Genetic Characterization of <i>tet</i> (X4)-Positive Multidrug-Resistant <i>Escherichia coli</i> Isolated from Weaning Pigs in Thailand

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