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Both a penicillinase and a cephalosporinase were purified from a strain of Serratia marcescens (GN7647) resistant to β-lactam antibiotics. The penicillinase was identical to the type I penicillinase, mediated by Rms212 and R-TEM. The purified cephalosporinase, a typical chromosomally mediated enterobacterial β-lactamase, gave a single protein band on polyacrylamide gel electrophoresis and immunoelectrophoresis; the pI was 9·56 and its molecular weight was approximately 37000. It contained tryptophan but not cysteine. The specific activity was 374 units (mg protein)−1 for the hydrolysis of cephaloridine, and the optimal pH was 8·5. Rabbit antisera raised against the purified cephalosporinase showed no cross-reaction in a neutralization test with cephalosporinases produced by other species of Enterobacteriaceae.
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