Virulence of human tubercle bacilli

An experimental basis for estimating the virulence of tubercle bacilli

1. Mice infected intracerebrally with M. tuberculosis H 37 Rv in doses of 400,000–1,300,000 bacillary units gave a graduated series of responses whose regression provided an arbitrary 100% standard of mouse virulence for human tubercle bacilli.

2. Under certain experimental conditions this standard remained stable and its confidence limits could be defined.

3. The relative virulence of other strains of tubercle bacilli could be assessed quantitatively in relation to this standard, and expressed as a (percentage) virulence ratio.

4. 100% virulence was defined as that of a strain showing a threshold of response of 600,000–1,300,000 bacillary units, lethal cerebral lesions at 8–20 days or necrotic pulmonary lesions at 20–30 days, and a mean 50% mortality time of less than 36 days.

5. Qualitative and quantitative responses characteristic of intermediate and very low virulence were also defined.

6. Higher proportions of virulent cells were obtained from 3 to 7 days than from older cultures in Tween-albumin liquid media, but inocula were more suitably standardized by obtaining cells from 11 to 12-day cultures.

7. Passage in the mouse and guinea-pig failed to alter the virulence of H 37 Rv. A slight increase was effected by passage in the lung of the rabbit.

8. The technique employed was a measure essentially of the ability of the organism to multiply in, and damage, resistant tissues.

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.