Evidence for nonspecific induction of beta-lactamase in overproducing variants ofEnterobacter cloacae andCitrobacter freundii

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Abstract
Induction of chromosomally mediatedβ-lactamases was studied in clinical isolates ofEnterobacter cloacae andCitrobacter freundii. Whereas isolates resistant to ampicillin and cefoxitin exhibited an inducible enzyme, those sensitive to both agents did not. Cefoxitin and above all imipenem proved the most efficacious enzyme inducers. Variants among these inducible isolates which produced large amounts ofβ-lactamase could be selected in the presence of cefamandole. In each of these selected variants constitutive enzyme production was markedly enhanced by induction, even with compounds lacking induction potency for the corresponding wild strains. Moreover, these variants could also be induced non-specifically, i.e. by non-β-lactam compounds. Production of large amounts of enzyme was associated with resistance to all availableβ-lactam compounds.