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...