Comparative study for the virulence of Mycobacterium avium isolates from patients with nodular-bronchiectasis- and cavitary-type diseases

Mycobacterium avium (Mav) lung infections, called nodular-bronchiectasis (NB)-type M. avium complex (MAC) disease, are globally increasing. To elucidate whether there are unusual populations of Mav, causing NB-type disease rather than cavitary (CA)-type disease, we compared the virulence of Mav isolates from patients with NB-type (NB-Mav) and those from CA-type (CA-Mav) diseases, based on intracellular growth in various types of human cells. Five strains each of NB-Mav and CA-Mav were compared with each other for their invasiveness and ability to intracellularly replicate in various types of cultured cells of human origin. The two types of Mav isolates showed a similar ability, on average, to replicate in macrophages and lung epithelial cells. Moreover, they showed a similar ability to induce the production of reactive nitrogen intermediates and reactive oxygen intermediates by macrophages and susceptibility to antimicrobial molecules. Therefore, it appears that there is no essential difference in virulence in terms of infectivity to human macrophages and lung cells between Mav strains isolated from NB-MAC disease and those from CA-MAC disease. These findings indicate the importance of further studies to elucidate the mechanism for the establishment of NB-type MAC diseases based on host immunological conditions rather than the pathogenic nature of MAC organisms themselves.