Pseudomonas aeruginosa beta-lactamase as a defence against azlocillin, mezlocillin and piperacillin.

Azlocillin, mezlocillin and piperacillin are weak substrates for the chromosomal beta-lactamase of Pseudomonas aeruginosa, and hydrolysis kinetics were calculated. Enzyme function in the living cell was studied by comparing antibiotic activity against a typical Ps. aeruginosa strain with inducible beta-lactamase expression with antibiotic activity against beta-lactamase uninducible and constitutive mutants. The inducible organism was less sensitive than its uninducible mutant to all three agents; this difference was more apparent at high inocula than low and in broth than in agar. These differences involved both enzyme induction and selection of genotypically enzyme derepressed variants. The penicillins were not, however, efficient beta-lactamase inducers at low concentrations and their activity against the inducible organism was antagonized by more potent inducers. Secondary inducers did not antagonize antibiotic activity against beta-lactamase uninducible and constitutive organisms. The beta-lactamase constitutive mutants were highly resistant to the three antibiotics tested.