Mycobacterium tuberculosis is strikingly more virulent for mice when given via the respiratory than via the intravenous route.

Fewer Mycobacterium tuberculosis colony-forming units given by aerosol were substantially more virulent for mice than much larger numbers inoculated intravenously (iv), as shown by a faster rate of bacillary growth in the lungs and much shorter survival of the host. Earlier death of mice infected by aerosol was associated with faster development of lung pathology, even though the number of M. tuberculosis given iv resulted in the same number of bacilli initially implanting in the lungs as the number given by aerosol. Mice depleted of CD4 T cells died of infection much sooner than immunocompetent mice but at about the same time after being infected via either route. The results indicate that a small number of M. tuberculosis colony-forming units given by aerosol is less immunogenic than a larger number given iv.