OXA-24, a novel class D beta-lactamase with carbapenemase activity in an Acinetobacter baumannii clinical strain

Acinetobacter baumannii RYC 52763/97, a clinical isolate involved in a prol onged nosocomial outbreak at our hospital, was resistant to all beta-lactam s tested, including imipenem and meropenem, which had MICs of 128 and 256 m u g/ml, respectively. This strain synthesized three beta-lactamases: a plas mid-mediated TEM-1 beta-lactamase (pI 5.4), an AmpC-type chromosomal cephal osporinase (pI 9.4), and a novel, presumptively chromosomally mediated OXA- related enzyme (pl 9.0) named OXA-24. After cloning and sequencing, the ded uced amino acid sequence of the OXA-24 beta-lactamase shelved 40% homology with the OXA-10 (PSE-2) and OXA-7 beta-lactamases, 39% homology with the OX A-11 and OXA-5 enzymes, and 33% homology with the LCR-1 beta-lactamase. The amino acid sequence of the OXA-24 beta-lactamase contained the STFK motif found in serine beta-lactamases, but the typical class D triad KTG was repl aced by KSG and the motif YGN was replaced by FGN. The OXA-24 beta-lactamas e hydrolyzed benzylpenicillin and cephaloridine but lacked activity against oxacillin, cloxacillin, and methicillin. The enzymatic activity was inhibi ted by chloride ions and by tazobactam (50% inhibitory concentration [IC50] , 0.5 mu M), sulbactam (IC50. 40 mu M), and clavulanic acid (IC50, 50 mu M) . Carbapenem MICs for an Escherichia coli transformant (pBMB-1) expressing the cloned OXA-24 enzyme had a fourfold increase. Relative V-max/K-m values of 13 and 6 were obtained with imipenem and meropenem, respectively, and a positive microbiological assay result with imipenem was obtained with a pu rified enzymatic extract of this transformant strain. Therefore, we conside r this new beta-lactamase to be involved in the carbapenem resistance of A. baumannii RYC 52763/97.