A novel and hybrid composite transposon at the origin of acquisition of bla(RTG-5) in Acinetobacter baumannii.

The genetic context of the gene coding for the novel variant of the class A β-lactamase RTG-5 (CARB-14), exhibiting two amino acid substitutions compared with the closest RTG-3 enzyme, was analysed in a clinical isolate of Acinetobacter baumannii. This novel variant exhibited a narrow-spectrum profile, as observed for RTG-3. Sequence analysis of the region flanking the 5' end of bla(CARB-14) identified a novel insertion sequence ISAba21 belonging to the IS3 family. Downstream of bla(CARB-14), another insertion sequence belonging to the IS3 family named ISAba14 was identified. Analysis of the left-end extremity of ISAba21 and the right-end extremity of ISAba14 identified direct repeat sequences of 3 bp, likely being the signature of a transposition process. This is one of the few examples of a composite transposon made of different insertion sequences bracketing a gene coding for this novel carbenicillinase.