Overexpression of the Naturally Occurring blaOXA-51 Gene in Acinetobacter baumannii Mediated by Novel Insertion Sequence ISAba9

Resistance to carbapenems in Acinetobacter baumannii is increasingly reported and is mostly associated with expression of carbapenemases (3, 6). Those carbapenemases are most frequently carbapenem-hydrolyzing class D -lactamases, which have been identified worldwide, including in Greece (4, 10). Four major subgroups of carbapenem-hydrolyzing class D -lactamases have been identified in A. baumannii: the natu-rally occurring OXA-51/69-type -lactamase and the acquired OXA-23, OXA-24/40 and OXA-58-type -lactamases. Despite a weak catalytic activity, the naturally occurring OXA-51/69-type -lactamases might contribute to carbapenem resistance by insertion of ISAba1 in the promoter region of the blaOXA-51/69-like genes (1, 8, 11). Twenty nonrepetitive carbapenem-resistant A. baumannii clinical isolates were isolated from August 2001 to Novembe