The β-lactamase of the soil-borne strain 108 (parental strain) of Nocardia asteroides is a non-inducible enzyme mainly associated with the cells; it can be efficiently extracted by ultra-sonication and SDS treatment. Crude enzyme preparations showed penicillinase and cephalosporinase activity. The kinetics of β-lactamase production and in-vitro susceptibility to combinations of β-lactam antibiotics plus β-lactamase inhibitors have been studied in two stable overproducer mutants (A14 and B1) obtained by mutagenization of the parental strain with nitrosoguanidine. The cell-associated enzyme increased with bacterial growth in parental and mutant strains and was particularly abundant in stationary phase cells. The β-lactamase inhibitors sulbactam and clavulanic acid decreased MIC values of penicillins more efficiently in the parental strain than in mutants, thus indicating some involvement of the enzyme in the resistance of N. asteroides strain 108 to β-lactam antibiotics.
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