Efflux-Related Carbapenem Resistance in Acinetobacter baumannii Is Associated with Two-Component Regulatory Efflux Systems' Alteration and Insertion of AbaR25-Type Island Fragment

The efflux pumps, beside the class D carbapenem-hydrolysing enzymes (CHLDs), are being increasingly investigated as a mechanism of carbapenem resistance in Acinetobacter baumannii. This study investigates the contribution of efflux mechanism to carbapenem resistance in 61 acquired bla(CHDL)-genes-carrying A. baumannii clinical strains isolated in Warsaw, Poland. Studies were conducted using phenotypic (susceptibility testing to carbapenems +/- efflux pump inhibitors (EPIs)) and molecular (determining expression levels of efflux operon with regulatory-gene and whole genome sequencing (WGS)) methods. EPIs reduced carbapenem resistance of 14/61 isolates. Upregulation (5-67-fold) of adeB was observed together with mutations in the sequences of AdeRS local and of BaeS global regulators in all 15 selected isolates. Long-read WGS of isolate no. AB96 revealed the presence of AbaR25 resistance island and its two disrupted elements: the first contained a duplicate ISAba1-bla(OXA-23), and the second was located between adeR and adeA in the efflux operon. This insert was flanked by two copies of ISAba1, and one of them provides a strong promoter for adeABC, elevating the adeB expression levels. Our study for the first time reports the involvement of the insertion of the Delta AbaR25-type resistance island fragment with ISAba1 element upstream the efflux operon in the carbapenem resistance of A. baumannii.