Emergence of Linezolid Resistance Associated with 23S rRNA G2576T Mutations in Vancomycin-Resistant Enterococcus faecium during Oxazolidinone Therapy.

Linezolid is a key option for vancomycin-resistant enterococci (VRE), yet resistance can emerge during therapy. We aimed to characterize the emergence of linezolid resistance in vanA-positive Enterococcus faecium using serial isolates and genomic analysis. Serial VRE isolates obtained during persistent bacteremia after allogeneic hematopoietic stem cell transplantation underwent broth microdilution MIC testing and short- and long-read whole-genome sequencing. Mutations in 23S rRNA domain V and ribosomal protein genes (rplC/rplD) and acquired oxazolidinone resistance genes (cfr, optrA, poxtA) were assessed. The four isolates subjected to genomic analysis had linezolid MICs of 1, 2, 4, and 8 μg/mL, respectively, over ∼2 months of oxazolidinone exposure. G2576T mutations were detected in two of six 23S rRNA gene copies in both V131 and V2271, involving different rrl loci. No G2505A mutation, previously reported amino acid alterations in L3/L4 encoded by rplC/rplD, or acquired resistance genes (cfr, optrA, poxtA) were identified. Higher linezolid MICs in the later isolates were temporally associated with the detection of G2576T mutations in 23S rRNA genes, while the other investigated resistance determinants were not identified. These findings document the emergence of linezolid resistance during prolonged oxazolidinone exposure and suggest that serial susceptibility testing during therapy may be important for early detection of emerging resistance and timely clinical management.