BackgroundIncreasing reports of multidrug resistance (MDR) in clinicalPseudomonas aeruginosahave led to a necessity for new antimicrobials. Ceftazidime-avibactam (CZA) is indicated for use against MDRP. aeruginosaacross a broad range of infection types and particularly those that are carbapenem resistant. This study sought to determine the molecular mechanisms of CZA and imipenem (IPM)-resistance in clinicalP. aeruginosaisolates obtained from Swiss hospitals.MethodsClinicalP. aeruginosaisolates were obtained from inpatients in three hospitals in Switzerland. Susceptibility was determined by either antibiotic disc testing or broth microdilution according to EUCAST methodology. AmpC activity was determined using cloxacillin and efflux activity was determined using phenylalanine-arginine β-naphthylamide, in agar plates. Whole Genome Sequencing was performed on 18 clinical isolates. Sequence types (STs) and resistance genes were ascertained using the Centre for Genomic Epidemiology platform. Genes of interest were extracted from sequenced isolates and compared to reference strainP. aeruginosaPAO1.ResultsSixteen different STs were identified amongst the 18 isolates in this study indicating a high degree of genomic diversity. No carbapenemases were detected but one isolate did harbor the ESBLblaPER-1. Eight isolates were CZA-resistant with MICs ranging from 16-64 mg/L, and the remaining ten isolates had either low/wildtype MICs (n=6; 1-2 mg/L) or elevated, but still susceptible, MICs (n=4; 4-8 mg/L). Ten isolates were IPM-resistant, seven of which had mutations resulting in truncations of OprD, and the remaining nine IPM-susceptible isolates had intactoprDgenes. Within CZA-R isolates, and those with reduced susceptibility, mutations resulting inampCderepression, OprD loss,mexABoverexpression and ESBL (blaPER-1) carriage were observed in various combinations and one harbored a truncation of the PBP4dacBgene. Within the six isolates with wildtype-resistance levels, five had no mutations that would affect any antimicrobial resistance (AMR) genes of interest when compared to PAO1.ConclusionThis preliminary study highlights that CZA-resistance inP. aeruginosais multifactorial and could be caused by the interplay between different resistance mechanisms including ESBL carriage, increased efflux, loss of permeability and derepression of its intrinsicampC.