A novel PhoPQ-potentiated mechanism of colistin resistance impairs membrane integrity in Pseudomonas aeruginosa.

Abstract

Polymicrobial communities are often recalcitrant to antibiotic treatment because interactions between different microbes can dramatically alter their responses and susceptibility to antimicrobials. However, the mechanisms of evolving antimicrobial resistance in such polymicrobial environments are poorly understood. We previously reported that Mg 2+ depletion caused by the fungus Candida albicans can enable Pseudomonas aeruginosa to acquire significant resistance to colistin, a last-resort antibiotic targeting bacterial membrane. Here, we dissect the genetic and biochemical basis of this increased colistin resistance. We show that P. aeruginosa cells can acquire colistin resistance using three distinct evolutionary trajectories involving mutations in genes involved in lipid A biosynthesis, lipid A modifications that are dependent on low Mg 2+ , and a putative Mg 2+ transporter, PA4824. These mutations confer colistin resistance by altering acyl chains, hydroxylation, and aminoarabinose modification of lipid A moieties on the bacterial outer membrane. In all cases, enhanced colistin resistance initially depends on the low Mg 2+ -responsive PhoPQ pathway, which potentiates the evolution of resistance mutations and lipid A modifications that do not occur without Mg 2+ depletion. However, the PhoPQ pathway is not required to maintain high colistin resistance in all cases. In most cases, the genetic and biochemical changes associated with these novel forms of colistin resistance also impair bacterial membrane integrity, leading to fitness costs. Our findings provide molecular insights into how nutritional competition drives a novel antibiotic resistance mechanism and its ensuing fitness tradeoffs.