COMPARISON OF CEFEPIME, CEFPIROME, AND CEFACLIDINE BINDING AFFINITIES FOR PENICILLIN-BINDING PROTEINS IN ESCHERICHIA-COLI K-12 AND PSEUDOMONAS-AERUGINOSA SC8329

The relative binding affinities of the extended-spectrum cephalosporins cefepime, cefpirome, and cefaclidine for the penicillin-binding proteins (PBPs) of Escherichia coli K-12 and Pseudomonas aeruginosa SC8329 were determined. Affinities were calculated from competition experiments between these antibiotics and [H-3]benzylpenicillin in isolated membrane preparations. The concentrations which reduced binding to a PBP by 50% (IC50S) were determined. For E. coli, all three antibiotics displayed good PBP 3 binding (IC50S of 0.5-mu-g/ml or less), and MICs roughly correlated with these values. Cefepime had a greater than 20-fold-lower IC50 for PBP 2 of E. coli than the other antibiotics. For P. aeruginosa, all of the antibiotics bound poorly (> 25-mu-g/ml) to PBP 2 but showed excellent pseudomonal (< 0.0025-mu-g/ml) PBP 3 binding. No correlations were seen between IC50S and MICs for P. aeruginosa. Despite differences in PBP binding, cefepime, cefpirome, and cefaclidine all displayed similar bactericidal activity for E. coli K-12 over the initial 3 h after antibiotic addition. All three caused E. coli to form filaments at values close to the MICs. In addition, cefepime induced "bleb" formation along the filaments at concentrations > 10x the MIC.