Linezolid resistance in immunocompromised patients: Co-occurrence of optrA and fexA genes harbored by a novel plasmid in Enterocococus faecalis ST300.

Aims: Linezolid resistance in Gram-positive cocci has been reported in clinical settings but it is still a rare phenomenon. This study investigated the molecular epidemiology of linezolid-resistant Enterococcus faecalis strains and the genetic environment of linezolid-resistance genes, in immunocompromised patients.

Methods and results: This study investigated two linezolid-resistant Enterococcus faecalis strains (Minimal inhibitory concentration = 16 mg/L). These colonizing strains were isolated from patients at Tunisia's National Bone Marrow Transplant Center. Both strains exhibited a multidrug resistant phenotype. Whole-genome sequencing revealed the co-occurrence of optrA and fexA resistance genes, for the first time, in ST300 and ST861 Enterococcus faecalis. Concerning the ST300 strain, optrA and fexA were located on a novel plasmid pS1987-optrA sharing high similarity with plasmids from clinical isolates in Japan and the Netherlands. Comparative genomics highlighted genetic diversity in the optrA region among geographically linked isolates, suggesting plasmid-mediated spread.

Conclusion: This study reported, for the first time, the co-occurrence of optrA and fexA genes in linezolid-resistant Enterococcus faecalis ST300 and ST861, harbored by a novel plasmid in the ST300 strain.