The ISVsa3-ORF2-abh-tet(X4) circular intermediate-mediated transmission of tigecycline resistance in Escherichia coli isolates from duck farms

Tigecycline is a last-resort drug used to treat serious infections caused by multidrug-resistant bacteria. <i>tet</i>(X4) is a recently discovered plasmid-mediated tigecycline resistance gene that confers high-level resistance to tigecycline and other tetracyclines. Since the first discovery of <i>tet</i>(X4) in 2019, it has spread rapidly worldwide, and as a consequence, tigecycline has become increasingly ineffective in the clinical treatment of multidrug-resistant infections. In this study, we identified and analyzed <i>tet</i>(X4)-positive <i>Escherichia coli</i> isolates from duck farms in Hunan Province, China. In total, 976 samples were collected from nine duck farms. Antimicrobial susceptibility testing and whole-genome sequencing (WGS) were performed to establish the phenotypes and genotypes of <i>tet</i>(X4)-positive isolates. In addition, the genomic characteristics and transferability of <i>tet</i>(X4) were determined based on bioinformatics analysis and conjugation. We accordingly detected an <i>E. coli</i> strain harboring <i>tet</i>(X4) and seven other resistance genes in duck feces. Multi-locus sequence typing analysis revealed that this isolate belonged to a new clone, and subsequent genetic analysis indicated that <i>tet</i>(X4) was carried in a 4608-bp circular intermediate, flanked by IS<i>Vsa3</i>-ORF2-<i>abh</i> elements. Moreover, it exhibited transferability to <i>E. coli</i> C600 with a frequency of 10^-5. The detection of <i>tet</i>(X4)-harboring <i>E, coli</i> strains on duck farms enhances our understanding of tigecycline resistance dynamics. The transferable nature of the circular intermediate of <i>tet</i>(X4) contributing to the spread of tigecycline resistance genes poses a substantial threat to healthcare. Consequently, vigilant monitoring and proactive measures are necessary to prevent their spread.