Background: Plasmid-mediated multi-drug resistance (MDR) has been widely found in Citrobacter freundii. C. freundii  P10159 was isolated from a human case of postoperative urinary tract infection in a Chinese teaching hospital.
Methods: The complete nucleotide sequences of five resistance plasmids pP10159-1, pP10159-2, pP10159-3, pP10159-4 and pP10159-5 from C. freundii  P10159 were determined through high-throughput genome sequencing, and then compared with related plasmids sequences. Plasmid transfer, CarbaNP test of carbapenemase activity, and bacterial antimicrobial susceptibility test were performed to characterize resistance phenotypes mediated by these plasmids.
Results: pP10159-1 carrying bla _NDM-1 and pP10159-2 harboring bla _IMP-4 plus qnrS1  were almost identical to IncX3 plasmid pNDM-HN380 and IncN1 plasmid pP378-IMP, respectively. The bla _KPC-2-carrying plasmids pP10159-3, pHS062105-3 and pECN49-KPC were highly similar to each other, and constituted a novel group of plasmids belonging to an unknown incomparability group. The MDR plasmids pP10159-4 and pP10159-5 had the backbones highly similar to IncHI4 plasmid pNDM-CIT and type 2 IncC plasmid pR55, respectively, but their accessory resistance regions differed from pNDM-CIT and pR55, respectively. The five plasmids from the P10159 isolate contained a total of 24 different genes or gene loci, which contributed to resistance to 13 distinct antibiotic molecules or toxic compounds.
Conclusion: This is the first report of co-occurrence of five different resistance plasmids, with determination of their complete sequences. Data presented here provide a deeper insight into co-selection and maintenance of multiple plasmids and an extremely large number of resistance genes in a single bacterial isolate.
Keywords: Citrobacter freundii , multi-drug resistance, plasmids, mobile elements