Effect of cortisone administration on the pathogenicity of Nocardia asteroides, N. caviae, and N. brasiliensis for white mice has been investigated by using the intravenous route of inoculation. The observations indicated that the susceptibility of white mice to nocardiosis was enhanced by cortisone. Test strains of the three species of Nocardia caused a higher and more rapid mortality, as well as more extensive lesions, in the cortisone-treated than in the normal mice. The mean lethal dose (LD50) values of N. asteroides and N. caviae for the cortisonetreated group were found to be seven and eight times lower than their respective values for the normal group. N. asteroides and N. caviae were more virulent than N. brasiliensis, the LD50 of N. brasiliensis for cortisone-treated mice being 30 and 26 times higher than that of the former two species, respectively. N. brasiliensis also differed from the other two species in its inability to infect the brain. In the untreated animals, N. asteroides and N. caviae showed a tendency to form conglomerate growth, in contrast to formation of freely dispersed growth in the lesions of cortisone-treated animals.
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