Persistent infection with virulent but not avirulent Mycobacterium tuberculosis in the lungs of mice causes progressive pathology.

A strain of Mycobacterium tuberculosis (H37Rv) considered virulent for mice and a strain (R1Rv) considered relatively avirulent were compared for their ability to survive host immunity in the lungs and to induce lung pathology. Although both strains of M. tuberculosis were capable of causing a slowly progressive infection in the lungs of immunocompetent mice, only the H37Rv strain was capable of inducing progressive destructive pathology and of causing loss of lung function over a 300-day period. Therefore, the ability to survive host immunity in the lungs and the ability to cause lung pathology are separate manifestations of mycobacterial virulence.