ABSTRACTInhaled aztreonam is increasingly used for chronicPseudomonas aeruginosasuppression in patients with cystic fibrosis (CF), but the potential for that organism to evolve aztreonam resistance remains incompletely explored. Here, we performed genomic analysis of clonally related pre- and posttreatment CF clinical isolate pairs to identify genes that are under positive selection during aztreonam therapyin vivo. We identified 16 frequently mutated genes associated with aztreonam resistance, the most prevalent beingftsIandampC, and 13 of which increased aztreonam resistance when introduced as single gene transposon mutants. Several previously implicated aztreonam resistance genes were found to be under positive selection in clinical isolates even in the absence of inhaled aztreonam exposure, indicating that other selective pressures in the cystic fibrosis airway can promote aztreonam resistance. Given its potential to confer plasmid-mediated resistance, we further characterized mutantampCalleles and performed artificial evolution ofampCfor maximal activity against aztreonam. We found that naturally occurringampCmutants conferred variably increased resistance to aztreonam (2- to 64-fold) and other β-lactam agents but that its maximal evolutionary capacity for hydrolyzing aztreonam was considerably higher (512- to 1,024-fold increases) and was achieved while maintaining or increasing resistance to other drugs. These studies implicate novel chromosomal aztreonam resistance determinants while highlighting that different mutations are favored during selectionin vivoandin vitro, show thatampChas a high maximal potential to hydrolyze aztreonam, and provide an approach to disambiguate mutations promoting specific resistance phenotypes from those more generally increasing bacterial fitnessin vivo.
Loss of CFTR Chloride Channels Alters Salt Absorption by Cystic Fibrosis Airway Epithelia In Vitro