Objective      To investigate high-level linezolid resistance-associated mutation and copy number of mutation gene in domain V region of 23S rRNA through inducing Enterococcus faecalis (E. faecalis) with linezolid in vitro.       Methods      The linezolid resistance was assessed in 5 strains of E. faecalis that had minimum inhibitory concentration (MIC) ranging from 4 μg/mL to 16 μg/mL and were induced with linezolid in vitro. After induction with linezolid

PCR and DNA sequencing were used to detect the mutations in 23S rRNA gene and genes encoding L3 and L4 ribosomal proteins in the induced strains and control strain E. faecalis ATCC29212. The copy number of the mutation gene was determined by DNA sequencing.       Results      After serial passages in the presence of linezolid in vitro

the MIC of the 5 strains were successfully increased by 8 to 32 times

and the maximum MIC was more than 256 μg/mL. The G2576T mutation was detected in 3 strains

while all possible mutations were not found in the domain V region of 23S rRNA gene in the other 2 strains. There were no mutations in the genes encoding L3 and L4 proteins in the strains.       Conclusion      Linezolid can rapidly induce the MIC increase in E. faecalis in vitro

and the principal mechanism may be associated with G2576T mutation in the domain V region of 23S rRNA gene. The copy number of mutation is positively correlated with MIC elevation.