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.
Keywords: IncX3; JARBS; bla NDM.