Linezolid resistance in Enterococcus faecium isolated in Ontario, Canada

Recent studies have described linezolid-resistant MRSA and vancomycin-resistant enterococci (VRE) occurring worldwide, including an outbreak of linezolid-resistant MRSA. The objective of this study was to determine if linezolid-resistant enterococci are present in clinical isolates in Ontario, Canada. From January 2010 to June 2012, all enterococcal isolates submitted to the Public Health Ontario Laboratory (PHOL) for confirmation of VRE and susceptibility testing were included in this study. Of 2829 enterococcal isolates tested, 12 Enterococcus faecium were found to be resistant to linezolid. All linezolid-resistant isolates were also resistant to ampicillin, ciprofloxacin, and vancomycin. In addition, 33% of isolates were non-susceptible to daptomycin, whereas 41% were resistant to quinupristin/dalfopristin. Molecular characterization of these isolates showed that 8/12 isolates (66.7%) contained the mutation G2576T in 23S rRNA, which has been associated with linezolid resistance. Amplification and sequencing of L3- and L4-coding genes did not reveal mutations associated with linezolid resistance. One isolate contained the cfr gene, which is associated with linezolid resistance, and has been found in staphylococcal species and E. faecalis. These data show that occurrence of linezolid resistance is still rare among enterococcal isolates referred to PHOL though detection of cfr in E. faecium is concerning as it has the potential to disseminate among other enterococci.