , respectively. The mutation ofwalRK was most frequently carried by VISA strains. Together, these results suggested that the mutations ofwalK andclpP identified in LR5P1-V3 constitute a new combination of genetic events causing vancomycin resistance inStaphylococcus aureus .