Purpose: The continuous rise in carbapenemase-producing Enterobacteriaceae infections is a major public health concern. However, there is limited information available on New Delhi metallo-β-lactamase-1 (NDM-1) producing Citrobacter koseri. In this study, we isolated a bla_NDM-1-carrying C. koseri from a stool sample of an inpatient. Our aim was to investigate the phenotypic and genomic features of this clinically derived carbapenem-resistant C. koseri isolate and to characterize the transmission pattern of the IncFII/IncN plasmid that carries the bla_NDM-1 gene.
Methods and Results: S1-PFGE, Southern blot and conjugation assay confirmed the presence of bla_NDM-1 gene in a conjugative plasmid. C. koseri L2395 and transconjugant L2395-EC600 strains showed similar resistance spectrum. Whole-genome analysis revealed that pL2395_NDM is an IncFII/IncN plasmid with a length of 67,839 bp. Moreover, bla_NDM-1 gene was found encoded in the ISKpn19-bla_NDM-1-ble-tnpF-dsbD-cutA-ISKpn19 cassette array. Phylogenetic analysis revealed that strain L2395 was close to an IMP-4-bearing C. koseri from Australia.
Conclusion: Ongoing surveillance will be essential to control and prevent the spread of carbapenem-resistant Citrobacter spp. in the future.

Keywords: Citrobacter koseri, carbapenem-resistant, NDM-1, whole-genome sequencing