Abstract
Non-tuberculous mycobacteria (NTM) are paradigmatic colonizers of the total environment, circulating at the interfaces of the atmosphere, lithosphere, hydrosphere, biosphere, and anthroposphere. Their striking adaptive ecology on the interconnection of multiple spheres results from the combination of several biological features related to their exclusive hydrophobic and lipid-rich impermeable cell wall, transcriptional regulation signatures, biofilm phenotype, and symbiosis with protozoa. This unique blend of traits is reviewed in this work, with highlights to the prodigious plasticity and persistence hallmarks of NTM in a wide diversity of environments, from extreme natural milieus to microniches in the human body. Knowledge on the taxonomy, evolution, and functional diversity of NTM is updated, as well as the molecular and physiological bases for environmental adaptation, tolerance to xenobiotics, and infection biology in the human and non-human host. The complex interplay between individual, species-specific and ecological niche traits contributing to NTM resilience across ecosystems are also explored. This work hinges current understandings of NTM, approaching their biology and heterogeneity from several angles and reinforcing the complexity of these microorganisms often associated with a multiplicity of diseases, including pulmonary, soft-tissue, or milliary. In addition to emphasizing the cornerstones of knowledge involving these bacteria, we identify research gaps that need to be addressed, stressing out the need for decision-makers to recognize NTM infection as a public health issue that has to be tackled, especially when considering an increasingly susceptible elderly and immunocompromised population in developed countries, as well as in low- or middle-income countries, where NTM infections are still highly misdiagnosed and neglected.
Highlights
Mycobacteria and the HostThe Mycobacterium genus includes a high diversity of species, with differential phenotypic and genotypic traits, as well as epidemiological relevance, including several important human and animal pathogens [1]
Mycobacterium species can be included in three groups: (1) mycobacteria that cause tuberculosis (TB), (2) mycobacteria that cause leprosy, and (3) the non-tuberculous mycobacteria (NTM), a wider diverse group of species referred in the literature as atypical or environmental mycobacteria [2]
Plasmids isolated from M. marinum and M. abscessus contain mercury resistance genes [26,27], plasmids from M. scrofulaceum are associated with both mercury and copper resistance [28,29], and plasmids from M. ulcerans contain genes involved in toxin production [30]
Summary
The Mycobacterium genus includes a high diversity of species, with differential phenotypic and genotypic traits, as well as epidemiological relevance, including several important human and animal pathogens [1]. Afterwards, reports describe taller, slender, and elderly women (and some men) as being more affected [7] These bacteria only attracted more attention from the medical community when disseminated Mycobacterium avium complex (MAC) was recognized as one of the major opportunistic bacterial infections affecting acquired-immunodeficient syndrome (AIDS) patients [7]. After this acknowledgment, pulmonary infection in immunocompetent patients by NTM became a concern, namely in developed countries, wherein trends for NTM infection have increased as pulmonary TB decreases [7]. Microorganisms 2020, 8, 1380 biological traits of mycobacteria foreordained by evolution currently distinguish NTM from any other microbes, being at the basis of their emergence as strict or opportunistic pathogens
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