Dissemination of OXA-23 carbapenemase-producing and is driven by transposon-carrying lineages in the UK.

Carbapenem-resistant are a significant threat to global public health. Here, we characterize -positive (=8) and (=3) isolates from human clinical samples collected between 2021 and 2024 in the UK. Whole-genome sequencing (WGS) was used to generate data, and a phylogenetic tree inferred from SNPs filtered for recombination was constructed to assess the genomic relatedness among the isolates. To provide an international context, we included publicly available genomes. Short-read mapping to a reference genome enabled reconstruction of the genomic neighbourhood around . Minimum inhibitory concentration (MIC) determination was performed using broth microdilution and results interpreted using the European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines. UK isolates belonged to ST142 and formed a sub-clade descending from an ancestral cluster of French isolates with relatively few SNPs between them (9-39). ST38 isolates harboured and showed close genetic relatedness (12-15 SNPs) among themselves. In , was associated with transposon Tn, while harboured a novel composite transposon, designated Tn, bordered by two copies of IS and with three copies of . was integrated into the chromosome in all isolates. All isolates were resistant to amoxicillin/clavulanic acid (>32 mg l) and with meropenem MICs above the EUCAST screening cut-off (0.5-1 mg l). In conclusion, UK -positive isolates belong to the same clonal lineage (ST142) previously reported in Belgium, Germany, Switzerland and France, suggesting introduction of this lineage into the UK. This is the first report of an ST38 lineage with chromosomally encoded located within a novel transposon Tn. WGS plays an important role in identifying the mechanism(s) of transmission of emerging carbapenemase genes.