Emergence and Characterization of the High-Level Tigecycline Resistance Gene <i>tet</i> (X4) in <i>Salmonella enterica</i> Serovar Rissen from Food in China

The plasmid-mediated <i>tet</i>(X4) gene has exhibited a high-level resistance to tigecycline (TGC), which has raised concerns globally regarding antibiotic resistance. Although the widespread <i>tet</i>(X4) has been found widely in <i>Escherichia coli</i>, it is scarcely found in other Enterobacteriaceae. This study aimed to characterize a ST469 <i>Salmonella enterica</i> serovar Rissen (<i>S.</i> Rissen) isolate harboring <i>tet</i>(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 <i>S.</i> Rissen isolates of 223 <i>Salmonella</i> spp. isolates were isolated from food samples in China during 2021-2023. One of 10 <i>S</i>. Rissen isolates, SM2301, carrying <i>tet</i>(X4) conferred high-level resistance to TGC (minimum inhibitory concentration > 8 µg/mL). The <i>tet</i>(X4) could be conjugated into different recipients, including <i>E. coli</i>, <i>S. enteritidis</i>, and <i>K. pneumoniae</i> isolates. Plasmid curing confirmed that <i>tet</i>(X4) was plasmid-mediated. Genetic analysis revealed that the <i>tet</i>(X4) in the SM2301 isolate was located in the IncFIA(HI1)-IncHI1A-IncHI1B(R27) hybrid plasmid, and the structure of <i>tet</i>(X4) was <i>abh</i>-<i>tet</i>(X4)-IS<i>CR2</i>. To the best of our knowledge, this is the first report of a <i>tet</i>(X4)-positive food-derived <i>S. Rissen</i> isolate. The extending bacterial species of <i>tet</i>(X4)-bearing plasmids suggested the increasing transmission risk of the mobile TGC resistance gene <i>tet</i>(X4) beyond <i>E. coli</i>. 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 <i>tet</i>(X4) in <i>Salmonella</i> spp. and other bacterial species.