Definition of a Family of Nonmobile Colistin Resistance (NMCR‐1) Determinants Suggests Aquatic Reservoirs for MCR‐4

Polymyxins, a family of cationic antimicrobial peptides, are recognized as a last‐resort clinical option used in the treatment of lethal infections with carbapenem‐resistant pathogens. A growing body of mobile colistin resistance (MCR) determinants renders colistin ineffective in the clinical and human sectors, posing a challenge to human health and food security. However, the origin and reservoir of the MCR family enzymes is poorly understood. Herein, a new family of nonmobile colistin resistance (from nmcr‐1 to nmcr‐1.8) from the aquatic bacterium Shewanella is reported. NMCR‐1 (541aa) displays 62.78% identity to MCR‐4. Genetic and structural analyses reveal that NMCR‐1 shares a similar catalytic mechanism and functional motifs, both of which are required for MCR action and its resultant phenotypic resistance to polymyxin. Phylogeny and domain‐swapping demonstrate that NMCR‐1 is a progenitor of MCR‐4 rather than MCR‐1/2. Additionally, the experiment of bacterial growth and viability reveals that NMCR‐1 promotes fitness cost as MCR‐1/4 does in the recipient Escherichia coli. In summary, the finding suggests that the aquatic bacterium Shewanella (and even its associated aquaculture) is a reservoir for MCR‐4 mobile colistin resistance.