Factors involved in the enhanced efficacy against gram-negative bacteria of fourth generation cephalosporins.
The fourth generation cephalosporins, cefpirome and cefepime, demonstrate better activity against strains of Enterobacter cloacae with derepressed beta-lactamase than the third generation compounds cefotaxime and ceftriaxone. Several methodological refinements were used to measure the parameters, predicted by the Zimmermann-Rosselet equation to be important in the efficacy of beta-lactams. Outer membrane permeability was measured by a novel HPLC method. The kinetics of interaction of purified beta-lactamase with beta-lactams were estimated to calculate the inhibition and catalytic constants. The periplasmic concentration of beta-lactams leading to growth inhibition of cells was determined by substituting the above parameters into the Zimmermann-Rosselet equation. Consideration of these three factors allowed accurate prediction of MICs in isogenic E. cloacae strains with differing porin or beta-lactamase contents. The fourth generation cephalosporins had markedly reduced affinity for beta-lactamase and increased outer membrane permeability when compared to the third generation cephalosporins. Such advantages were only partly offset by a lower stability of complexes with beta-lactamase and reduced affinity for their targets.
Hancock
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Canadian Bacterial Diseases Network, University of British Columbia, Vancouver.
Bellido
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D000900:Anti-Bacterial Agents / Q000378:metabolism / Q000494:pharmacology*; D002463:Cell Membrane Permeability; D002511:Cephalosporins / Q000494:pharmacology*; D006090:Gram-Negative Bacteria / Q000187:drug effects* / Q000378:metabolism; D006868:Hydrolysis; D007700:Kinetics; D008826:Microbial Sensitivity Tests
D000900:Anti-Bacterial Agents; D002511:Cephalosporins