Molecular Characterization of -Carrying IncX3 Plasmids: Likely Emerged from a Mutation of on IncX3 Plasmid.

Dissemination of , which is carried on the IncX3 plasmid, among has been reported worldwide. In particular, -carrying IncX3 plasmids can spread among several hosts, facilitating their dissemination. Other variants, such as -, -, -, -, and -carrying IncX3 plasmids, have also been reported. Here, we characterized, using whole-genome sequencing (WGS), a -carrying IncX3 plasmid harbored by Escherichia coli strain TA8571, which was isolated from a urine specimen of a hospital inpatient in Tokyo, Japan. The differed in sequence from (C > T at site 698, resulting in an Ala233Val substitution). This -carrying IncX3 plasmid (pTMTA8571-1) is 46,161 bp in length and transferred via conjugation. Transconjugants showed high resistance to β-lactam antimicrobials (except for aztreonam). Because pTMTA8571-1, which carries the Tnrelated region containing and conjugative transfer genes, was similar to the previously reported IncX3 plasmids, we performed phylogenetic analysis based on the sequence of 34 shared genes in 142 -carrying IncX3 plasmids (22,846/46,923 bp). Comparative analysis of the shared genes revealed short branches on the phylogenetic tree (average of 1.08 nucleotide substitutions per shared genes), but each variant was divided into separate groups, and the structure of the tree correlated with the flowchart of nucleotide substitutions. The -carrying IncX3 plasmids may thereby have evolved from the same ancestral plasmid with subsequent mutation of the . Therefore, pTMTA8571-1 likely emerged from a -carrying IncX3 plasmid. This study suggested that the spread of -carrying IncX3 plasmids may be a hotbed for the emergence of novel variants of . -carrying IncX3 plasmids have been reported worldwide. Harbored variants were mainly , but there were also rare variants like , , , , and , including detected in this study. For these plasmids, previous reports analyzed whole genomes or parts of sequences among a small number of samples, whereas, in this study, we performed an analysis of 142 -carrying IncX3 plasmids detected around the world. The results showed that regardless of the variants, -carrying IncX3 plasmids harbored highly similar shared genes. Because these plasmids already spread worldwide may be a hotbed for the emergence of rare or novel variants of , increased attention should be paid to -carrying IncX3 plasmids in the future.