In vivo and in vitro efficacy of rifampicin and tetracycline hydrochloride, used alone and in combination, was tested against one strain of Brucella suis. The minimal inhibitory concentrations of rifampicin and tetracycline hydrochloride against this strain were 2 and 0.2 microgram/ml, respectively. In vitro efficacy was evaluated by inhibition of bacterial growth in liquid medium. Rifampicin killed 99.9% of the Brucella after 18 h of incubation of concentrations of 2 and 4 microgram/ml. Tetracycline hydrochloride was bacteriostatic at concentrations of 0.2 and 0.4 microgram/ml. Combinations of rifampicin and tetracycline hydrochloride had the same bactericidal efficacy as rifampicin alone when bactericidal concentrations of rifampicin (greater than 2 microgram/ml) were used. In contrast, a highly additive effect was observed with combinations containing non-bactericidal concentrations of rifampicin (less than 2 microgram/ml). In vivo efficacy was tested in experimentally infected mice by enumeration of B. suis in the spleen. Administration of rifampicin (25 mg/kg/day) or tetracycline hydrochloride (200 mg/kg/day) for 21 days cured 12 of 22 mice and 1 of 22 mice, respectively. Administration of both rifampicin and tetracycline hydrochloride for 10 days did not improve efficacy when compared to rifampicin alone. Concentrations of rifampicin and tetracycline hydrochloride in the serum and spleen of mice after single or combined administration were assayed using the large plate agar diffusion method. The presence of one antibiotic did not reduce serum and spleen concentrations of the other. The spontaneous mutation rate of B. suis against rifampicin (25 microgram/ml) was estimated to be 2.5 X 10(-9) per cell division. No rifampicin-resistant colony was detected out of 3,800 CFU originated from spleens of mice which were infected than treated with rifampicin.
