Microbial kinetics of drug action against gram-positive and gram-negative organisms. III: Effect of lincomycin and clindamycin combinations on Staphylococcus aureus and Escherichia coli.

The functional dependencies of apparent first-order generation rate constants kapp, of drug-affected cultures on drug concentrations indicate that lincomycin and clindamycin possess the same mechanism of action, which is bacteriostatic, against Staphylococcus aureus. Clindamycin also possesses another mechanism of action, which is bactericidal, at high concentration levels. However, clindamycin possesses only one of the two mechanisms of lincomycin action, which is bacteriostatic, against Escherichia coli. The relative potency of action of a clindamycin-lincomycin combination against Staph. aureus is variable, and the effective ratio ranges between 5:1 and 9:1; the effective ratio against E. coli is fixed at 6:1 over a wide concentration range. This difference is attributed to differences in bioavailability and/or binding characteristics of the drugs for bioreceptors, as a consequence of structural modifications in the drug molecules, and to differences in modes of action in the respective organisms. Mixtures containing equipotent fractions of clindamycin and lincomycin show "equivalence" or "indifference" of effects on Staph. aureus. The combined action of the mixtures can be quantitatively predicted from the separate dose-response curves of either component drug alone. Therefore, it is concluded that clindamycin and lincomycin may bind to the same receptor site that is engaged in microbial protein synthesis to inhibit the generation of Staph. aureus. However, combinations of clindamycin and lincomycin are less active than the a priori equipotent concentration of either drug alone in their action against E. coli, demonstrating unequivocally an antagonism of effects. Furthermore, the degree of antagonism is dependent on the order of addition of the drugs, which is attributed to the possibility that clindamycin and lincomycin bind differently on active and allosteric loci of the same receptor site functionally engaged in protein synthesis in E. coli. A rational approach to the quantification and prediction of combined antibiotic action must, therefore, be based not only on the kinetics and mechanisms of action as well as on the dose-response relationship over a wide concentration range for the separate antibiotics but also on the strain and species of the test organism.