Mycobacterium abscessus has emerged as an important pathogen in people with chronic inflammatory lung diseases such as cystic fibrosis, and recent reports suggest that it may be transmissible by fomites. M. abscessus exhibits two major colony morphology variants: a smooth morphotype (MaSm) and a rough morphotype (MaRg). Biofilm formation, prolonged intracellular survival, and colony variant diversity can each contribute to the persistence of M. abscessus and other bacterial pathogens in chronic pulmonary diseases. A prevailing paradigm of chronic M. abscessus infection is that MaSm is a noninvasive, biofilm-forming, persistent phenotype and MaRg an invasive phenotype that is unable to form biofilms. We show that MaRg is hyperaggregative and forms biofilm-like aggregates, which, like MaSm biofilm aggregates, are significantly more tolerant than planktonic variants to acidic pHs, hydrogen peroxide (H2O2), and treatment with amikacin or azithromycin. We further show that both variants are recalcitrant to antibiotic treatment inside human macrophage-like cells and that MaRg is more refractory than MaSm to azithromycin. Our results indicate that biofilm-like aggregation and protracted intracellular survival may each contribute to the persistence of this problematic pathogen in the face of antimicrobial agents regardless of morphotype. Biofilms of each M. abscessus variant are rapidly killed, however, by acetic acid, which may help to prevent local fomite transmission.

Mycobacterium intracellulare is an opportunistic pathogen which causes disease in immunocompromised individuals. Like several other nontuberculous mycobacteria (NTM) species, a single strain of M. intracellulare forms bacterial colonies with varying morphologies; however, the relative virulence and immunogenicity of differing M. intracellulare morphotypes are unknown. We have obtained a strain of M. intracellulare isolated from an infected patient and further subcultured it to distinguish the component colony morphotypes present within the parent isolate. To determine whether these distinct colony morphotypes have the ability to stimulate differential immune responses in an infected host, C57BL/6J mice were aerosol infected with either the parent clinical strain or each of the isolated component colony morphotypes. Differential immune response and virulence characteristics of the morphotypes were evaluated by tracking host bacterial load, cytokine expression profiles, and histology. In addition, bone marrow-derived macrophages were isolated and infected in vitro with the different morphotypes and parent isolate. From this study, we have found that the component colony morphotypes are able to differentially activate macrophages and affect the host immune response to infection. These results may establish an important immunological basis for patient clinical outcomes.