Kinetics and Mechanisms of Drug Action on Microorganisms XX: Integrated Receptor Site Model Rationalizing Observed Microbial Rate Dependencies on Drug Concentration and Microbial Kinetics Affected by Sodium Fusidate

Abstract

The steady-state generation of Escherichia coli (ATCC 12407) was inhibited by the antibiotic sodium fusidate in the drug concentration range of 0–100 μg/ml. There was no significant change in drug activity when the initial inoculum size was varied between 5 × 105 and 2 × 106E. coli/ml. The drug activity was unaffected by varying the concentrations of ammonium ions, magnesium ions, amino acids, dextrose, or citrate components of the nutrient broth in Anton’s medium over a fourfold concentration range and did not implicate binding to the components. The variation of drug activity with pH could be assigned completely to the action of the unionized species. Sodium fusidate was bacteriostatic up to 500 μg/ml, about five times the minimum inhibitory concentration, and the action of the drug was readily reversible on dilution. Resistant mutants of E. coli appeared after exposure of the organisms to sodium fusidate, and the numbers of such resistant organisms appeared to be a function of the inoculum size at the time of drug addition. No significant inactivation of the drug by resistant organisms was observed. The dependence of the apparent generation constant, kapp, on drug concentration in the medium is an S-shaped curve, which can be rationalized on the premise of a saturable enzymatic degradation of drug in the biophase of the microorganism. The same model, which considers the steady-state microbial generation as a consequence of rates of drug permeation into the biophase of the organism, the saturable biophasic degradation of the drug, and the reaction of drug with saturable receptor sites that leads to generation inhibition, can serve as a proper rationalization of all previously observed various generation rate dependencies on drug concentrations for different drugs. The action of sodium fusidate in combination with tetracycline, clindamycin, and 1′-demethyl-4′-depropyl-4′-[(R) and (S)]-n-pentylclindamycin is indifferent; with erythromycin and oleandomycin, it is mildly synergistic; and with novobiocin, it is antagonistic.