Diversity of mechanisms conferring resistance to β-lactams among OXA-23-producing Acinetobacter baumannii clones

A total of 31 unrelated OXA-23-producing Acinetobacter baumannii strains isolated from 14 hospitals located in distinct Brazilian regions were evaluated in this study. These isolates were grouped into 12 different sequence types (STs), of which 7 had unique allelic sequences (ST188, ST189, ST190, ST191, ST192, ST228, and ST299). Most isolates belonged to the clonal complex CC79 followed by CC15 and CC1. Only polymyxin B and minocycline showed good activity against the OXA-23-producing A. baumannii clones. The ISAba1 upstream blaOXA-23, blaOXA-51-like, or ampC was found in 100%, 54.8%, and 77.4% of the isolates, respectively. High resistance rates to ceftazidime and cefotaxime were observed among those isolates possessing ISAba1 upstream ampC, in contrast to those isolates that did not carry this configuration. Moreover, a ≥2 Log2 decrease in the MICs of meropenem and ceftazidime was observed in the presence of phenyl-arginine-β-naphthylamide for 80.6% and 54.8% of isolates, respectively. Overexpression of the adeB was observed in 61.3% of isolates, particularly among those isolates belonging to the ST1 (CC1). It was also verified that ompW was down-regulated in all isolates belonging to the ST15 (CC15). On the other hand, carO and omp33-36 genes were overexpressed in 48.4% and 58.1% of the isolates, respectively. In this study, we show that overexpression of AdeABC system could significantly contribute for resistance to meropenem and ceftazidime among OXA-23-producing A. baumannii clones in Brazil, demonstrating the complexity involved in the β-lactam resistance in such isolates.