The virulence in the guinea-pig of isoniazid-sensitive tubercle bacilli isolated from South Indian patients before treatment and after three months of chemotherapy.

In order to find out whether chemotherapy with isoniazid affects the virulence in the guinea-pig of tubercle bacilli that do not develop resistance to the drug, virulence tests were carried out on isoniazid-sensitive cultures obtained from 20 South Indian tuberculous patients before treatment and after three months of chemotherapy with isoniazid. No significant difference in virulence was observed between the cultures obtained on admission to treatment and those obtained after three months of chemotherapy. This is a finding with important implications for large-scale surveys of the distribution of attenuated strains of tubercle bacilli from untreated patients in India and other countries. Detailed and repeated inquiries as to previous chemotherapy are not important in such surveys, provided that sensitivity tests are done on all the cultures.