Identification of atypical Escherichia coli as hidden reservoir of tet(X4)-positive IncX1 plasmid.

The horizontal dissemination of antimicrobial resistance genes (ARGs) poses a significant threat to public health. In this study, we investigated the presence and transferability of a tet(X4)-positive IncX1 plasmid in an atypical Escherichia coli isolate, GX-3-49. Genomic analysis revealed that GX-3-49 is markedly distinct from conventional E. coli, characterized by a reduced gene repertoire, fewer toxin genes, and a distinct phylogenetic lineage. Phenotypic characterization further demonstrated that, compared to the reference strain EC25922, GX-3-49 exhibits diminished motility, growth, and virulence, alongside a marked increase in biofilm formation and pH tolerance, but greater sensitivity to high temperature. These findings suggested that GX-3-49 might have evolved a fitness trade-off, reducing its virulence in exchange for enhanced resilience under acidic and biofilm-forming conditions. Additionally, a tet(X4)-positive IncX1 plasmid co-harboring blaSHV-12, tet(A), and floR, was identified in GX-3-49 and remained stable during serial passaging. This plasmid exhibited high sequence similarity to IncX1 plasmids previously identified in E. coli and Klebsiella pneumoniae and was capable of conjugative transfer to E. coli EC600, suggesting its potential for interspecies dissemination. In conclusion, this study identified a stable and transferable tet(X4)-positive IncX1 plasmid in an atypical E. coli strain exhibiting reduced virulence and increased susceptibility to heat, yet enhanced stress resilience under specific environmental conditions. These findings highlight the potential of such bacteria to serve as hidden reservoirs for antimicrobial resistance and facilitate the dissemination of ARGs.