Drug-resistant strains of Mycobacterium tuberculosis exhibit a range of virulence for mice.

A panel of clinical isolates of Mycobacterium tuberculosis, several of which were resistant to one or more antimycobacterial drugs, were tested for their capacity to give rise to active disease following aerogenic infection of normal immunocompetent mice. The panel exhibited a range of virulence in this model, which followed no clear trend in terms of geographical source, degree of drug resistance, or rate of growth in vitro. Several isolates grew very quickly over the first 20 days in mouse lungs before being contained by emerging immunity. In view of this latter observation, we hypothesize that it is possible that such so-called fast growers may be responsible for the rapid fatality sometimes seen in immunocompromised patients with tuberculosis. Moreover, the results of the study do not support the belief that increased drug resistance usually associates with loss of virulence of the isolate.