Polymyxin antibiotics, specifically colistin, are agents of last resort for treating infections by these species and, despite toxicity concerns , their use has increased in recent years . Polymyxins disrupt the Gram-negative envelope by a sequential process that remains incompletely understood : initial interaction with outer membrane lipopolysaccharide (LPS) is followed by uptake into the bacterial periplasm and effects upon the inner membrane that may include interference with LPS before or during trafficking to the outer membrane and/or direct permeabilization. In consequence, modifications to LPS that disrupt polymyxin interactions are determinants of polymyxin resistance; specifically, these reduce negative charges on the lipid A disaccharide by covalent modification of phosphate groups, thereby preventing electrostatic interactions with cationic components of the polymyxin peptide. Such modifications most commonly involve addition of phosphoethanolamine (PEtN) (Scheme 1A) or 4-amino-4-deoxy-L-arabinose (L-Ara-4N) to the 1' or 4' phosphates .