Detection and genomic characterization of tet(X4)-positive Escherichia coli in retail pig products in Xuancheng, China

Tigecycline is considered a last-resort antibiotic against multidrug-resistant Gram-negative bacteria. The emergence of the mobile resistance gene tet (X4), which confers high-level resistance to tigecycline, has raised growing concern due to its rapid dissemination among humans, animals, and the environment. To determine the occurrence and genomic characteristics of tet (X4)-positive Escherichia coli in retail pig products from Xuancheng and assess their potential public health implications. Forty-nine retail pig-derived samples collected from supermarkets and farmers’ markets were enriched and cultured on MacConkey agar supplemented with tigecycline (2 mg/L). Colonies were identified by MALDI-TOF MS and tet (X) genes were detected by PCR, followed by sequencing confirmation. Antimicrobial susceptibility testing and whole-genome sequencing were performed to characterize resistance profiles, MLST, plasmid replicons, and tet (X4) genetic structures. Core-genome SNP phylogenomic analysis was performed to assess the genetic relatedness of the study isolates to publicly available E. coli genomes belonging to the same sequence types (STs). Twelve tet (X4 ) -positive E. coli isolates were recovered from 49 retail pig-derived products (24.5%, 12/49). All isolates were resistant to tigecycline, and 11 exhibited multidrug resistance. The isolates frequently co-harbored bla TEM−1B , tet (A), floR , qnrS1 , and sul3 . MLST assigned the isolates to ten distinct STs, including globally disseminated lineage (e.g., ST10) and regionally clustered lineages (e.g., ST2144). Six study isolates showed close genomic relatedness to publicly available same-ST E. coli genomes from diverse geographical regions and hosts, whereas other isolates showed greater within-ST genomic divergence, indicating heterogeneity in the genomic backgrounds of the analyzed lineages. The tet (X4) genes were embedded in conserved “IS CR2 – catD – tet (X4)–IS CR2 ” modules with structural variations associated with IS 26 or IS 1 elements. Tet(X4)-positive E. coli were detected among retail pig-derived products in Xuancheng and occurred across diverse STs with multiple resistance genes. These findings indicate that tet (X4) is present in genetically diverse E. coli at the retail level and warrant continued surveillance to better understand its distribution and potential public health implications.