Mycobacterium abscessus: An emerging pulmonary pathogen

M. abscessus is the most common rapidly growing mycobacterium (RGM) pathogenic for man. It is by far the leading RGM causing lung infections in patients with an underlying disease (cystic fibrosis, COPD, bronchial dilatation), but also in healthy subjects free of lung disease. M. abscessus is also the most common RGM found in bronchial secretions in patients with cystic fibrosis. Besides their frequency, M. abscessus lung infections are generally true lung infections using the criteria established by the American Thoracic Society (ATS). The genome of the typical strain of M. abscessus has been sequenced and now provides means for molecular studies of the species with construction of deletion mutants. In addition, M. abscessus has a characteristic genotypic and phenotypic heterogeneity. M. abscessus, which was long considered to be a sub-species of M. chelonae, is now considered to be a complex including M. abscessus species stricto senso, M. bolletii and M. massiliense. Molecular identification being the only reliable identification method for M. abscessus, the study of different identification targets evidenced the mosaic makeup of some isolates, raising questions about the speciation process within the complex. Regarding the phenotype of M. abscessus sensu lato, two morphotypes can be described on solid media: the smooth (5) and the rough (R) morphotype. This heterogeneity is correlated with variable virulence, some isolates only being associated with true mycobacterial disease, these strains commonly displaying an R morphotype. The biochemical basis underlying the S and R phenotypes remain obscure, but recent studies shows that the R morphotype is associated with a small or highly diminished capacity for these bacteria to produce glycopeptidolipid (GPL) surface molecules. These recent data show that the R variants induce hyperproduction of TNF by murine macrophages in culture but also that the activation pathway leading to this process involves TLR2 receptors. The host response has been studied in mice. In this model, mycobacteria are typically controlled by the T cell compartment. An original humoral immune response has been demonstrated in the control of the emergence of rough strains. The study of the interaction between M. abscessus and other cell or animal models presenting a CFTR-/- or Delta F508 phenotype will be useful in understanding the specific sensitivity of children with cystic fibrosis to this mycobacterium. (C) 2010 Elsevier Masson SAS. All rights reserved.