W222 (serological group 3) synthesized two different intracellular β-lactamases, called A and B. Enzyme B was more sensitive than A to inhibition by cloxacillin. The minimum inhibitory concentrations of various β-lactam antibiotics for strains of of groups 3 and 9 and the effect of cloxacillin on these concentrations suggested differential roles for β-lactamases of types A and B in penicillin and cephalosporin resistance. Type B enzymes protected against cephalothin and cephalosporin C, whereas type A enzymes protected very efficiently against carbenicillin. Protection against other β-lactam antibiotics was exerted by both enzymes. However, while both enzymes readily hydrolysed cephaloridine and showed no crypticity with this substrate, they only conferred a very weak protection against it. This may be because cephaloridine reached its target quickly, before it was degraded. The resistance of strains of from groups 1, 2 and 16 was also explicable in terms of a two-enzyme system, whereas strains belonging to group 5b produced only a type B lactamase and were sensitive to carbenicillin.


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