Genetic Contexts of bla NDM-1 in Patients Carrying Multiple NDM-Producing Strains

The carbapenem resistance determinant bla NDM-1 has been found in various Gram-negative bacteria and upon different plasmid replicon types (Inc). Here, we present four patients within two hospitals in Pakistan harboring between two and four NDM-1-producing Gram-negative bacilli of different species coresident in their stool samples. We characterize the bla NDM-1 genetic contexts of these 11 NDM-1-producing Gram-negative bacilli in addition to other antimicrobial resistance mechanisms, plasmid replicon profiles, and sequence types (STs) in order to understand the underlying acquisition mechanisms of carbapenem resistance within these bacteria. Two common plasmid types (IncN2 and IncA/C) were identified to carry bla NDM-1 among the six different bacterial species isolated from the four patients. Two of these strains were novel Citrobacter freundii ST 20 and ST 21. The same IncN2-type bla NDM-1 genetic context was found in all four patients and within four different species. The IncA/C-type bla NDM-1 genetic context was found in two different species and in two of the four patients. Combining genetic context characterization with other molecular epidemiology methods, we were able to establish the molecular epidemiological links between genetically unrelated bacterial species by linking their acquisition of an IncN2 or IncA/C plasmid carrying bla NDM-1 for carbapenem resistance. By combining plasmid characterization and in-depth genetic context assessment, this analysis highlights the importance of plasmids in antimicrobial resistance. It also provides a novel approach for investigating the underlying mechanisms of bla NDM-1 -related spread between bacterial species and genera via plasmids.