Purpose: To investigate the potential mechanism and molecular characteristics of linezolid-non-sensitive Enterococcus faecium  from a tertiary hospital in southwest China and characterize the relevant plasmids.
Patients and Methods: Linezolid-non-sensitive Enterococcus faecium  (LNSEFM) isolates collected from January 2014 to December 2018 were screened for resistant genes 23s rRNA, rplC, rplD, rplV, optrA, cfr, poxtA , by PCR. Molecular epidemiological analysis was performed by multilocus sequence typing (MLST). The optrA -and-poxtA  co-harboring strain EFM_7150  was subjected to the whole genome sequencing (WGS) by Illumina HiSeq and Oxford Nanopore MinION.
Results: A total of 15 LNSEFM with linezolid MICs ranging from 4 to 16 mg/L were identified. About 66.7% (10/15) of isolates were linezolid-resistant. About 46.7% (7/15) of strains were positive for optrA . Two types of optrA  variants (P and EYDNDM) were identified. About 13.3% (2/15) of isolates had poxtA . 1 harbored a L22 protein alteration (Ser77Thr). One isolate coharbored optrA  (EYDNDM variant) and poxtA . There was no mutation in the gene that encoded the ribosomal protein L3 /L4  or the domain V of 23S rRNA. No cfr  gene was detected. Based on WGS data, optrA  was associated with Tn558 inserted to radC  gene and poxtA  was flanked by IS1216E.
Conclusion: OptrA  is primary mechanism in linezolid-resistant Enterococcus faecium . This is the first report of optrA  variants P and EYDNDM identified in Enterococcus faecium  and optrA -and-poxtA  co-harboring Enterococcus faecium  clinically in southwest China. Besides, Tn558 and IS1216Es may play an important role in the dissemination of optrA  and poxtA , respectively. The findings revealed the potential threat to nosocomial infection by optrA  and coexistence of optrA  and poxtA  in Enterococcus faecium . Thus, clinical surveillance of linezolid-resistant Enterococcus  is urgently needed.
Keywords: optrA , poxtA , linezolid-non-sensitive Enterococcus faecium , WGS