was initially reported as the first plasmid-mediated colistin resistance gene in clinical isolates of and in China and has subsequently been identified worldwide in various species of the family encodes a phosphoethanolamine transferase, and its expression has been shown to generate phosphoethanolamine-modified bis-phosphorylated hexa-acylated lipid A in Here, we investigated the effects of on colistin susceptibility and on lipopolysaccharide structures in laboratory and clinical strains of the Gram-negative ESKAPE (, ,, ,, and species) pathogens, which are often treated clinically by colistin. The effects of on colistin resistance were determined using MIC assays of laboratory and clinical strains of ,, , and Lipid A structural changes resulting from MCR-1 were analyzed by mass spectrometry. The introduction of led to colistin resistance in, , and but only moderately reduced susceptibility in Phosphoethanolamine modification of lipid A was observed consistently for all four species. These findings highlight the risk of colistin resistance as a consequence of expression among ESKAPE pathogens, especially in and Furthermore, the observation that lipid A structures were modified despite only modest increases in colistin MICs in some instances suggests more sophisticated surveillance methods may need to be developed to track the dissemination of or plasmid-mediated phosphoethanolamine transferases in general.