PBP binding and periplasmic concentration as determinants of the antibacterial activities of three new oral cephalosporins in Escherichia coli.

The antibacterial activities of cefetamet, cefixime and cefuroxime were investigated in Escherichia coli with regard to their penetration rates through the outer membrane and their affinities for PBPs in Escherichia coli. The permeability coefficient of cefetamet was measured in E. coli C600 carrying a pUC18 plasmid derivative, in which the gene of a transferable cephamycinase (CMY-2) was cloned, whereas diffusion of cefixime and cefuroxime was measured in E. coli C600 harbouring the OXA-1 beta-lactamase gene. It was found that cefetamet penetrated 5 and 9 times faster than cefixime and cefuroxime, respectively. The correlation between antibacterial activities and PBP affinities was studied in E. coli C600 not producing beta-lactamases. In this strain, cefetamet and cefixime shared the same inhibitory activity (MIC = 1 microgram/ml for both antibiotics) and the same affinity for PBP 3 (ID50 = 0.25 micrograms/ml). Cefuroxime had a lower inhibitory activity (MIC = 4 micrograms/ml) and a lower affinity for PBP 3 (ID50 = 0.5 microgram/ml). Cefixime and cefuroxime had 20 and 10 times higher affinity, respectively, than cefetamet for PBP 1s. It was concluded that the superior PBP affinity of cefixime can counterbalance the better penetration of cefetamet through the outer membrane, whilst differences in either permeability or PBP affinity seem sufficient to explain the lower antibacterial activity of cefuroxime compared to cefixime, which might well be related to the poor stability of the cefuroxime molecule.