Novel transposon Tn <i>8026</i> acts as a global driver of transmissible linezolid resistance in <i>Enterococcus</i> via a linear plasmid

Linezolid is a critical last-resort antimicrobial for multidrug-resistant <i>Enterococcus faecium</i> , particularly against vancomycin-resistant lineages where therapeutic options are severely limited. While resistance has historically arisen through <i>de novo</i> chromosomal mutations, the global emergence of transferable resistance mechanisms threatens to render more infections untreatable. Here, we characterise a recent (2023-2024) hospital-associated outbreak of linezolid-resistant <i>E. faecium</i> in Queensland, Australia. Although the cohort comprised a variety of sequence types, the outbreak was primarily driven by the clonal expansion of an ST80 lineage carrying the plasmid-borne <i>poxtA-Ef</i> gene. Standard short-read genomic surveillance failed to resolve the genetic context of the resistance determinant. However, long-read sequencing revealed that <i>poxtA-Ef</i> was carried within a novel transposon, Tn <i>8026</i> , situated on a linear plasmid. Structural analysis defined Tn <i>8026</i> as a unique element flanked by IS <i>1678</i> and the novel insertion sequence IS <i>Efa26</i> . Furthermore, we identified an instance of Tn <i>8026</i> integration into the chromosome, providing functional evidence of its mobility and capacity for stabilisation within the genome. Global genomic screening demonstrated that Tn <i>8026</i> significantly predates the local outbreak, identified in a historical Norwegian isolate from 2012, indicating a long-standing yet unrecognised global reservoir. Phylogenomic analysis provided strong evidence that the linear plasmid was imported from the Indian subcontinent, initiating a chain of silent dissemination in eastern Australia where the lineage circulated undetected prior to clinical recognition. Crucially, we also confirmed the presence of the linear plasmid in <i>Enterococcus gallinarum</i> , demonstrating its capacity to mobilise transmissible linezolid resistance across enterococcal species boundaries. These findings emphasise the need for detailed long-read-based surveillance of mobile genetic elements, with a particular focus on identifying linear plasmids that are often overlooked.