Citrobacter freundii carrying bla(KPC-2) and bla(NDM-1): characterization by whole genome sequencing

A carbapenem-resistant Citrobacter freundii strain WCHCF65 was recovered from hospital sewage and was characterized by genome sequencing and conjugation experiments. The strain carried nine genes encoding β-lactamases including two carbapenemase genes, bla(NDM-1) and bla(KPC-2). bla(NDM-1) was carried on an IncX3 plasmid, which was identical to a plasmid found in a local Escherichia coli, suggesting interspecies horizontal transfer. bla(KPC-2) was bracketed by two copies of insertion sequence ISKpn19, which could form a composite transposon with the potential to mobilize bla(KPC-2), on a new type of plasmid. The coexistence of bla(NDM-1) and bla(KPC-2) conferred higher levels of resistance to carbapenems compared with bla(NDM-1) or bla(KPC-2) alone. The coexistence of these carbapenemase genes, on two different plasmids, in one strain may allow new genetic platforms to be generated to mediate their spread.