Studies from Madras had shown that the strain of M.tuberculosis isolated from south India were low virulent to guineapigs. The relationship between virulence in guineapigs and pathogenesis in humans could not be established earlier. A study was conducted to investigate the relationship of virulence with the pathogenesis by comparing the virulence of isolates from pulmonary tuberculosis with that from patients with TB meningitis. The strains of bacilli were obtained from three different sources: a) Sputum from rural tuberculosis patients living near Bangalore city, b) sputum of TB patients living in the city and c) from Cerebrospinal fluid (CSF) of patients suffering from tuberculous meningitis and admitted in different institutions in Bangalore city. The specimens were processed by standard recommended procedures and cultured on Lowenstein Jensen medium. The identification of an isolate as M. tuberculosis was based on the niacin test. Albino Guinea pigs of both sexes (who were bred and raised at this Institute) were used for the tests. The virulence assay and the calculation of the root-index of the virulence (RIV) were carried out according to the Mitchison method.

1) As per the RIV method, virulence has been classified into low, moderate and high. The study showed that the percentages of cultures having isolates of low, moderate and high virulence, were the same as that of isolates obtained from patients in Madras, reported by Mitchison et al., in 1960. 2) The distribution of the RIV of virulence of isolates from patients living in the city of Bangalore was significantly different (p < 0.05) from that of isolates from patients living in rural Bangalore. 3) The number of cultures classified as high virulent were significantly greater in isolates from patients with tuberculous meningitis compared with those from patients with pulmonary tuberculosis. However, 36% of the isolates from the meningitis group were of low virulence.