Growth characteristics of recent sputum isolates of Mycobacterium tuberculosis in guinea pigs infected by the respiratory route.

The consideration of virulence must distinguish between infectivity and the ability to cause progressive disease once the infection is established. Several investigators have reported the presence of naturally occurring isolates which differ in virulence for guinea pigs. Isolates from south India which differed with respect to gross disease and number of bacilli recovered from spleen after an intramuscular infection also differed in their efficiencies to initiate an infection, once inhaled and retained. Also, this difference was correlated with differences in the rate of multiplication at the site of implantation and rate of multiplication at sites of hematogenous seeding, as well as the extent of hematogenous seeding. The number of metastatic foci was identified as a quantitative measure of hematogenous seeding, which was not confounded by the rate of multiplication of bacilli. Even allowing for the fourfold-reduced efficiency of low-virulence tubercle bacilli to produce a lesion, this measure clearly revealed a significantly reduced ability of the low-virulence tubercle bacilli to disseminate via the bloodstream.