A comparison of aztreonam and imipenem induction of Class I β-lactamase in Enterobacter cloacae ATCC 13047

Aztreonam and imipenem were shown to induce Class I beta-lactamase in Enterobacter cloacae ATCC 13047 to a similar extent. Quantitatively, however, aztreonam was far less efficient as an inducer than imipenem. Optimum induction by aztreonam required a concentration of 200 mg/l, which was 800-fold greater than the concentration of 0.25 mg/l of imipenem which resulted in the optimum induction. The differences in the concentrations of aztreonam and imipenem that gave optimum induction were related to the inherent antibacterial activities of the antibiotics when these were determined under the conditions of broth culture. The beta-lactamase activity of sonicated cell samples following induction was inhibited by the presence of aztreonam but not by imipenem. The inhibition was overcome by first washing the cell samples from induced cultures and then incubating the sonicates for a prolonged period at 4 degrees C. It is proposed that the phenomena of an optimum inducing concentration and the interference with the assay of beta-lactamase by the presence of residual antibiotic demonstrated in this study with aztreonam and imipenem would be of relevance when applied more broadly to studies of beta-lactamase induction. In particular these would have a profound effect on the results of studies which attempt to compare the efficacy of beta-lactams as inducers of Class I beta-lactamase.