Molecular mechanisms of polymyxin resistance: knowns and unknowns

•Colistin is a last-line drug against multidrug-resistant Gram-negative pathogens.•Chromosomal mutations and plasmid-borne mcr-1 remain the common modes of resistance.•Not all mechanisms mediating colistin resistance are currently known.

Colistin, also referred to as polymyxin E, is an effective antibiotic against most multidrug-resistant Gram-negative bacteria and is currently used as a last-line drug for treating severe bacterial infections. Colistin resistance has increased gradually for the last few years, and knowledge of its multifaceted mechanisms is expanding. This includes the newly discovered plasmid-mediated colistin resistance gene mcr-1, which has been detected in over 20 countries within 3 months of its first report. We previously reported all of the known mechanisms of polymyxin resistance in our first review in 2014, but an update seems necessary in 2016, considering the significant recent discoveries that have been made in this domain. This review provides an update about what is already known, what is new, and some unresolved questions with respect to colistin resistance.