Abstract
IncX3 plasmids harboring bla (NDM-5) play a major role in the spread of carbapenem resistance in Asia, particularly in China, in clinical and environmental settings. In this study, we present that Enterobacterales isolates carrying IncX3 plasmids harboring bla (NDM-5) have been disseminated in Japan, where their identification was previously rare. In addition, bla (NDM-16b), a single-nucleotide variant of bla (NDM-5), was found to be carried by an identical IncX3 plasmid. A comparative sequence analysis revealed that the bla (NDM-16b) gene emerged from a single nucleotide substitution on an IncX3 plasmid harboring bla (NDM-5). The bla (NDM-16b) gene did not confer elevated carbapenem resistance compared to bla (NDM-5) in our assay using transformants carrying the plasmid harboring either of these genes, although the A233V substitution was reported to confer stability to the enzyme in ion-depleted conditions. Nevertheless, vigilance regarding the emergence of novel variants is required.