Role of Chromosome- and/or Plasmid-Located bla NDM on the Carbapenem Resistance and the Gene Stability in Escherichia coli.

The spread of New Delhi metallo-β-lactamase (NDM)-producing Enterobacterales represents a public health risk. The horizontal transfer of plasmids encoding the NDM gene, bla NDM , usually mediates its spread to other bacteria within the family. In contrast, Enterobacterales with a chromosome-located bla NDM is rarely reported. The phenotypic differences between chromosome- and plasmid-located carbapenemase genes are poorly understood. To determine the significance in terms of the location of drug resistance genes, we examined carbapenemase activity and stability of chromosome- and plasmid-located bla NDM . Escherichia coli M719 possessing both chromosomes- and plasmid-located bla NDM genes was used as a wild-type strain (WT) for the construction of mutants, Δp bla NDM and Δc bla NDM , wherein chromosome- or plasmid-located bla NDM, was knocked out, respectively. The mutant Δp bla NDM showed lower hydrolyzing activity against imipenem and gene expression than the WT or Δc bla NDM mutant. The MICs of both mutant strains were still above the breakpoint of imipenem and meropenem. Moreover, the chromosome-located bla NDM gene was stable for at least 30 days in the absence of antimicrobial pressure, whereas the Δc bla NDM mutant lost bla NDM to 87% at 30 days compared to that of the initial inoculum. Organisms harboring the plasmid-located carbapenemase genes were found to provide a higher level of carbapenem resistance than those with chromosome-located genes. However, the latter organisms with chromosomal carbapenemase genes exhibited more stable carbapenem resistance than did the former ones. In summary, chromosomally located carbapenemase genes require further monitoring and more attention should be paid to them. IMPORTANCE Carbapenem-resistant Enterobacterales (CRE) carrying bla NDM have spread worldwide since they were first reported in 2009. Many studies using whole-genome sequencing have identified the genetic structures, plasmid scaffolds of bla NDM , and mechanisms of spread via horizontal transfer. Chromosome-located bla NDM and integration mechanisms from plasmids have rarely been reported, and their significance is not fully understood. Here, we showed that the chromosome-located bla NDM was associated with lower levels of carbapenem resistance and carbapenemase activity than the plasmid-located bla NDM . However, it conferred carbapenem resistance above the breakpoints and the loss of chromosome-located bla NDM was not observed in the absence of antibiotic pressure. This study suggests that CRE strains carrying chromosome-located bla NDM may persist in clinical and environmental settings for a long period even without antibiotic pressure and need to be monitored along with plasmid-located bla NDM .