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Both a penicillinase and a cephalosporinase are present in a strain of Citrobacter freundii (GN7391) resistant to β-lactam antibiotics. The penicillinase was identical to the type Ia penicillinases (Type III by Richmond classification), mediated by Rms212 and R-TEM. A cephalosporinase, typical of enterobacteriaceae chromosomal β-lactamase (Type I by Richmond classification), was purified from the strain. It gave a single protein band on polyacrylamide gel electrophoresis and immunoelectrophoresis; the pI was 8·6 and its molecular weight was approximately 38000. Cysteine was not found among its amino acids. The specific activity was 388 units (mg protein)−1for the hydrolysis of cephaloridine, and the optimal pH was 8·0. Rabbit antiserum obtained against the purified enzyme showed cross-reaction with cephalosporinases produced by strains of Enterobacter cloacae in a neutralization test.
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