Novel ST3871 Escherichia coli exhibits diverse transmission modes, carrying the tet(X4) resistance gene.

Although tigecycline is strictly restricted for use in animals, plasmid-mediated tet(X) is increasingly being detected in swine and poultry production, which may be associated with the highly intensive farming model and the use of antibiotics as feed additives. Escherichia coli, as one of the important reservoirs of tet(X), has attracted worldwide attention due to the complex plasmid background and the diversified mobile genetic elements (MGEs). This study isolated nine tet(X)-positive E. coli belonging to a novel sequence type (ST), ST3871, from a commercial swine farm in Hebei, China. Phylogenetic analysis and pulsed-field gel electrophoresis (PFGE) demonstrated that clonal transmission drove the farm-level prevalence of these strains. The ST3871 tet(X)-positive E. coli exhibited resistance to multiple antibiotics and carried various resistance genes. Whole-genome sequencing (WGS) and southern blotting confirmed that these strains carried the plasmid-mediated tet(X4) gene. Conjugation assays revealed that tet(X4)-positive plasmids could be transferred into E. coli and Salmonella. Although the limited sampling range of this study limits further epidemic analysis, the emergence of tet(X)-mediated tigecycline resistance in a novel ST of E. coli undeniably confirms that the tet(X) gene has resided in a wide variety of ST clones, highlighting the significant contribution of E. coli to the dissemination of tet(X).IMPORTANCEE. coli carrying tet(X) has spread globally, with the most extensive distribution observed in Asia. This study revealed the prevalence of a novel ST3871 E. coli carrying the plasmid-mediated tet(X4) gene on a commercial swine farm in Hebei, China, indicating that the tet(X) gene, particularly plasmid-mediated tet(X), has been distributed across a wide variety of E. coli ST clones. This undoubtedly poses a threat to public health, necessitating comprehensive strategies and continuous monitoring to control it.