Synergistic action of streptomycin with other antibiotics of intracellular Brucella abortus in vitro

Richardson, Marvis (Michigan State University, East Lansing) and Jane N. Holt. Synergistic action of streptomycin with other antibiotics on intracellular Brucella abortus in vitro. J. Bacteriol. 84:638-646. 1962.-It has been found that streptomycin acts synergistically with penicillin and tetracycline to inhibit the growth of Brucella abortus within bovine cells cultured in vitro. Extracellular penicillin at 10 to 50 mug/ml was essentially bacteriostatic for intracellular brucellae. A combination of 5 to 50 mug/ml of streptomycin, which alone did not prevent the multiplication of intracellular brucellae, and 50 mug/ml of penicillin was bactericidal. Tetracycline at 0.5 to 1.0 mug/ml, which permitted growth or was bacteriostatic for brucellae within tissue cells, proved markedly bactericidal when combined with 10 mug/ml of streptomycin. Within certain limits, the synergistic effect of streptomycin and tetracycline did not appear to be a function of concentration. When the addition of streptomycin was delayed until 24 hr after the addition of tetracycline, the antibiotics acted synergistically on intracellular brucellae. To achieve intracellular synergism, five to ten times the effective extracellular concentration of streptomycin was required. Penicillin and tetracycline acted at the same concentration on extracellular and intracellular brucellae. These results suggest that tissue cell systems may prove useful for study of multiple drug therapy to be used in vivo against intracellular bacteria.

Transmissible drug resistance in an epidemic strain of Salmonella typhimurium

Among 309 cultures ofSalmonella typhimurium, phage-type 27, fifteen, isolated from eight patients, were found to be resistant to the three drugs, streptomycin, tetracycline and sulphathiazole. This triple resistance could be transferred by growth in mixed broth culture to a strain ofSkigella sonneiand back again to sensitive cultures ofS. typhimurium. In whole cultures the resistance was stable, but spontaneous loss could be demonstrated in a small proportion of the organisms in such cultures. No elimination of resistance was demonstrated after treatment with acriflavine. Resemblances to the multiple drug resistance in enteric bacteria reported from Japan are noted.

The author is most grateful to Dr E. S. Anderson, Director of the Enteric Reference Laboratory, Colindale, N.W. 9, for phage-typing the cultures ofSalmonella typhimurium, and to Dr K. Patricia Carpenter, Director of the Dysentery Reference Laboratory, for supplying cultures ofSh. sonneiwhich were essential for the experimental work.