Features of plasmids carrying blaNDM-1 and blaNDM-3 in Australia

Background: Plasmids carrying carbapenemase gene, such as blaNDM-1, are an emerging threat to human health and often carry multiple antimicrobial resistance genes. We previously reported the presence of a blaNDM-1 carrying plasmid in K. pneumoniae. Here, we investigate additional Australian isolates of E. coli and Enterobacter cloacae harboring blaNDM comparing the genetic structure of blaNDM plasmids from all three species. Methods: Genes encoding resistance to b-lactams, aminoglycosides and insertion element (ISCR) types were determined. Plasmids carrying the NDM gene from K. pneumoniae, E. coli and E. cloacae were transformed into E. coli Top10 and studied by plasmid cloning, primer walking, plasmid profiling and amplification of the IncA/C plasmid backbone. E. coli sequence type (ST) was determined. Results: The E. coli, typed as ST 443, had a variant of blaNDM-1, blaNDM-3, which differed by one nucleotide mutation. E. cloacae possessed blaNDM-1. All three blaNDM plasmids also harbored rmtC and aac-6r-1b. The replicon type of E. cloacae the blaNDM-1 plasmid was IncFII. The plasmids of K. pneumoniae and E. coli were type A/C (IncA/C) and also carried the beta-lactamase blaCMY-6. These two plasmids possessed identical tra regions and a high number of the 12 IncA/C markers - plasmid backbone regions - previously described in a variety of human and animal/food Enterobacteriaceae isolates (K. pneumoniae=5 regions, E. coli=6, E. cloacae=0). The plasmid profiles of the two IncA/C plasmids were similar; in contrast, the blaNDM-1 plasmid of E. cloacae was distinct. The E. coli, blaNDM-3 plasmid also had ISCR1. In addition, E. coli contained a separate plasmid (replicon type FIA and FIB) carrying blaCTX-M-15. Conclusions: The similarities of two plasmids harbouring NDM genes suggest an affinity for lateral acquisition of this gene by existing blaCMY harboring IncA/C plasmids. This may explain the reported dissemination of the gene through human pathogenic isolates already harboring IncA/C plasmid.