Genetic determinants of nonsusceptibility to novel β-lactam/β-lactamase inhibitors in non-metallo-β-lactamase-producing Pseudomonas aeruginosa.

OBJECTIVE

Novel β-lactam/β-lactamase inhibitor (BL/BLI) agents including ceftazidime-avibactam, ceftolozane-tazobactam, and imipenem-relebactam have expanded treatment options for multidrug-resistant Pseudomonas aeruginosa. However, resistance mechanisms beyond acquired metallo-β-lactamases (MBLs) remain incompletely defined. We investigated the genomic basis of BL/BLI nonsusceptibility in carbapenem-resistant P. aeruginosa isolates collected at a tertiary-care hospital in South Korea.

METHODS

Antimicrobial susceptibility testing was performed for 71 carbapenem-resistant P. aeruginosa bloodstream isolates collected between 2021 and 2023. Ten isolates selected based on BL/BLI nonsusceptibility profiles and MBL production, including seven non-MBL-producing isolates and three MBL-producing isolates, underwent carbapenemase screening, whole-genome sequencing, and core-genome SNP analysis.

RESULTS

All non-MBL-producing isolates nonsusceptible to novel BL/BLI agents carried alterations in the outer membrane porin OprD. Many also harboured mutations involving AmpC (PDC) regulatory pathways and efflux-associated regulators (mexR and nalD). In contrast, MBL-producing isolates primarily carried acquired carbapenemase genes and exhibited fewer chromosomal mutations. No functionally relevant AmpC (PDC) structural mutations or known resistance-associated penicillin-binding proteins alterations were identified among non-MBL-producing isolates. Core-genome single-nucleotide polymorphism (SNP) analysis demonstrated substantial genetic diversity, suggesting independent emergence across multiple lineages.

CONCLUSIONS

Reduced susceptibility to BL/BLI agents among non-MBL-producing isolates appeared to be associated with chromosomal adaptations involving altered AmpC regulation, OprD dysfunction, and efflux-associated mechanisms. These findings highlight the importance of genomic surveillance beyond carbapenemase detection as the clinical use of these agents expands.