Retail pork markets as underrecognized hubs of tigecycline resistance: Genomic epidemiology of tet(X4)-harboring Escherichia coli and global ST195 dissemination.

Antimicrobial resistance in the food chain threatens public health, yet the tigecycline resistance gene tet(X4) remains under-characterized in retail pork markets. Here, we investigated the presence, genetic features, and dissemination of tet(X4)-harboring bacteria in retail pork markets, systematically analyzed tet(X4)-positive strains globally and further assessed the global spread dynamics of the epidemic Escherichia coli ST195 lineage, a common ST type harboring tet(X4). We collected 109 samples (pork, processing tools, and sewage water) from three retail pork markets in Heyuan, Guangdong, China. In total, 26 samples tested positive for tet(X4) (23.85%, 26/109), with the highest positivity in processing tools (38.10%, 16/42), followed by pork (18.18%, 6/33) and sewage water (11.76%, 4/34). From these positive samples, we isolated 26 tet(X4)-positive E. coli spanning 16 sequence types, including ST3321 and ST195. Antimicrobial susceptibility testing against 17 agents showed that all isolates were multidrug-resistant, with the highest resistance rates to tigecycline, tetracycline, and florfenicol. Whole-genome sequencing and comparative genomics indicated that IncX1 plasmids were predominant carriers of tet(X4) and displayed high structural similarity across retail sources and global isolates, suggesting transnational dissemination. Core-genome SNP analysis showed that the two retail-derived ST195 isolates were closely related to 76 tet(X4)-positive ST195 genomes in GenBank (5-29 SNPs). Bayesian phylogenetic and phylodynamic analyses further suggested that tet(X4)-positive ST195 emerged in the mid-2000s, with swine acting as a key convergence host and suspected bidirectional transmission between China and Thailand (Bayesian factors = 4735.04 and 5.63), contributing to regional and cross-border spread. Together, our findings highlight retail pork markets in southern China as potential reservoirs and dissemination nodes for tet(X4)-harboring bacteria, supporting strengthened hygiene and One Health-integrated surveillance to mitigate foodborne antimicrobial resistance risks.