Antibacterial activity of four cephalosporins in an experimental infection in relation to in vitro effect and pharmacokinetics.

The in vitro activity of four cephalosporins was compared with their effects in an experimental thigh infection (cefuroxime and cefamandole against Escherichia coli and cefamandole, ceftriaxone, and ceftazidime against Klebsiella pneumoniae) in granulocytopenic mice. The effect in vitro (ER) was defined as the difference between the growth rate without antibiotic and the growth rate at the steepest part of a 3-h growth curve in the presence of an antibiotic. The relation between concentration and ER was described with the Hill equation. Using pharmacokinetic parameters of the plasma concentrations in vivo and those of the Hill equation the corresponding time course of ER was calculated and by integration with respect to time (0tERdt), an estimate was obtained of the effect on bacteria. For all four antibiotics this estimate was significantly correlated with the actual values of the effect in vivo (EN), defined as the difference in numbers of bacteria between controls and antibiotic-treated animals at 4 h.

Comparison of in vivo and in vitro activities of antibiotics with different modes of action against a tolerant and a non-tolerant Staphylococcus aureus strain.

The antibacterial efficacies of 4 antibiotics with different modes of action against a penicillin-tolerant and a non-tolerant strain of Staphylococcus aureus were investigated. For the in vitro studies the minimum inhibitory concentration (MIC) and the minimum bacterial concentration (MBC) were determined and short-term growth experiments at different antibiotic concentrations were performed. For the in vivo studies, antibacterial efficacy in an experimental infection in normal and granulocytopenic mice was evaluated. For erythromycin, rifampicin and ciprofloxacin, there was no difference in the MIC and MBC values for the 2 strains. Benzylpenicillin had an MBC value for the tolerant strains which was 256 times higher than the MIC; with the non-tolerant strain there was no difference. EC50 values, calculated from the in vitro short-term growth curves, gave similar results. Only benzylpenicillin exhibited a difference in activity against the tolerant strain, as reflected by the EC50 that was 290 times the EC50 for the non-tolerant strain. Studies in normal and granulocytopenic mice gave similar results: benzylpenicillin was 268 times less active against the tolerant strain than against the non-tolerant strain. Erythromycin, rifampicin and ciprofloxacin were 2-3 times less active against the tolerant strain than against the non-tolerant strain. The presence of granulocytes is important for the antibacterial effect of all antibiotics studied, since in the absence of granulocytes higher doses of the antibiotics are needed in order to obtain the same antibacterial effect as when granulocytes are present.

The efficacy of rifampicin against Staphylococcus aureus in vitro and in an experimental infection in normal and granulocytopenic mice.

The effect of rifampicin on Staphylococcus aureus in vitro was assessed as the difference between the logarithms of the numbers of colony forming units (CFU) with and without 3 h of exposure to the drug. The efficacy was expressed as the EC50, i.e. the concentration at which 50% of the maximal effect was obtained, calculated according to the Hill equation. The value found for the EC50 was 3.8 micrograms/l and the mean maximal effect was a log ratio of 5.03 (SEM 0.33). In vivo experiments were performed in normal mice and in mice made granulocytopenic by irradiation. The effect of rifampicin was assessed as the CFU count 5 h after the injection of a suspension of bacteria into the thigh muscle and 4 h after the administration of rifampicin. The efficacy was expressed as the ED50, i.e. the dose at which 50% of the maximal effect is obtained. This value was 0.18 mg/kg for the normal mice and 0.15 mg/kg for the granulocytopenic mice. The corresponding mean plasma concentrations of non-protein-bound drug were 28 and 24 mg/l, respectively. Thus, the EC50 was found to be much higher in vivo than that in vitro. This difference should be taken into account when parameters of in-vitro efficacy are applied to establish dosage schedules.

Quantitative effect of granulocytes on antibiotic treatment of experimental staphylococcal infection.

The quantitative relation between granulocytopenia and antibiotic treatment was established for a short-term Staphylococcus aureus infection in the thighs of mice, using rifampin, benzylpenicillin, and erythromycin. Granulocytopenia was induced by total-body irradiation; the number of granulocytes decreased gradually during the first 5 days after irradiation to 10% of the number in normal mice. Experimental infections were established on each of the 5 days after irradiation. In animals not treated with antibiotics, the number of granulocytes in peripheral blood and the number of CFU at the site of infection exhibited a strong negative correlation. The influence of granulocytes on the effect of antibiotics on the number of CFU differed for the three antibiotics. For erythromycin the slope of the dose-effect relation was rather flat, but a decrease in the number of granulocytes caused a significant, nearly parallel, shift in the dose-effect relation, resulting in an increase in the number of CFU. For benzylpenicillin the slope of the dose-effect relation for normal mice was also flat, but as the number of granulocytes decreased the slope became significantly steeper, resulting in a diminishing influence of granulocytes at higher dosages. For rifampin the slope of the dose-effect relation, which was already steep for nonirradiated animals, increased significantly. Here too the effect of granulocytes decreased as the dose increased.