Molecular analysis of carbapenemase-producing Pseudomonas aeruginosa strains in a tertiary care hospital in Riyadh

BACKGROUND: Carbapenem-resistant Pseudomonas aeruginosa (CRPA) presents a significant obstacle to effective antimicrobial treatment, particularly when resistance is driven by carbapenemase-producing strains (CPPA). While global carbapenem resistance rates in P. aeruginosa vary widely, from 10 % to 50 %, understanding local and regional patterns is crucial for guiding antimicrobial stewardship efforts. We used whole-genome sequencing (WGS) to characterize the genetic mechanisms of resistance in CRPA isolates. METHODS: From 2019-2021, 221 P. aeruginosa isolates were collected from clinical specimens. Antimicrobial susceptibility was determined using VITEK®2 and E-test. Sixty-six isolates were randomly selected for carbapenemase gene detection via whole-genome sequencing (WGS) after DNA extraction (QIAamp® kit). RESULTS: Whole-genome sequencing identified a predominance of intrinsic β-lactamases particularly OXA-50-like enzymes, including OXA-50, OXA-486 (53 %), OXA-488, and OXA-485) do not exhibit clinically significant carbapenem-hydrolyzing activity. In contrast, OXA-10 and OXA-232 are acquired OXA-type β-lactamases and are not intrinsic to P. aeruginosa. True carbapenemases were infrequently detected and were limited to IMP-34 (6 %) and VIM-28 (1.5 %). Notably, DHA-1 was identified for the first time in Riyadh. AmpC-like β-lactamases, such as PDC-3, were detected in 45.4 % of isolates. Class A extended-spectrum β-lactamases (ESBLs), including GES-14 (4.5 %) and VEB-9 (7.6 %), were also detected but are not considered carbapenemases. Predominant sequence types (STs) included ST357 (9.8 %), ST235 (8 %), and ST260 and ST244 (6.5 % each). High-risk clones, particularly ST235 and ST357, exhibited elevated carbapenem MICs attributable to the combined effects of intrinsic β-lactamase expression, efflux pump overexpression, and outer membrane permeability defects, rather than carbapenemase production alone. CONCLUSION: Carbapenem resistance in the studied P. aeruginosa isolates was predominantly mediated by intrinsic resistance mechanisms, including OXA-50-like and AmpC β-lactamases, together with efflux pump overexpression and permeability defects. True carbapenemases were infrequently detected and were limited mainly to IMP-34 and VIM-28. High-risk clones such as ST235 and ST357 exhibited elevated carbapenem MICs largely independent of carbapenemase production.