Genetic alterations responsible for reduced susceptibility to vancomycin in community-associated MRSA strains of ST72.

Abstract

We previously reported the first case of vancomycin treatment failure due to development of vancomycin-intermediate resistance in a patient with an MRSA of ST72, a community genotype in Korea. We investigated two isogenic MRSA strains from this patient, who experienced treatment failure with vancomycin and rifampicin. We tracked the genetic alterations that confer reduced susceptibility to vancomycin on those two isogenic MRSA strains by WGS. Five non-synonymous mutations were identified, including rpoB (H481Y), dprA (G196C), femA (F92C), vraR (E127K) and agrC (E391stop). We further studied the role of a mutation of vraR in reduced susceptibility to vancomycin. Introduction of the mutated vraR (E127K) into a vancomycin-susceptible Staphylococcus aureus strain resulted in an increase in vraSR mRNA expression and vancomycin MIC and development of the hetero-VISA phenotype, which was confirmed by the population analysis profile (PAP)/AUC. Electron microscopy showed increased cell wall thickness in the strains with mutated vraR. Based on the genomic data, molecular experiments and PAP and cell wall analyses, we propose that a single mutation of vraR is associated with the reduced susceptibility to vancomycin in MRSA and further treatment failure.