Detection of an Escherichia coli Sequence Type 167 Strain with Two Tandem Copies of bla NDM-1 in the Chromosome

New Delhi metallo-β-lactamase-1 (NDM-1)-producing Enterobacteriaceae has disseminated rapidly throughout the world and poses an urgent threat to public health. Previous studies confirmed that the bla NDM-1 gene is typically carried in plasmids but rarely in chromosome. We discovered a multidrug-resistant Escherichia coli strain Y5, originating from a urine sample and containing the bla NDM-1 gene, which did not transfer by either conjugation or electrotransformation. We confirmed the possibility of its chromosome location by S1-pulsed-field gel electrophoresis (PFGE) and XbaI-PFGE, followed by Southern blotting. To determine the genomic background of bla NDM-1 , the genome of Y5 was completely sequenced and compared to other reference genomes. The results of our study revealed that this isolate consists of a 4.8-Mbp chromosome and three plasmids, it is an epidemic clone of sequence type (ST) 167, and it shows 99% identity with Escherichia coli 6409 (GenBank accession no. CP010371), which lacks the same bla NDM-1 gene-surrounding structure as Y5. The bla NDM-1 gene is embedded in the chromosome along with two tandem copies of an insertion sequence common region 1 (ISCR1) element (sul1-ARR-3-cat-bla NDM-1 -bleo-ISCR1), which appears intact in the plasmid from Proteus mirabilis (GenBank accession no. KP662515). The genomic context indicates that the ISCR1 element mediated the bla NDM-1 transposition from a single source plasmid to the chromosome. Our study is the first report of an Enterobacteriaceae strain harboring a chromosomally integrated bla NDM-1 , which directly reveals the vertical spreading pattern of the gene. Close surveillance is urgently needed to monitor the emergence and potential spread of ST167 strains that harbor bla NDM-1 .