Effects of inner-membrane-associated beta-lactamase on penicillin-binding protein assays. Study of stably derepressed Pseudomonas aeruginosa strains from experimental endocarditis.

We studied penicillin-binding protein (PBP) profiles of two Pseudomonas aeruginosa strains stably derepressed for constitutive, type Id beta-lactamase overproduction. Substantial levels of beta-lactamase were found to be strongly associated with isolated inner membranes of these two strains, as well as from a plasmid-encoded, TEM-2 beta-lactamase-producing control strain of P. aeruginosa. The inner membrane-associated beta-lactamase resulted in significant decreases in the intensity on autoradiographs of PBPs labelled with 35S-penicillin, yielding spurious PBP profiles for these strains. Inner membrane beta-lactamases could be substantially removed by a sonication-ultracentrifugation step, producing the bona fide PBP profiles.