Characterization of NDM-5-positive extensively resistant Escherichia coli isolates from dairy cows

The aim of this study was to investigate the prevalence of bla_NDM-5 gene in Escherichia coli isolates from dairy cows and to characterize the molecular traits of the bla_NDM-5-positive isolates. A total of 169 cows were sampled (169 feces and 169 raw milk samples) in three dairy farms in Jiangsu Province and 203 E. coli isolates were recovered. Among these strains, three isolates carried bla_NDM-5 gene, including one co-harboring mcr-1, which belonged to sequence type 446 and the other two belonged to ST2. Susceptibility testing revealed that the three bla_NDM-5-positive isolates showed extensive resistance to antimicrobials. The bla_NDM-5 gene was located on a ∼46-kb IncX3 transferrable pNDM-MGR194-like plasmid in all three isolates, while mcr-1 was located on a ∼260-kb IncHI2 plasmid pXGE1mcr. Competition experiments revealed that acquisition of bla_NDM-5 or mcr-1-bearing plasmid can incur fitness cost of bacterial host, however, plasmid stability testing showed that both bla_NDM-5 and mcr-1-carrying plasmid maintained stable in the hosts after ten passages without antimicrobial selection. Whole genome sequencing revealed that the mcr-1 gene coexisted with multiple resistance genes in pXGE1mcr and the backbone of this plasmid was similar to that of previously reported mcr-1-positive plasmid pHNSHP45-2. Moreover, pXGE1mcr could be conjugated into clinical NDM-5-positive E. coli isolates in vitro, thereby generating strains that approached pan-resistance. Active surveillance efforts are imperative to monitor the prevalence of bla_NDM-5 and mcr-1 in carbapenem-resistant Enterobacteriaceae from dairy farms throughout China.