Quantitative and qualitative determinations of the combined effect of tetracycline and oleandomycin. II. In vivo effect.

The combination effect of tetracycline (TC) and oleandomycin (OM) on acute infection of mice with four strains of Staphylococcus aureus including TC or OM resistant ones was examined by the quantitative determination of protective potencies of single and combined drugs. The grade of synergism was expressed by the synergistic ratio (SR), a ratio of experimentally determined potency of the combined drug over a hypothetical potency in which additive effect of the both drugs is assumed. With 3 out of the 4 strains of S. aureus synergism between TC and OM or triacetyloleandomycin (TAO) was demonstrated by the determination of the 50% effective dose and by statistical examination of the SR. The grade of synergistic protection by these drugs varied with the strains infected and it did not depend upon the sensitivity to antibiotics or grade of synergism in vitro. There was no synergistic enhancement of acute toxic action in the combined administration of TC and OM to mice.