ABSTRACT Objectives This study aimed to measure the variability in carbapenem susceptibility conferred by different OxaAb variants, characterise the molecular evolution of oxaAb and elucidate the contribution of OxaAb and other possible carbapenem resistance factors in the clinical isolates using WGS and LC-MS/MS. Methods Disc susceptibility and MIC broth microdilution tests were performed on ten clinical A. baumannii isolates and interpreted according to CLSI guidelines. Imipenem and meropenem MICs were evaluated for all oxaAb variants cloned into susceptible A. baumannii CIP70.10 and increased adeABC efflux pump gene expression BM4547 backgrounds, with and without their natural promoters. Molecular evolution analysis of the oxaAb variants was performed using FastTree and SplitsTree4. Resistance determinants were studied in the clinical isolates using WGS and LC-MS/MS analysis. Results OxaAb(82), OxaAb(83), OxaAb(107), and OxaAb(110) carrying I129L and L167V substitutions increased carbapenem MICs when transferred into susceptible A. baumannii backgrounds without an upstream IS element. Carbapenem resistance was conferred with the addition of their natural upstream IS Aba1 promoter. LC-MS/MS analysis on the original clinical isolates showed no differences in expression levels of proteins commonly associated with carbapenem resistance. Conclusions IS Aba1 -driven overexpression of OxaAb variants with substitutions I129L and L167V confers carbapenem resistance with no need for additional resistance mechanisms.

Variability in carbapenemase activity of intrinsic OxaAb (OXA-51-like) beta-lactamase enzymes in Acinetobacter baumannii