Increased prevalence of Escherichia coli strains from food carrying bla (NDM) and mcr-1-bearing plasmids that structurally resemble those of clinical strains, China, 2015 to 2017

INTRODUCTION: Emergence of resistance determinants of bla (NDM) and mcr-1 has undermined the antimicrobial effectiveness of the last line drugs carbapenems and colistin. AIM: This work aimed to assess the prevalence of bla (NDM) and mcr-1 in E. coli strains collected from food in Shenzhen, China, during the period 2015 to 2017. METHODS: Multidrug-resistant E. coli strains were isolated from food samples. Plasmids encoding mcr-1 or bla (NDM) genes were characterised and compared with plasmids found in clinical isolates. RESULTS: Among 1,166 non-repeated cephalosporin-resistant E. coli strains isolated from 2,147 food samples, 390 and 42, respectively, were resistant to colistin and meropenem, with five strains being resistant to both agents. The rate of resistance to colistin increased significantly (p < 0.01) from 26% in 2015 to 46% in 2017, and that of meropenem resistance also increased sharply from 0.3% in 2015 to 17% in 2017 (p < 0.01). All meropenem-resistant strains carried a plasmid-borne bla (NDM) gene. Among the colistin-resistant strains, three types of mcr-1-bearing plasmids were determined. Plasmid sequencing indicated that these mcr-1 and bla (NDM)-bearing plasmids were structurally similar to those commonly recovered from clinical isolates. Interestingly, both mcr-1-bearing and bla (NDM)-bearing plasmids were transferrable to E. coli strain J53 under selection by meropenem, yet only mcr-1-bearing plasmids were transferrable under colistin selection. CONCLUSION: These findings might suggest that mobile elements harbouring mcr-1 and bla (NDM) have been acquired by animal strains and transmitted to our food products, highlighting a need to prevent a spike in the rate of drug resistant food-borne infections.