South Indian Variant strain of M.tuberculosis has been found to be less virulent to guineapigs through various studies from Madras and Bangalore. It was not known whether the low virulent nature of the tubercle bacilli was a fixed character or a change due to serial passages inside the body over a period of time. Hence, a study was conducted to see the behaviour of low virulent tubercle bacilli over a period of eight years during twenty passages in animals.

Ten low virulent cultures of M.tuberculosis isolated from patients belonging to Bangalore area were injected intramuscularly into guineapigs. The extent of lesions was assessed through Mitchison's Virulence scoring method. Bacilli recovered from the spleen of these guineapigs were passed into another set of animals and virulence scored. Twenty serial passages were thus performed over a period of eight years. The findings revealed that throughout the study in all the passages, the south Indian Variant of M.tuberculosis maintained its low virulent character.