LAUSR

The emergence of vancomycin-resistant Staphylococcus aureus (VRSA) threatens global health. The mechanism of vancomycin resistance of VRSA without vanA gene acquisition was not fully elucidated. Therefore, we aimed to determine the mechanism of vancomycin resistance of VRSA besides that by vanA gene acquisition. In this study, we obtained vancomycin-resistant strains (V036-V64; MIC = 64 µg /ml) from susceptible strain (V036; MIC = 0.5 µg /ml) by exposure of vancomycin in vitro and examined the phenotypic characteristics and antibiotic susceptibility profiles of the resistant strain (V036-V64). To identify the genetic variations caused vancomycin resistance, we determined the complete genome sequences of V036 and V036-V64 and analyzed for single-nucleotide polymorphisms (SNPs) between two strains. Morphologically, V036-V64 had a twofold thicker cell wall compared with V036. Linezolid, rifampicin, and ceftaroline had similar MIC ranges against V036-V64 and V036, but V036-V64 showed lower susceptibilities to daptomycin and telavancin. We detected eight single-nucleotide polymorphisms differing between V036-V64 and V036: rimM (G16D), ssaA2 (G128A), rpsK (P60R), rpoB (R917C), walK (T492R), d-alanyl-d-alanine carboxypeptidase (L307I), vraT (A152V), and chromosome segregation ATPase (T440I). This study demonstrates that, under selective pressure, by the accumulation of mutations in genes related to cell wall synthesis, vancomycin-susceptible S. aureus can develop thicker cell walls and, hence, develop high vancomycin resistance. Thus, we highlight a novel vanA-negative mechanism for VRSA emergence.