Acid Fasting: Modulation of Mycobacterium tuberculosis Metabolism at Acidic pH

Abstract

Mycobacterium tuberculosis (Mtb) senses and adapts to acidic host environments during the course of pathogenesis. Mutants defective in acidic pH-dependent adaptations are often attenuated during macrophage or animal infections, supporting that these pathways are essential for pathogenesis and represent important new targets for drug discovery. This review examines a confluence of findings supporting that Mtb has restricted metabolism at acidic pH that results in the slowing of bacterial growth and changes in redox homeostasis. It is proposed that induction of the PhoPR regulon and anaplerotic metabolism, in concert with the restricted use of specific carbon sources, functions to counter reductive stress associated with acidic pH.

Keywords: metabolism; microbial pathogenesis; persistence; regulatory networks; stress response.

Figure 1.. Mtb signaling and redox homeostasis adaptations at acidic pH.

Acidic pH promotes transcriptional and physiological adaptations that are associated with changes in intracellular redox homeostasis. This model is a synthesis of the observed adaptations and presents a speculative framework for how these changes promote pH-dependent adaptations. The PhoPR regulon is induced by acidic pH and induces the redox sensor whiB3 and genes for synthesis of acylated trehaloses (e.g. pks2 and pks3) which promote anabolism of cell envelope lipids[17, 18, 24, 27]. Mutations in phoP and whiB3 are associated with reductive stress, supporting that this lipid synthesis may mitigate reductive stress by oxidizing NADPH[17, 28, 29]. Reductive stress may also be mitigated by direct oxidation of NADH by NADH oxidases, such as KatG, which will generate ROS[32]. KatG is induced at acidic pH[17], and enhanced intracellular ROS have been observed at acidic pH[31], suggesting that ROS accumulation may be associated with reductive stress. Low molecular weight (LMW) thiols, such as mycothiol or ergothioneine, could function to mitigate this ROS stress and also directly mitigate reductive stress by consuming NADPH[29, 78, 79]. Transcriptional changes in the respiratory chain (shown as up or down arrows) are observed at acidic pH, supporting remodeling of respiration at acidic pH[17]. It is hypothesized that electron transport is altered at acidic pH, due to the need to maintain cytoplasmic pH homeostasis by efflux of protons. In support of this hypothesis, proton translocating type I NADH dehydrogenase genes are repressed, while the non-proton translocating type II enzyme is induced[17]. These adaptations could enable oxidation of NADH without further increasing proton-motive force. Downregulation of ATP synthase genes[17] at acidic pH could function to slow growth and limit proton translocation into the cytoplasm. These combined adaptations may function to replenish pools of oxidized cofactors and maintain redox- and pH-homeostasis.

Figure 2.. Mtb restricts its metabolism at acidic pH.

At acidic pH, Mtb exhibits arrested growth on various single carbon sources (shown in blue), that fully support growth at neutral pH[17]. In contrast, carbon sources that feed the anaplerotic node (shown in red) promote growth at both acidic and neutral pH. Notably, icl and pckA, genes that encode enzymes that promote anaplerotic metabolism are strongly induced at acidic pH (in both permissive and non-permissive conditions). Mutations in these genes cause reduced growth at acidic pH in rich medium and altered accumulation of metabolites in defined media[60]. Together, these data suggest that at acidic pH Mtb may shift its metabolism to the PEP-glyoxylate pathway (highlighted in red), which would limit metabolism around the oxidative branch of the TCA cycle and decrease synthesis of NADH and CO₂ release. These changes would directly mitigate reductive stress and conserve carbon which can then be used to synthesize lipids or other metabolites to further mitigate reductive stress. Notably, mutations in ppe51 enable growth on glycerol at acidic pH, suggesting that the bacterium is adapted to limit metabolism at acidic pH[60].