SUMMARYTwo P-lactamases, A and B, have been shown to be present in a strain of Yersinia enterocolitica (w222). P-Lactamase A hydrolyses a variety of penicillins and cephalosporins. This enzyme is sensitive to thiol reagents, is only partially inhibited by 0-1 mM-cloxacillin and has a molecular weight of approximately 20000. P-Lactamase B shows strong cephalosporinase activity but does not hydrolyse some of the penicillins. It is more resistant than P-lactamase A to thiol reagents, is completely inhibited by 0-1 mM-cloxacillin and has a molecular weight of about 34000. With cephaloridine as a substrate, which is readily hydrolysed by both enzymes, about 85 % of the total activity of a cell extract is due to P-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 P-lactamase B.Two P-lactamases similar to enzymes A and B have been found in five other strains of Y. enterocolitica. In contrast, only one p-lactamase, similar to enzyme B, has been detected in a different strain of Y. enterocolitica (~66), 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 p-lactamase production.

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The P-lactamase isoelectric focusing patterns of 37 strains of Yersinia enterocolitica from various serological and biochemical groups were examined. Strains of different serological groups generally had different patterns, but those of serological groups I , 2, 3 and g were identical.

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