The virulence of tubercl becilli to experimental animals varies widely according to the conditions of host and parasite. The study was undertaken with the purpose of analyzing the relation between the environment for the growth of tubercle bacilli (Ravenel strain) and their virulence. The following conditions were kept as constant as possible such as the inoculum dosis, the state of dispersion of bacterial cells in suspension, the age of culture, and some conditions of experimental animals (strain and age) .
I. Observations on the organismsin vitro
a) A culture in tween-albumin liquid medium (Dubos' liquid medium) induced more severe tuberculsis in mice, rabbits or guinea pigs than a culture in Sauton's synthetic medium or Ogawa's solid medium. (see Table 1, 2 and 3)
b) The difference of virulence between Dubos' cultlre and Ogawa's culture was kept unchanged through a series of passages in mouse lung. (see Table 4)
c) The virulence of Dubos' culture was lowered easily by subculture in Ogawa's medium. (see Table 5)
II. Observations on organismsin vivo
a) A strain from lung of the mouse, injected with Dubos' culture, was more virulent to mice, rabbits or guinea pigs than a strain from liver of the same mouse. (see Table 6and7)
b) The strain from lung was decreased in virulence by passage through liver of mice, on the other hand, the strain from liver was increased in virulence by passage through lung. (see Table 8)
c) The strain from lung was increased in virulence by subculture in Dubos' medium, while decreased by subculture in Ogawa's medium. Though the strain from liver was increased in virulence by subculture in Dubos' medium, this subculture did not show the same level of virulence as the subculture of the strain from lung in Dubos' medium. No difference in virulence was observed between the strain from liver and subculture of the same strain in Ogawa's medium. (see Table 9)