Oxacillin-hydrolyzing β-lactamase involved in resistance to imipenem in Acinetobacter baumannii

Abstract Acinetobacter baumannii strain A148, a clinical isolate resistant to imipenem (MIC=32 mg l−1), synthesized two β-lactamases with pIs 6.3 and >9.2. The pI 6.3 enzyme hydrolyzed the penicillins, including isoxazoylpenicillins, first-, second- and, to a lesser extent, third-generation cephalosporins. It was inhibited by chloride ions and by the penem β-lactamase inhibitor BRL 42715. Clavulanate was a weak inhibitor and EDTA did not affect the β-lactamase activity. This enzyme also hydrolyzed imipenem with a catalytic efficiency (kcat/Km) of 1500 mM−1 s−1. Moreover, this purified β-lactamase produced a positive microbiological clover-leaf test with imipenem. Therefore, the pI 6.3 β-lactamase was considered to be involved in the imipenem resistance of A. baumannii strain A148.