Abstract: ObjectiveThe methicillinresistant  Staphylococcus aureus (MRSA) with reduced susceptibility  to vancomycin which were selected  during  mutant prevention concentration(MPC) determination were characterized to compare the changes  in  the  graS  gene  and  cell  ultrastructure.MethodsThe strains selected  were characterized by pulsedfield gel  electrophoresis  (PFGE) and SauPCR  assay. The graS gene was detected by polymerase chain reaction method; Denaturing gradient gel electrophoresis (DGGE) was used to probe for mutations in the graS gene. The electron  microscopy was used to compare the differences on cell   wall.ResultsCompared with the parental strains， some strains with reduced susceptibility to vancomycin had different DNA profiles by SauPCR assay and PFGE. Electron  microscopy showed  that  the mutant  strains had  slightly  thicker cell walls  than  those of the wildtype  strains. DDGE was used to probe for mutations  in  the graS gene， but no different banding patterns were observed on the electrophoretogram  spectrum. ConclusionThe strains with reduced susceptibility to vancomycin were not caused by genetic mutations of graS， there may be mutations in genes other than the  graS gene or existence of other resistant mechanisms.

Key words: antibacterial agents, Staphylococcus aureus；drug resistance, bacterial