Coexistence of a novel KPC-2-encoding MDR plasmid and an NDM-1-encoding pNDM-HN380-like plasmid in a clinical isolate of Citrobacter freundii

Objectives: The objective of this study was to characterize the molecular mechanism of coproduction of KPC-2 and NDM-1 in Citrobacter freundii. Methods: C. freundii strain 112298 was isolated from a human case of septic shock in a Chinese teaching hospital. The major carbapenemase and ESBL genes were detected by PCR. The MIC values were determined by using VITEK 2 and antimicrobial susceptibility was judged by CLSI standards. The resistance plasmid was transferred into Escherichia coli by electroporation, followed by plasmid DNA isolation from the electroporant, and then fully sequenced and compared with closely related plasmids. Results: Strain 112298 produces KPC-2 and NDM-1, encoded by the novel non-typeable plasmid p112298-KPC and an IncX3-type plasmid p112298-NDM, respectively. In p112298-KPC, a Tn1722-based bla(KPC-2)-carrying transposon is associated with several additional resistance modules, constituting a single MDR region. Assembly of these resistance modules is likely mediated by homologous recombination between five copies of IS26 elements at different sites within the MDR region. p112298-NDM is a very close relation of pNDM-HN380. bla(NDM-1) in p112298-NDM is carried by a Tn125 variant, which differs from the prototype Tn125 as observed in pNDM-BJ01 by disruption of an upstream copy of ISAba125 by IS5 and absence of a downstream copy of ISAba125. Conclusions: Production of KPC-2 and NDM-1 by p112298-KPC and p112298-NDM, respectively, makes C. freundii 112298 highly resistant to carbapenems and, moreover, these two plasmids still harbour genes for resistance to cephalosporins, chloramphenicol, chromate, fosfomycin, quaternary ammonium, rifampicin and sulphonamides.