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Reduced susceptibility to vancomycin in isogenic Staphylococcus aureus strains of sequence type 59: tracking evolution and identifying mutations by whole-genome sequencing.

OBJECTIVES
Vancomycin-intermediate Staphylococcus aureus (VISA) and heterogeneous VISA (hVISA) phenotypes are increasingly reported in methicillin-resistant S. aureus (MRSA) strains of distinct genetic backgrounds. This study tracked genetic evolution during the development of vancomycin non-susceptibility in a prevalent Asian community-associated MRSA clone of sequence type (ST) 59.


METHODS
ST59 strains were consecutively isolated from a patient who failed chemotherapy for a septic knee over 15 months. The genetic mutations associated with the VISA phenotype were identified by whole-genome sequencing of two strains, which had the vancomycin-susceptible S. aureus (VSSA) and VISA phenotypes. The mutations were subsequently screened in other strains. By correlating the accumulated mutations with vancomycin susceptibility, genetic evolution was tracked at the whole-genome scale.


RESULTS
Nine non-synonymous mutations and two steps of genetic evolution were identified during the development of the VISA phenotype. The first step involved a nonsense mutation in agrC and point mutations at five other loci, which were associated with the VSSA-to-hVISA conversion. Mutations of rpoB and fusA following the use of rifampicin and fusidic acid were identified in the second step of evolution, which corresponded to the development of dual resistance to rifampicin and fusidic acid and the conversion of hVISA to VISA.


CONCLUSIONS
In vivo genetic evolution of S. aureus occurred in stepwise order during the development of incremental vancomycin non-susceptibility and was related to the use of antimicrobial agents.