1989 Volume 37 Issue 12 Pages 1458-1466
To examine the effect of β-lactams on the induction of β-lactamase in Pseudomonas aeruginosa, we used the disc method. When an imipenem disc was placed adjacent to a β-lactam disc the inhibition zone produced by the β-lactam disc was truncated on the side closest to the imipenem disc. When enzyme synthesis was induced by these double disc tests, the activity of most β-lactams was antagonised significantly against β-lactamase-inducible strains. A mpicillin, penicillin G, cephaloridine and cefmetazole in addition to imipenem also induced β-lactamase production in P.aeruginosa. In more than 90% of clinical isolates of P.aeruginosa strains, β-lactamase synthesis was induced by β-lactam disc. Using the disc method, we estimated the optimal concentration of β-lactam for β-lactamase induction.
β-Lactam higly resistant mutants were isolated at a frequency of about 10-5 to 10-6 by the disc method. Most of the mutants selected by the β-lactams produced inducible β-lactamases at a high level. However, the mutants isolated by imipenem disc were resistant to imipenem only, and the enzyme activities of these resistant mutants were almost the same as those of theparent strains.
