Detection of mcr-1 Gene among Escherichia coli Isolates from Farmed Fish and Characterization of mcr-1-Bearing IncP Plasmids.

The presence of the mcr-1 gene in Escherichia coli isolated from retail freshwater fish was investigated. Seven (3.65%) clonally unrelated original E. coli isolates from grass carp were positive for mcr-1 The mcr-1 genes were encoded by either chromosomes (n = 2) or conjugative plasmids (2 IncI2, 2 IncP, and 1 IncX4). The IncP plasmids were similar to other mcr-1-harboring IncP plasmids from China, though the insertion sites varied. Our report warrants further surveillance of resistance genes in aquaculture.