The sensitivity of 25 strains of , comprising strains of the biotypes commonly encountered in the United Kingdom, was determined to six antibiotics and co-trimoxazole (a combination of sulphamethoxasole and trimethoprim) by means of four methods, including a modification of the ditchdiffusion method. The ditch method was shown to be satisfactory for routine sensitivity testing of brucellae. The strains were almost uniformly sensitive to tetracycline, and the MIC values were between four and eight times less than the expected peak plasma levels. The strains were not as sensitive to streptomycin as those previously reported by Spink (1956), and the MIC values to this antibiotic were distributed over a wider range of concentrations than those of gentamicin or kanamycin. A comparison of MIC, peak plasma levels and plasma half-lives suggest that gentamicin and kanamycin may be more effective than streptomycin in the treatment of brucellosis when used in combination with tetracycline.

The failure of ampicillin in the treatment of brucellosis accords with the finding that of 25 strains 17 had MIC values in excess of the peak plasma levels. An experiment with carbenicillin indicated that it was unlikely to be effective in the treatment of infection.

Synergism between sulphamethoxazole and trimethoprim was demonstrated on all strains examined. The strains were less sensitive to trimethoprim than sulphamethoxazole, indicating that a high plasma value for trimethoprim should be maintained, but further clinical trials of co-trimoxazole are necessary.


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