PmrB Mutations Promote Polymyxin Resistance of Pseudomonas aeruginosaIsolated from Colistin-Treated Cystic Fibrosis Patients

ABSTRACTPseudomonas aeruginosacan develop resistance to polymyxin and other cationic antimicrobial peptides. Previous work has shown that mutations in the PmrAB and PhoPQ regulatory systems can confer low to moderate levels of colistin (polymyxin E) resistance in laboratory strains and clinical isolates of this organism (MICs of 8 to 64 mg/liter). To explore the role of PmrAB in high-level clinical polymyxin resistance, P. aeruginosaisolates from chronically colistin-treated cystic fibrosis patients, most with colistin MICs of >512 mg/liter, were analyzed. These cystic fibrosis isolates contained probable gain-of-function pmrBalleles that conferred polymyxin resistance to strains with a wild-type or pmrABdeletion background. Double mutant pmrBalleles that contained mutations in both the periplasmic and dimerization-phosphotransferase domains markedly augmented polymyxin resistance. Expression of mutant pmrBalleles induced transcription from the promoter of the arnBoperon and stimulated addition of 4-amino-l-arabinose to lipid A, consistent with the known role of this lipid A modification in polymyxin resistance. For some highly polymyxin-resistant clinical isolates, repeated passage without antibiotic selection pressure resulted in loss of resistance, suggesting that secondary suppressors occur at a relatively high frequency and account for the instability of this phenotype. These results indicate that pmrBgain-of-function mutations can contribute to high-level polymyxin resistance in clinical strains of P. aeruginosa.