Disruption of the blaOXA-51-like gene by ISAba16 and activation of the blaOXA-58 gene leading to carbapenem resistance in Acinetobacter baumannii Ab244

Objectives: This study examines the mechanism of carbapenem resistance in Acinetobacter baumannii isolate Ab244. 

Methods: A multiplex PCR for the detection of the blaOXA-23-like, blaOXA-40-like, blaOXA-51-like and blaOXA-58-like families was performed. MICs of imipenem and meropenem were determined by the agar dilution method. The sequence surrounding the blaOXA-132 gene was determined by amplification with primer pairs encompassing a part of fxsA and an acetyltransferase gene (GNAT). The sequence upstream of the blaOXA-58 gene was determined by sequencing. SDS–PAGE and carO PCR were performed to check the integrity of the outer membrane proteins. RT–PCRs for the expression of the blaOXA-132 gene and the blaOXA-58 gene were performed. 

Results: Isolate Ab244 harboured blaOXA-132 belonging to the blaOXA-51-like gene cluster and a blaOXA-58 gene. The 4239 bp region between fxsA and GNAT showed an insert of ISAba16 (where IS stands for insertion sequence) after the first 15 nucleotides of the blaOXA-132 gene, with an 8 bp target site duplication at the 5′ and 3′ ends of ISAba16. The sequence oriented in the 5′→3′ direction caused insertional inactivation of the blaOXA-132 gene. The blaOXA-58 gene was highly expressed by the promoters provided by an ISAba3-like structure found upstream of the gene. The isolate was resistant to meropenem and had intermediate resistance to imipenem, and was also positive for ISAba1. 

Conclusions: This is the first report showing ISAba16-mediated inactivation of the blaOXA-132 gene in strain Ab244. The resistance to carbapenems in strain Ab244 is related to the acquisition of the blaOXA-58 gene, here governed by an ISAba3-like element.