Mycobacterium abscessus Smooth and Rough Morphotypes Form Antimicrobial-Tolerant Biofilm Phenotypes but Are Killed by Acetic Acid

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.