Genomic features of an Escherichia coli ST156 strain harboring chromosome-located mcr-1 and plasmid-mediated bla_NDM-5

The emergence of Escherichia coli strains coharboring the bla_NDM and mcr genes has become a new challenge in clinical therapy because of their resistance to most antibiotics. This study reports the emergence of an E. coli strain GZEC065 isolated from blood sample of a patient in China with both chromosome-located mcr-1 and plasmid-mediated bla_NDM-5 genes. Antimicrobial susceptibility testing showed that GZEC065 was resistant to most tested antimicrobials, including imipenem and polymyxin B. S1-nuclease-pulsed-field gel electrophoresis, Southern blotting analysis and conjugation assay was performed to determine the location and conjugative ability of mcr-1 and bla_NDM-5 genes. Whole genome sequencing and analysis showed that GZEC065 belonged to sequence type ST156 and contained three different plasmids (46 kb, 88 kb and 142 kb) with multiple resistance genes, such as mcr-1, bla_NDM-5 and bla_TEM-1. The bla_NDM-5 gene was carried by the 46 kb conjugative IncX3 plasmid, which has been reported frequently in China. The mcr-1 gene was located on the chromosome mediated by the Tn6330 mobile element and showed genetic complexity among different strains. The emergence of E. coli strains coharboring both chromosome-located mcr-1 and plasmid-mediated bla_NDM-5 genes highlights the urgent need for alternative antibiotic treatment and continuous active surveillance.