Comprehensive Understanding of the Plasmid-Mediated Colistin Resistance Gene mcr-1 in Aquatic Environments.

The emergence of plasmid-mediated colistin resistance gene mcr-1 has attracted global attention and raised serious concerns about its possible cross-environment dissemination. However, the systematic exploration of mcr-1 both by monitoring and genetic dissection in aquatic environments has not been conducted. This study addresses the gap related to the occurrence and distribution of mcr-1 in watersheds, eastern China. The results showed an abundance of mcr-1 gene in four watersheds, and the highest level of mcr-1 reached 1.8 × 109 gene copies per liter of water. Furthermore, the transfer frequencies of the plasmids in isolated Escherichia coli were 2.76 × 10-6-6.11 × 10-4 within genera and minimal inhibitory concentrations of polymyxin resistance were 8-16 mg/L for transconjugants. Mass spectrometry data allowed visualization of the function of mcr-1 expression, rendering bacterial resistance to colistin. The genetic details of six mcr-1-harboring plasmids in E. coli isolates of aquatic origin were obtained by single-molecule real-time sequencing. These plasmids were closely associated with E. coli strains of pig and human origin, supporting the concept of mcr-1 dissemination across natural environments, livestock farms, and humans. In conclusion, this study provides the first glimpse of the profile of mcr-1-harboring plasmids and their genetic environment in aquatic ecosystems.