Evidence for multiple forms of type I chromosomal beta-lactamase in Pseudomonas aeruginosa

Abstract

The multiple stages of derepression of the type I chromosomal beta-lactamase in Pseudomonas aeruginosa were examined. Mutants partially and fully derepressed for beta-lactamase were selected from a wild-type clinical isolate. An analysis of the beta-lactamase produced by these mutants and the induced wild type revealed significant differences in the products of derepression at each stage. Beta-lactamase produced by the fully derepressed mutant showed a lower affinity (Ki,m 0.113 mM) for cephalothin than that produced by the partially derepressed mutant (Km, 0.049 mM). However, due to a very large Vmax, the former possessed a much greater hydrolytic efficiency. Differences in substrate profile were also noted. Only beta-lactamase from the fully derepressed mutant hydrolyzed cefamandole, cefoperazone, and cefonicid. The partially derepressed mutant possessed a single beta-lactamase band with a pl of 8.4. The fully derepressed mutant possessed this band and an additional major band with a pl of 7.5. Induction of the wild type with cefoxitin produced both bands. The changes in physiologic parameters of the enzymes produced in the different stages of derepression suggest a complex system for beta-lactamase expression in P. aeruginosa. This may involve at least two distinct structural regions, each of which is under control of the same repressor.

Antibacterial-ResistantPseudomonas aeruginosa: Clinical Impact and Complex Regulation of Chromosomally Encoded Resistance Mechanisms

“…Although other investigators have suggested that imipenem hypersusceptibility is a result of disrupted ampC induction (151), the observation VOL. 22,2009 ANTIBACTERIAL-RESISTANT PSEUDOMONAS AERUGINOSA 601 that AmpC overproduction does not significantly increase imipenem MICs (63,131,169,274) argues against this hypothesis. Furthermore, our own studies have shown that imipenem-hypersusceptible mutants can be selected from P. aeruginosa parent strains that are partially and fully derepressed for ampC expression, without any changes in either the expression of ampC or AmpC hydrolysis activity (274).…”

“…In contrast, published data have suggested that overproduction of AmpC does not play a discernible role in the development of carbapenem resistance among P. aeruginosa isolates. AmpC overproduction among isogenic mutants selected with ␤-lactams does not significantly decrease P. aeruginosa susceptibility to carbapenems (63,131,169,274). In addition, AmpC overproduction in carbapenemsusceptible clinical isolates of P. aeruginosa has been reported.…”

Antibiotic combinations: should they be tested?

“…Derepression of chromosomally mediated beta-lactamases has recently been shown to be a more complex process than initially appreciated, involving two or more stages of genetic control and possibly synthesis of multiple enzyme forms (62).…”

AmpC and OprD Are Not Involved in the Mechanism of Imipenem Hypersusceptibility among Pseudomonas aeruginosa Isolates Overexpressing the mexCD - oprJ Efflux Pump

“…Masuda et al suggested imipenem hypersusceptibility was due to a disruption in the induction pathway of AmpC cephalosporinase (13). However, imipenem is relatively stabile to AmpC hydrolysis, and large increases in AmpC activity alone do not significantly decrease imipenem susceptibility (2,10,11,18,23). Therefore, a disruption of ampC expression should not be solely responsible for imipenem hypersusceptibility among mexCD-oprJ-overexpressing P. aeruginosa.…”