Genetic Characterization of tet(X4)-Positive Multidrug-Resistant Escherichia coli Isolated from Weaning Pigs in Thailand.

Tigecycline, a last-resort antibiotic against multidrug-resistant bacteria, has recently faced emerging resistance mediated by tet(X) genes. Livestock, particularly swine, are considered potential reservoirs for these resistance determinants. This study investigated the prevalence and molecular characteristics of tigecycline-resistant Escherichia coli in healthy weaning pigs in Thailand. Fecal samples were collected from healthy weaning pigs in 2024. Detection of tet(X) genes was performed using PCR. Among the 305 fecal samples, four were positive for the tet(X4) gene. A total of four tet(X)-positive E. coli isolates exhibited low-level resistance to tigecycline (minimum inhibitory concentration: 0.5-4 mg/L). Whole-genome sequencing analysis identified the tet(X4)-positive isolates as belonging to sequence types (STs) rST-153719, rST-59143, rST-45855, and rST-86073. Multiple antimicrobial resistance genes coexisting with tet(X4) were detected, including aph(6)-Id, aph(3'')-Ib, blaTEM-1B, blaTEM-215, sul2, floR, tet(A), and qnrS1. Plasmids carrying tet(X4) in the E. coli isolates were identified as IncN (in strains NO25 and T8) and IncR (in strains C2 and OS57). This study reports the first detection of tigecycline-resistant E. coli from weaning pigs in Thailand, raising significant concerns for both food animal production and public health.