Mechanism of horizontal transmission of tet(A)multicopy structures in Escherichia coli of chicken origin.

Abstract

Tigecycline is a vital antimicrobial to treat bacterial infection. Our previous studies have demonstrated that a variant of the tet(A) gene, tet(A), can enhance Escherichia coli resistance to tigecycline through tandem amplification. However, the mechanism of tet(A) multicopy structure transfer remains unclear. In this study, we report the mechanism of tet(A)multicopy structure transfer with the assistance of plasmid recombination. S1-PFGE shows that the transconjugant 573_16xJ53-TC1 (hereinafter referred to as the "TC1") carries two plasmids. Interestingly, the sizes of these two plasmids do not match any plasmids in donor strain E. coli 573_16. Whole genome sequencing showed that TC1 carries only one plasmid, named p573_16xJ53-TC1 (hereinafter referred to as the "pTC1"). Sequence alignment indicates that the tet(A)multicopy structure was transferred from the donor strain to the recipient strain via plasmid p573_16-3, rather than its original p573_16-2. Genetic environment analysis indicated that the cross-plasmid transfer of the tet(A)gene was mediated by the unconventional circularizable structure (UCS) formed by homologous recombination of its upstream and downstream ΔTnAs1. Conjugation experiments indicate that the hybrid plasmid pTC1 still retains the ability for horizontal transfer. The tet(A)gene can cause cross-resistance to tetracycline and tigecycline, and its horizontal spread through different plasmids indicates a complex mode of transmission.