Sensitivity to drugs of Australian leptospiral serotypes.

The in vitro susceptibilities to antibiotics of stock strains of various Australian leptospiral serotypes were established. Leptospira pomona was highly susceptible to penicillin and erythromycin, susceptible to streptomycin and tetracyclines, but resistant to chloramphenicol and novobiocin. In general, the twelve serotypes tested were susceptible to streptomycin, penicillin and tetracycline, but L. icterohaemorrhagiae, L. canicola and L. zanoni were more resistant than the other serotypes, especially to penicillin. A newly isolated strain of L. zanoni was more sensitive to penicillin, streptomycin, and tetracycline than was the stock strain. Against L. australis high concentrations both of penicillin and of streptomycin were bactericidal. In lower concentrations penicillin was bacteriostatic. In high concentrations penicillin caused lysis of leptospirae, but streptomycin and the tetracyclines did not. These results do not agree well with those on the chemotherapy of experimental infections.