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There are strong evidences that  Mycobacterium tuberculosis  survives in a non-replicating state in the absence of oxygen in closed lesions and granuloma  in vivo . In addition,  M. tuberculosis  is acid-resistant, allowing mycobacteria to survive in acidic, inflamed lesions. The ability of  M. tuberculosis  to resist to acid was recently shown to contribute to the bacillus virulence although the mechanisms involved have yet to be deciphered. In this study, we report that  M. tuberculosis  resistance to acid is oxygen-dependent; whereas aerobic mycobacteria were resistant to a mild acid challenge (pH 5.5) as previously reported, we found microaerophilic and hypoxic mycobacteria to be more sensitive to acid. In hypoxic conditions, mild-acidity promoted the dissipation of the protonmotive force, rapid ATP depletion and cell death. Exogenous nitrate, the most effective alternate terminal electron acceptor after molecular oxygen, protected hypoxic mycobacteria from acid stress. Nitrate-mediated resistance to acidity was not observed for a respiratory nitrate reductase NarGH knock-out mutant strain. Furthermore, we found that nitrate respiration was equally important in protecting hypoxic non-replicating mycobacteria from radical nitrogen species toxicity. Overall, these data shed light on a new role for nitrate respiration in protecting  M. tuberculosis  from acidity and reactive nitrogen species, two environmental stresses likely encountered by the pathogen during the course of infection.

Nitrate Respiration Protects Hypoxic Mycobacterium tuberculosis Against Acid- and Reactive Nitrogen Species Stresses