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An isolate of the methicillin-resistant  Staphylococcus aureus  (MRSA) clone USA300 with reduced susceptibility to vancomycin (SG-R) (i.e, vancomycin-intermediate  S. aureus , VISA) and its susceptible “parental” strain (SG-S) were recovered from a patient at the end and at the beginning of an unsuccessful vancomycin therapy. The VISA phenotype was unstable  in vitro  generating a susceptible revertant strain (SG-rev). The availability of these 3 isogenic strains allowed us to explore genetic correlates of antibiotic resistance as it emerged  in vivo . Compared to the susceptible isolate, both the VISA and revertant strains carried the same point mutations in  yycH ,  vraG ,  yvqF  and  lspA  genes and a substantial deletion within an intergenic region. The revertant strain carried a single additional frameshift mutation in  vraS  which is part of two component regulatory system VraSR. VISA isolate SG-R showed complex alterations in phenotype: decreased susceptibility to other antibiotics, slow autolysis, abnormal cell division and increased thickness of cell wall. There was also altered expression of 239 genes including down-regulation of major virulence determinants. All phenotypic properties and gene expression profile returned to parental levels in the revertant strain.  Introduction  of wild type  yvqF  on a multicopy plasmid into the VISA strain caused loss of resistance along with loss of all the associated phenotypic changes.  Introduction  of the wild type  vraSR  into the revertant strain caused recovery of VISA type resistance. The  yvqF/vraSR  operon seems to function as an on/off switch: mutation in  yvqF  in strain SG-R turns on the  vraSR  system, which leads to increase in vancomycin resistance and down-regulation of virulence determinants. Mutation in  vraS  in the revertant strain turns off this regulatory system accompanied by loss of resistance and normal expression of virulence genes. Down-regulation of virulence genes may provide VISA strains with a “stealth” strategy to evade detection by the host immune system.

Genetic Pathway in Acquisition and Loss of Vancomycin Resistance in a Methicillin Resistant Staphylococcus aureus (MRSA) Strain of Clonal Type USA300