Selection and properties ofPseudomonas aeruginosa variants resistant to beta-lactam antibiotics

The relation between basal and inducible-lactamase production and resistance to-lactam compounds was studied in five clinicalPseudomonas aemginosa isolates and their corresponding resistant variants selected in the presence of either piperacillin, ceftazidime or aztreonam. In all wild-type strains enzyme levels were barely detectable in the uninduced state and most-lactams, including sulbactam and clavulanic acid, exhibited poor induction potency. Imipenem proved to be the most potent inducer in both these strains and their resistant variants. In the variants selected by either piperacillin or ceftazidime enzyme production amounted to 1.28 units/mg protein of the cell-free supernatants following the addition of-lactams as inducers. Additionally, these variants exhibited the phenomenon of non-specific induction, i.e. the increase of enzyme production by either a complex nutrient medium or by addition of vitamins. Enzyme production in the aztreonam-resistant variants was identical to that in the wild-type strains with a single exception, where the entire derepression of-lactamase production in one of the variants took place. Derepression of the chromosomally mediated enzyme affects the susceptibility to ureidopenicillins more than that to carboxy-penicillins and cephalosporins, whereas the-lactamase-independent resistance results in increased resistance to all-lactams with the single exception of imipenem.