Infection with Mycobacterium avium-intracellulare and the protective effects of Bacille Calmette-Guérin.

The protective efficacies of bacille Calmette-Guérin (BCG) and Mycobacterium avium-intracellulare (MAI) were examined in guinea pigs that were infected by the respiratory route with one of three strains of Mycobacterium tuberculosis: two strains obtained as recent sputum isolates from an ongoing BCG trial in south India and laboratory strain H37Rv. Groups of animals were given all combinations of two treatments with BCG, MAI, or placebo and than challenged. The numbers of tubercle bacilli recovered from the primary lung lesions, primary lesion-free lung lobes, and spleens were used to estimate the relative protective effects of the various treatments. In general, BCG and MAI protected equally well against the low-virulence strain of M. tuberculosis. For the two more virulent strains the results were less clear; however, a substantial protective effect of MAI compared with BCG was noted. Infection with MAI did not significantly alter the capacity of BCG to protect against tuberculous infection.