Co-introduction of plasmids harbouring the carbapenemase genes, bla(NDM-1) and bla(OXA-232), increases fitness and virulence of bacterial host

BACKGROUND: Bacterial isolates with multiple plasmids harbouring different carbapenemase genes have emerged and been identified repeatedly, despite a general notion that plasmids confer fitness cost in bacterial host. In this study, we investigated the effects of plasmids with carbapenemase genes on the fitness and virulence of bacteria. METHODS: Different plasmids harbouring the carbapenemase genes, bla(NDM-1) and bla(OXA-232), were isolated from a carbapenem-resistant K. pneumoniae strain. Each plasmid was conjugated into the Escherichia coli strain DH5α, and a transconjugant with both plasmids was also obtained by transformation. Their in vitro competitive ability, biofilm formation, serum resistance, survival ability within macrophage and fruit fly, and fly killing ability were evaluated. RESULTS: The transconjugants with a single plasmid showed identical phenotypes to the plasmid-free strain, except that they decreased fly survival after infection. However, significantly increased fitness, virulence and biofilm production were observed consistently for the transconjugant with both plasmids, harbouring bla(NDM-1) and bla(OXA-232). CONCLUSIONS: Our data indicate that bacteria carrying multiple plasmids encoding different carbapenemases may have increased fitness and virulence, emphasizing the need for diverse strategies to combat antimicrobial resistance.