Inactivation of the Glycoside Hydrolase NagZ Attenuates Antipseudomonal β-Lactam Resistance in Pseudomonas aeruginosa

Abstract

ABSTRACT The overproduction of chromosomal AmpC β-lactamase poses a serious challenge to the successful treatment of Pseudomonas aeruginosa infections with β-lactam antibiotics. The induction of ampC expression by β-lactams is mediated by the disruption of peptidoglycan (PG) recycling and the accumulation of cytosolic 1,6-anhydro- N -acetylmuramyl peptides, catabolites of PG recycling that are generated by an N -acetyl-β- d -glucosaminidase encoded by nagZ (PA3005). In the absence of β-lactams, ampC expression is repressed by three AmpD amidases encoded by ampD , ampDh2 , and ampDh3 , which act to degrade these 1,6-anhydro- N -acetylmuramyl peptide inducer molecules. The inactivation of ampD genes results in the stepwise upregulation of ampC expression and clinical resistance to antipseudomonal β-lactams due to the accumulation of the ampC inducer anhydromuropeptides. To examine the role of NagZ on AmpC-mediated β-lactam resistance in P. aeruginosa , we inactivated nagZ in P. aeruginosa PAO1 and in an isogenic triple ampD null mutant. We show that the inactivation of nagZ represses both the intrinsic β-lactam resistance (up to 4-fold) and the high antipseudomonal β-lactam resistance (up to 16-fold) that is associated with the loss of AmpD activity. We also demonstrate that AmpC-mediated resistance to antipseudomonal β-lactams can be attenuated in PAO1 and in a series of ampD null mutants using a selective small-molecule inhibitor of NagZ. Our results suggest that the blockage of NagZ activity could provide a strategy to enhance the efficacies of β-lactams against P. aeruginosa and other gram-negative organisms that encode inducible chromosomal ampC and to counteract the hyperinduction of ampC that occurs from the selection of ampD null mutations during β-lactam therapy.