Emergence of Linezolid Resistance Genes optrA and poxtA in an Avian Enterococcus asini

Linezolid represents a critical last-resort treatment for severe multidrug-resistant (MDR) Gram-positive bacterial infections. Rising linezolid resistance in Enterococcus isolates threatens its efficacy; this study characterized the molecular features and transfer potential of plasmid-encoded linezolid resistance genes optrA and poxtA in a linezolid-resistant Enterococcus asini isolate from chickens. An E. asini strain was isolated during a surveillance program focusing on drug-resistant Gram-positive bacteria in poultry. PCR screened linezolid resistance genes, conjugation and plasmid stability assays evaluated gene transferability and stability, and whole-genome sequencing (WGS) was performed using both the Illumina and Nanopore platforms. We present the first detection of optrA and poxtA genes in E. asini recovered from chicken feces in China. Sequence analysis of the complete genome showed that poxtA and optrA were situated on two distinct plasmids. The poxtA positive plasmid, pHNGXN23C145Ea-1, also carried multiple resistance genes, including tet(S), fexB, erm(B), ant(6)-Ia, aph(3′)-III. Furthermore, the poxtA gene was flanked by IS1216E mobile elements. The optrA bearing plasmid, pHNGXN23C145Ea-2, harbours a common genetic array of ‘IS1216E fexA-optrA-erm(A)-IS1216E’. Conjugation experiments indicated that neither the poxtA- nor the optrA-bearing plasmid was transferred to recipient strains, which was consistent with sequence analysis showing that both plasmids lacked intact conjugative transfer regions. Stability assays confirmed that poxtA and optrA remained highly stable in the absence of selective pressure. Notably, this discovery was made in a livestock sample, despite the non-use of linezolid in food animals, suggesting that such niches may act as silent reservoirs for resistance genes, which could persist and potentially transfer to clinically relevant MDR pathogens.