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

Acinetobacter baumannii strain A148, a clinical isolate resistant to imipenem (MIC=32 mg l(-1)), synthesized two beta-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 beta-lactamase inhibitor BRL 42715. Clavulanate was a weak inhibitor and EDTA did not affect the beta-lactamase activity. This enzyme also hydrolyzed imipenem with a catalytic efficiency (k(cat)/K-m) of 1500 mM(-1) s(-1). Moreover, this purified beta-lactamase produced a positive microbiological clover-leaf test with imipenem. Therefore, the pI 6.3 beta-lactamase was considered to be involved in the imipenem resistance of A. baumannii strain A148.