BIOCHEMICAL-CHARACTERIZATION OF A BETA-LACTAMASE THAT HYDROLYZES PENEMS AND CARBAPENEMS FROM 2 SERRATIA-MARCESCENS ISOLATES

Reexamination of Serratia marcescens isolates obtained in 1982 revealed two organisms that were resistant to the penem FCE 22101 (MIC, 512 μg/ml) and imipenem (MIC, 16 μg/ml) and that had slightly reduced susceptibilities to meropenem (MIC, 0.12 μg/ml). MICs of these agents for typical S. marcescens isolates were 1 to 8, 0.25 to 0.5, and 0.03 μg/ml, respectively. The two isolates were fully susceptible to broad-spectrum cephalosporins, and only one was highly resistant to ampicillin and carbenicillin (MICs, > 1,024 μg/ml). Both isolates had β-lactamases that focused at pIs 8.2 and 9.7. The penicillin-resistant isolate additionally produced the TEM-1 enzyme. The enzymes with pIs of 8.2 and 9.7 were separated by cation-exchange chromatography. The pI 8.2 β-lactamase was a class I enzyme of the type found in most S. marcescens isolates. It was almost inactive against carbapenems and penems, as was the class I enzyme from another S. marcescens strain. The pI 9.7 enzyme hydrolyzed penems and carbapenems rapidly: k(cat) (turnover number) values for FCE 22101, imipenem, and meropenem were 3.4, 26, and 1% of the k(cat) value for cephaloridine, respectively; k(cat)/K(m) values were 140, 915, and 150% of the k(cat)/K(m) value for cephaloridine, respectively. Otherwise, the pI 9.7 enzyme had predominantly penicillinase activity. It was inhibited more readily by clavulanate than by tazobactam and was inactivated by the chelating agents EDTA and ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Expression of the pI 9.7 enzyme was not associated with any plasmid, and production was not transferred to Escherichia coli K-12 recipients, even after the mobilizing plasmid pUZ8 was inserted into the S. marcescens donor strains.