SUMMARY: Two β-lactamases, A and B, have been shown to be present in a strain of (W222). β-Lactamase A hydrolyses a variety of penicillins and cephalosporins. This enzyme is sensitive to thiol reagents, is only partially inhibited 0.1 mM-cloxacillin and has a molecular weight of approximately 20000. β-Lactamase B shows strong cephalosporinase activity but does not hydrolyse some of the penicillins. It is more resistant than β-lactamase A to thiol reagents, is completely inhibited 0.1 mM-cloxacillin and has a molecular weight of about 34000. With cephaloridine as a substrate, which is readily hydrolysed both enzymes, about 85% of the total activity of a cell extract is due to β-lactamase A and 15% to B. Addition of 6-aminopenicillanic acid to the culture during growth results in a 2- to 4-fold selective increase in the amount of β-lactamase B. Two β-lactamases similar to enzymes A and B have been found in five other strains of In contrast, only one β-lactamase, similar to enzyme B, has been detected in a different strain of (H66), which is abnormal in that it is sensitive to ampicillin. Addition of 6-aminopenicillanic acid to cultures of this strain results in an 8- to 10-fold increase in β-lactamase production.


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