Emergence and Characterization of the High-Level Tigecycline Resistance Gene (X4) in Serovar Rissen from Food in China.

The plasmid-mediated (X4) gene has exhibited a high-level resistance to tigecycline (TGC), which has raised concerns globally regarding antibiotic resistance. Although the widespread (X4) has been found widely in , it is scarcely found in other Enterobacteriaceae. This study aimed to characterize a ST469 serovar Rissen ( Rissen) isolate harboring (X4) from pork, which was identified and characterized via antimicrobial susceptibility testing, conjugation assays, plasmid curing testing, whole-genome sequencing, and bioinformatic analysis. Ten ST469 Rissen isolates of 223 spp. isolates were isolated from food samples in China during 2021-2023. One of 10 . Rissen isolates, SM2301, carrying (X4) conferred high-level resistance to TGC (minimum inhibitory concentration > 8 µg/mL). The (X4) could be conjugated into different recipients, including , , and isolates. Plasmid curing confirmed that (X4) was plasmid-mediated. Genetic analysis revealed that the (X4) in the SM2301 isolate was located in the IncFIA(HI1)-IncHI1A-IncHI1B(R27) hybrid plasmid, and the structure of (X4) was -(X4)-IS. To the best of our knowledge, this is the first report of a (X4)-positive food-derived isolate. The extending bacterial species of (X4)-bearing plasmids suggested the increasing transmission risk of the mobile TGC resistance gene (X4) beyond . This study highlights the emerging and evolution risk of novel resistance genes across various bacterial species. Therefore, further surveillance is warranted to monitor the prevalence of (X4) in spp. and other bacterial species.