Characteristics of Escherichia coli co-harboring bla NDM and mcr-1 genes from market chicken meat.

Carbapenems and colistin resistance in Enterobacterales is a global public health issue. In this study, we isolated seven Escherichia coli strains co-resistant to colistin and meropenem from market chicken meat in Zhejiang Province, China. Antimicrobial susceptibility testing showed that these strains exhibited extensive antimicrobial resistance, with a 100% multidrug-resistant rate. All seven strains harbored both bla NDM and mcr-1 genes, along with various additional resistance genes. Among the seven E. coli strains, the harboring bla NDM plasmids were IncX3 (4/7) and IncB/O/K/Z (3/7) plasmids, and the mcr-1 gene was primarily identified on IncI2 (3/7) plasmids. Notably, most plasmids harboring the bla NDM and mcr genes can be transferred to recipient bacteria via conjugation, highlighting their role as important vehicles for the dissemination of resistance genes. Meanwhile, we also identified a novel bla NDM variant, designated bla NDM-85, which exhibited amino acid substitutions E152K and M154L compared to bla NDM-1. Through the cloning and expression of bla NDM-1 to bla NDM-10 and bla NDM-85 genes, it was found that the resistance levels of bla NDM-85 were significantly higher than other bla NDM variants. It should be noted that the spread of bla NDM-85 will further increase the difficulty of treating clinically resistant bacteria. Our findings highlight the detection of bla NDM and mcr-1 producing E. coli in market chicken meat as a potential public health risk. And the urgent need for enhanced surveillance and intervention strategies to mitigate the spread of these multidrug-resistant pathogens in the food chain.