Selection and molecular characterization of ceftazidime/avibactam-resistant mutants in Pseudomonas aeruginosa strains containing derepressed AmpC

Abstract

Objectives: Pseudomonas aeruginosa is an important nosocomial pathogen that can cause a wide range of infections resulting in significant morbidity and mortality. Avibactam, a novel non-β-lactam β-lactamase inhibitor, is being developed in combination with ceftazidime and has the potential to be a valuable addition to the treatment options for the infectious diseases practitioner. We compared the frequency of resistance development to ceftazidime/avibactam in three P. aeruginosa strains that carried derepressed ampC alleles.

Methods: The strains were incubated in the presence of increasing concentrations of ceftazidime with a fixed concentration (4 mg/L) of avibactam to calculate the frequency of spontaneous resistance. The mutants were characterized by WGS to identify the underlying mechanism of resistance. A representative mutant protein was characterized biochemically.

Results: The resistance frequency was very low in all strains. The resistant variants isolated exhibited ceftazidime/avibactam MIC values that ranged from 64 to 256 mg/L. All of the mutants exhibited changes in the chromosomal ampC gene, the majority of which were deletions of various sizes in the Ω-loop region of AmpC. The mutant enzyme that carried the smallest Ω-loop deletion, which formed a part of the avibactam-binding pocket, was characterized biochemically and found to be less effectively inhibited by avibactam as well as exhibiting increased hydrolysis of ceftazidime.

Conclusions: The development of high-level resistance to ceftazidime/avibactam appears to occur at low frequency, but structural modifications in AmpC can occur that impact the ability of avibactam to inhibit the enzyme and thereby protect ceftazidime from hydrolysis.

Keywords: ESAC; cephalosporins; extended-spectrum cephalosporinases; Ω-loop.