Mycobacterium abscessus: An environmental mycobacteria being a human pathogen

Abstract

BackgroundMycobacterium abscessus is an opportunistic organism common in the environment. This species rarely cause infections in immunocompetent individuals. Certain patient groups, however, can get severe infections, e.g., in skin, soft tissues or lungs, as well as disseminated infections. During the last years, this organism has emerged as a major pathogen in patients with cystic fibroses. M. abscessus exhibits two different colony types: one with smooth and shining morphology, the other with rough and dry morphology. The smooth strains are considered wild-types, which become rough due to mutation. These features make the M. abscessus variants particularly interesting for scientific studies, since the results of such analyses can be a basis for creating theories also concerning other mycobacteria. AimThe aim of this study is to analyze potential differences between smooth and rough strains of M. abscessus concerning human host response. MethodsM. abscessus strains isolated at the Mycobacterial Laboratory in Gothenburg, Sweden were analyzed morphologically. The ability of monocytes to internalize the two colony variants of M. abscessus was studied, as well as the cytokine response. ResultsThe analyses showed that isolates from the wound mainly exhibit smooth colony formation, while those from airways generally are of the rough phenotype. Human monocytes easily internalized the smooth varieties of M. abscessus, whereas the rough ones rarely were internalized. Differences between the two types, concerning the capacity to induce cytokines, were also revealed. Discussion and conclusionsThe wound isolates of M. abscessus were mainly smooth, while the lung isolates were rough. The rough variants that lack certain surface glycolipids are likely to be more hydrophobic and aerosol transmissible than the smooth strains, which might explain why the rough ones are more often associated with infections in the airways than are the smooth ones. Studies on other mycobacteria have also shown a link between hydrophobicity and aerosol transmission.These studies showed that human monocytes easily internalize smooth M. abscessus cells, but rarely the rough ones. This capacity of the rough cells to avoid internalization is in all probability also due to absence or reduction of certain surface glycolipids. The lack of these surface lipids makes the rough bacteria stick to each other forming so-called cords, which are aggregates of many cells and thus difficult to internalize. The absence of glycolipids exposing other surface structures of the bacteria cell may also contribute to prevent internalization. Even if intracellular survival is regarded as the most important virulence factor of mycobacteria, it is nevertheless likely that the capacity to avoid internalization can contribute to pathogenesis at several mycobacterioses. It is possible that this phenomenon is important at tuberculosis, since Mycobacterium tuberculosis cells form rough colonies.