A carbapenem-resistant Acinetobacter baumannii strain was isolated in Toulouse, France, in 2003. Cloning and expression in Escherichia coli identified the carbapenem-hydrolyzing ␤-lactamase OXA-58, which is weakly related (less than 50% amino acid identity) to other oxacillinases. It hydrolyzed penicillins, oxacillin, and imipenem but not expanded-spectrum cephalosporins. The bla OXA-58 gene was located on a ca. 30-kb non-selftransferable plasmid. After electrotransformation in the A. baumannii CIP7010 T reference strain, it conferred reduced susceptibility to carbapenems. The bla OXA-58 gene was bracketed by two novel ISAba3-like insertion elements. This study describes a newly characterized ␤-lactamase that may contribute to carbapenem resistance in A. baumannii.Carbapenem resistance in Acinetobacter baumannii is observed increasingly in nosocomial isolates, especially in isolates recovered from intensive care units (4). This resistance phenotype is often associated with multidrug resistance, leading to limited choices for treating A. baumannii infections. This bacterial species naturally produces a chromosomally encoded cephalosporinase (6) that may be overexpressed due to insertion of ISAba1, which brings promoter sequences necessary for high-level expression of this ␤-lactamase (12, 36). Carbapenem resistance may be due to a reduced permeability related to porin deficiency and to modification of penicillin-binding protein affinity (10, 17, 39), but recent reports showed that ␤-lactamase-mediated carbapenem resistance is the most common mechanism (29). Only a few instances of metallo-␤-lactamase have been described for A. baumannii (9,15,33,37,40), but identification of several Ambler class D ␤-lactamases (oxacillinases) has been reported recently (23).Six oxacillinases with carbapenem-hydrolyzing activity have been sequenced from A. baumannii, and these isolates were responsible for nosocomial outbreaks in several cases (5, 14). OXA-23 (also named ARI-1) (16, 25) and OXA-27 (2) have 99% amino acid identity, whereas they share 60% identity with a second group of oxacillinases consisting of OXA-24, -25, -26, and -40, which differ by a few amino acid substitutions (2,7,18). In addition, OXA-48 was characterized recently from a Klebsiella pneumoniae clinical isolate that hydrolyzed imipenem to a significant extent (32). This plasmid-mediated Ambler class D ␤-lactamase likely originated from Shewanella spp., since it shared 92% amino acid identity with the naturally occurring ␤-lactamase OXA-54 from Shewanella oneidensis (31).In this study, we have characterized a novel imipenem-hydrolyzing oxacillinase identified in A. baumannii. MATERIALS AND METHODSBacterial strains and plasmids. A. baumannii clinical isolate MAD was isolated in 2003. It was identified with the API20NE system (bioMérieux, Marcyl'Etoile, France). Escherichia coli reference strain DH10B and plasmid pBK-CMV (Stratagene, Amsterdam, The Netherlands) were used for cloning experiments. A. baumannii CIP7010 T (Pasteur Institute, Paris, France) and E....

The spread of the plasmid-mediated carbapenem-hydrolyzing oxacillinase OXA-58 was detected in Acinetobacter sp. clinical isolates from southern Europe, the Balkans, and central Turkey. It may contribute significantly to the emergence of carbapenem resistance in Acinetobacter spp., at least in this part of the world.