Enterococcus faecium harbouring vanA, optrA and cfr(D) on a linear plasmid in Canada: a CANWARD surveillance case

Abstract

Objectives: VRE resistant to potential therapeutic agents such as linezolid are an increasing concern in Canada and worldwide. Infections produced by these isolates have limited treatment options and are associated with poor clinical outcomes. We report an isolate of vancomycin-resistant Enterococcus faecium (VREfm); identified through the Canadian surveillance program, CANWARD, which exhibited linezolid resistance (LVREfm); and carried the optrA and cfr(D) genes on a linear plasmid.

Methods: Antimicrobial susceptibility testing was performed by broth microdilution and MICs were interpreted by 2025 CLSI breakpoints. WGS was performed to characterize resistance determinants and the associated plasmid.

Results: In 2021, a single LVREfm isolate was recovered from a blood culture of a 67-year-old male patient admitted to an ICU. The isolate was XDR but remained susceptible dose dependent to daptomycin (MIC = 2 mg/L). MLST identified the isolate as ST817, a sequence type not commonly found in our national collection.Vancomycin resistance was mediated by the vanA gene cluster, while linezolid resistance determinants included optrA, cfr(D) and the 23S rRNA gene mutation, G2576T. Vancomycin and linezolid resistance genes were co-located on a linear plasmid that exhibited over 94% sequence identity to previously described linear plasmids from India and the USA. Notably, the plasmid also encoded four toxin-antitoxin systems, potentially contributing to its stability and persistence.

Conclusions: This case highlights the appearance of linezolid resistance in VRE mediated by linear plasmids in Canada and underscores the importance of ongoing genomic surveillance to track the dissemination of MDR determinants in clinical settings.