Extraordinary solute-stress tolerance contributes to the environmental tenacity of mycobacteria

Mycobacteria are associated with a number of well-characterized diseases, yet we know little about their stress biology in natural ecosystems. This study focuses on the isolation and characterization of strains from Yellowstone National Park (YNP) and Glacier National Park (GNP; USA), the majority of those identified were Mycobacterium parascrofulaceum, Mycobacterium avium (YNP) or Mycobacterium gordonae (GNP). Generally, their windows for growth spanned a temperature range of > 60 °C; selected isolates grew at super-saturated concentrations of hydrophobic stressors and at levels of osmotic stress and chaotropic activity (up to 13.4 kJ kg(-1) ) similar to, or exceeding, those for the xerophilic fungus Aspergillus wentii and solvent-tolerant bacterium Pseudomonas putida. For example, mycobacteria grew down to 0.800 water activity indicating that they are, with the sole exception of halophiles, more xerotolerant than other bacteria (or any Archaea). Furthermore, the fatty-acid composition of Mycobacterium cells grown over a range of salt concentrations changed less than that of other bacteria, indicating a high level of resilience, regardless of the stress load. Cells of M. parascrofulaceum, M. smegmatis and M. avium resisted the acute, potentially lethal challenges from extremes of pH (< 1; > 13), and saturated MgCl2 solutions (5 M; 212 kJ kg(-1) chaotropicity). Collectively, these findings challenge the paradigm that bacteria have solute tolerances inferior to those of eukaryotes.