Lipid A phosphoethanolamine transferases are widely distributed in Gram-negative bacteria, and are constitutively expressed in some species, including Neisseria meningitidis and Moraxella catarrhalis , that in consequence are intrinsically polymyxin-resistant. In others, including Acinetobacter baumannii , acquired polymyxin resistance can arise from upregulation of chromosomal PEtN transferases, but in E. coli this has remained rare. In 2015 however, a plasmid-borne polymyxin resistance determinant, the Mobile Colistin Resistance (MCR)-1 PEtN transferase, was identified in China in an E. coli isolate of porcine origin , since when multiple MCR isoforms have been discovered in multiple species worldwide . In E. coli MCR production remains the dominant polymyxin resistance mechanism.