Genetic characterisation of a complex class 1 integron in an NDM-1-producing Citrobacter freundii ST396 clinical strain isolated from a urine sample

Objectives: The aim of this study was to characterise a multidrug-resistant (MDR) Citrobacter freundii strain carrying a blaNDM-1-harbouring conjugative IncA/C2-type plasmid in China. Methods: The whole genome of C. freundii strain L75 was sequenced both on HiSeq® (Novogene Corp., China) and GridION X5 (GrandOmics Biosciences Co., Ltd.) platforms. De novo genome assembly, genome annotation, multilocus sequence typing (MLST) and plasmid replicon typing were performed. Results: Citrobacter freundii L75 was assigned to ST396 and contains a 5.1-Mb chromosome and two plasmids (∼146 kb and ∼73 kb) designated pCf75 and pCf76, respectively. The antibiotic resistance determinants blaNDM-1, sul1, sul2, arr-3, mph(A), aadA16, aph(3”)-Ib, aac(3)-IId, aph(6)-Id, aac(6')-Ib-cr and dfrA27 were located on plasmid pCf75, whilst blaCMY-78 was chromosomally encoded. A 22.1-kb MDR-encoding region (MDRR) in a complex class 1 integron contained all of the resistance genes of plasmid pCf75. Conclusions: We detected and characterised a plasmid carrying a novel MDRR in which a complex class 1 integron plays an essential role. In addition, this study provides fresh genomic insights into the diversity of C. freundii ST396 prevalent in Guangzhou Province, China, and underscores the increasing clinical relevance of the IncA/C2 plasmid family.