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.