Mycobacterium tuberculosis is a chronic, facultative intracellular pathogen that spends the majority of its decades-long life cycle in a non-or slowly replicating state. However, the bacterium remains poised to resume replicating so that it can transmit itself to a new host. Knowledge of the metabolic adaptations used to facilitate entry into and exit from nonreplicative states remains incomplete. Here, we apply 13 C-based metabolomic profiling to characterize the activity of M. tuberculosis tricarboxylic acid cycle during adaptation to and recovery from hypoxia, a physiologically relevant condition associated with nonreplication. We show that, as M. tuberculosis adapts to hypoxia, it slows and remodels its tricarboxylic acid cycle to increase production of succinate, which is used to flexibly sustain membrane potential, ATP synthesis, and anaplerosis, in response to varying degrees of O 2 limitation and the presence or absence of the alternate electron acceptor nitrate. This remodeling is mediated by the bifunctional enzyme isocitrate lyase acting in a noncanonical role distinct from fatty acid catabolism. Isocitrate lyase-dependent production of succinate affords M. tuberculosis with a unique and bioenergetically efficient metabolic means of entry into and exit from hypoxia-induced quiescence. Q uiescence, or exit from cell cycle, is a physiologic prerogative of all cells, executed irreversibly by some upon terminal differentiation and reversibly by others as they adapt to changing conditions (1). For Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), quiescence has emerged as a hallmark of its pathogenicity. M. tuberculosis infects approximately one in every three people worldwide and is the leading bacterial cause of death. Following infection, M. tuberculosis enters a clinically asymptomatic state of non-or slowly replicating physiology that often lasts decades, if not the lifetime, of the infected host, and exhibits a form of nonheritable resistance to nearly all TB drugs that has hindered mass eradication strategies (2). Clinical TB arises when M. tuberculosis reenters cell cycle and provokes an inflammatory response that inflicts host tissue damage and enables it to transmit itself to a new host. However, some of the M. tuberculosis in active TB is nonreplicating. This is thought to impose the need for chemotherapies that are longer and more complex than for virtually any other bacterial infection (2-7). However, biochemical knowledge of quiescent M. tuberculosis remains highly incomplete.Relieved of the requirement to double biomass, quiescent cells have generally been perceived to have minimal metabolic activity. However, quiescent cells often occupy ecological niches that are highly dynamic and face the challenge of preserving both their viability and their ability to reenter cell cycle. Fibroblasts induced into quiescence by contact inhibition metabolized glucose through all branches of central carbon metabolism at a rate similar to those of proliferating cells (8). Such studies ha...

SUMMARY The mechanisms by which Mycobacterium tuberculosis (Mtb) maintains metabolic equilibrium to survive during infection and upon exposure to antimycobacterial drugs are poorly characterized. Ergothioneine (EGT) and mycothiol (MSH) are the major redox buffers present in Mtb, but the contribution of EGT to Mtb redox homeostasis and virulence remains unknown. We report that Mtb WhiB3, a 4Fe-4S redox sensor protein, regulates EGT production and maintains bioenergetic homeostasis. We show that central carbon metabolism and lipid precursors regulate EGT production and that EGT modulates drug sensitivity. Notably, EGT and MSH are both essential for redox and bioenergetic homeostasis. Transcriptomic analyses of EGT and MSH mutants indicate overlapping, but distinct functions of EGT and MSH. Lastly, we show that EGT is critical for Mtb survival in both macrophages and mice. This study has uncovered a dynamic balance between Mtb redox and bioenergetic homeostasis, which critically influences Mtb drug susceptibility and pathogenicity.

Few mutations attenuate Mycobacterium tuberculosis (Mtb) more profoundly than deletion of its isocitrate lyases (ICLs). However, the basis for this attenuation remains incompletely defined. Mtb's ICLs are catalytically bifunctional isocitrate and methylisocitrate lyases required for growth on even and odd chain fatty acids. Here, we report that Mtb's ICLs are essential for survival on both acetate and propionate because of its methylisocitrate lyase (MCL) activity. Lack of MCL activity converts Mtb's methylcitrate cycle into a "dead end" pathway that sequesters tricarboxylic acid (TCA) cycle intermediates into methylcitrate cycle intermediates, depletes gluconeogenic precursors, and results in defects of membrane potential and intrabacterial pH. Activation of an alternative vitamin B 12 -dependent pathway of propionate metabolism led to selective corrections of TCA cycle activity, membrane potential, and intrabacterial pH that specifically restored survival, but not growth, of ICL-deficient Mtb metabolizing acetate or propionate. These results thus resolve the biochemical basis of essentiality for Mtb's ICLs and survival on fatty acids. However, our understanding of the pathways and physiologic functions they serve is incomplete. Enzymes catalyze reactions whose identities, rates, and directions are often difficult to determine and, where known, function within the context of complex and extensively interconnected physiologic networks (1). Thus, although recent genetic and biochemical approaches have made it possible to determine the essentiality of a given enzyme, our understanding of the biochemical mechanisms underlying their essentiality remains incomplete.Unlike the case for most microbial pathogens, humans serve as both host and reservoir for Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB). Within humans, Mtb resides within the biochemically stringent environment of the macrophage phagosome. Growing evidence has separately implicated Mtb's metabolic enzymes as a key determinant of its pathogenicity. Mtb's metabolic network thus appears to have evolved to serve interdependent physiologic and pathogenic roles (2, 3).Isocitrate lyase (ICL) is a metabolic enzyme used by bacteria to sustain growth on even-chain fatty acids through an anaplerotic pathway called the glyoxylate shunt. Mtb encodes two paralogous ICLs, each of which also functions as a methylisocitrate lyase (MCL), and serves a parallel pathway involved in the metabolism of propionyl CoA generated by β-oxidation of oddchain fatty acids, called the methylcitrate cycle. Mtb lacking both ICLs were shown to be incapable of either establishing or maintaining infection in a mouse model of pulmonary TB (4, 5). Although loss of ICL activity is expected to result in the inability to incorporate carbon from fatty acids (a form of starvation), simultaneous loss of MCL activity is predicted to result in the potentially toxic accumulation of propionyl-CoA metabolites (a form of intoxication). Here, we applied 13 C-based metabolomic tra...

Intracellular nucleic acid sensors often undergo sophisticated modifications that are critical for the regulation of antimicrobial responses. Upon recognition of DNA, the cytosolic sensor cyclic GMP-AMP (cGAMP) synthase (cGAS) produces the second messenger cGAMP, which subsequently initiates downstream signaling to induce interferon-αβ (IFNαβ) production. Here we report that TRIM56 E3 ligase-induced monoubiquitination of cGAS is important for cytosolic DNA sensing and IFNαβ production to induce anti-DNA viral immunity. TRIM56 induces the Lys335 monoubiquitination of cGAS, resulting in a marked increase of its dimerization, DNA-binding activity, and cGAMP production. Consequently, TRIM56-deficient cells are defective in cGAS-mediated IFNαβ production upon herpes simplex virus-1 (HSV-1) infection. Furthermore, TRIM56-deficient mice show impaired IFNαβ production and high susceptibility to lethal HSV-1 infection but not to influenza A virus infection. This adds TRIM56 as a crucial component of the cytosolic DNA sensing pathway that induces anti-DNA viral innate immunity.

Stochastic formation of Mycobacterium tuberculosis (Mtb) persisters achieves a high level of antibiotic-tolerance and serves as a source of multidrug-resistant (MDR) mutations. As conventional treatment is not effective against infections by persisters and MDR-Mtb, novel therapeutics are needed. Several approaches were proposed to kill persisters by altering their metabolism, obviating the need to target active processes. Here, we adapted a biofilm culture to model Mtb persister-like bacilli (PLB) and demonstrated that PLB underwent trehalose metabolism remodeling. PLB use trehalose as an internal carbon to biosynthesize central carbon metabolism intermediates instead of cell surface glycolipids, thus maintaining levels of ATP and antioxidants. Similar changes were identified in Mtb following antibiotic-treatment, and MDR-Mtb as mechanisms to circumvent antibiotic effects. This suggests that trehalose metabolism is associated not only with transient drug-tolerance but also permanent drug-resistance, and serves as a source of adjunctive therapeutic options, potentiating antibiotic efficacy by interfering with adaptive strategies.