Growth of <i>Mycobacterium tuberculosis</i> at acidic pH depends on lipid assimilation and is accompanied by reduced GAPDH activity.

Abstract

Acidic pH arrests the growth of <i>Mycobacterium tuberculosis</i> in vitro (pH < 5.8) and is thought to significantly contribute to the ability of macrophages to control <i>M. tuberculosis</i> replication. However, this pathogen has been shown to survive and even slowly replicate within macrophage phagolysosomes (pH 4.5 to 5) [M. S. Gomes <i>et al.</i>, <i>Infect. Immun.</i> 67, 3199-3206 (1999)] [S. Levitte <i>et al.</i>, <i>Cell Host Microbe</i> 20, 250-258 (2016)]. Here, we demonstrate that <i>M. tuberculosis</i> can grow at acidic pH, as low as pH 4.5, in the presence of host-relevant lipids. We show that lack of phosphoenolpyruvate carboxykinase and isocitrate lyase, two enzymes necessary for lipid assimilation, is cidal to <i>M. tuberculosis</i> in the presence of oleic acid at acidic pH. Metabolomic analysis revealed that <i>M. tuberculosis</i> responds to acidic pH by altering its metabolism to preferentially assimilate lipids such as oleic acid over carbohydrates such as glycerol. We show that the activity of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is impaired in acid-exposed <i>M. tuberculosis</i> likely contributing to a reduction in glycolytic flux. The generation of endogenous reactive oxygen species at acidic pH is consistent with the inhibition of GAPDH, an enzyme well-known to be sensitive to oxidation. This work shows that <i>M. tuberculosis</i> alters its carbon diet in response to pH and provides a greater understanding of the physiology of this pathogen during acid stress.