Mycobacterium tuberculosis survives in latently infected individuals, likely in a nonreplicating or dormancylike state. The M. tuberculosis DosR regulon is a genetic program induced by conditions that inhibit aerobic respiration and prevent bacillus replication. In this study, we used a mutant incapable of DosR regulon induction to investigate the contribution of this regulon to bacterial metabolism during anaerobic dormancy. Our results confirm that the DosR regulon is essential for M. tuberculosis survival during anaerobic dormancy and demonstrate that it is required for metabolic processes that occur upon entry into and throughout the dormant state. Specifically, we showed that regulon mechanisms shift metabolism away from aerobic respiration in the face of dwindling oxygen availability and are required for maintaining energy levels and redox balance as the culture becomes anaerobic. We also demonstrated that the DosR regulon is crucial for rapid resumption of growth once M. tuberculosis exits an anaerobic or nitric oxide-induced nonrespiring state. In summary, the DosR regulon encodes novel metabolic mechanisms essential for M. tuberculosis to survive in the absence of respiration and to successfully transition rapidly between respiring and nonrespiring conditions without loss of viability.Mycobacterium tuberculosis, a major human pathogen, infects nearly one-third of the people in the world and causes two million deaths per year (8). Most infections are latent, and a substantial number of new infections are transmitted by individuals in whom latent infections are being reactivated. Latency is a clinical term describing people that are infected with M. tuberculosis but lack symptoms of active disease. Traditionally, it has been thought that bacilli in latently infected individuals reside almost exclusively inside granulomas and mature tubercle lesions. Recent studies indicate that in latently infected individuals M. tuberculosis may also be found outside granulomas in places such as endothelial cells, fibroblasts, and adipose tissue (17, 28). The evidence for M. tuberculosis metabolic activity in vivo is more limited, but two studies by Lillebaek et al. are informative (24,25). In these studies the researchers used detailed records of tuberculosis epidemiology and strain types in the fairly static population of Denmark. They found that strains isolated from patients thought to have reactivated disease (rather than a primary infection) were nearly identical to strains present 30 years earlier in the same geographic population. The near-identity of the strains and the fact that infections were attributed to reactivation suggest that bacteria in latently infected individuals experience little genetic change during years of latent infection. The researchers concluded that during latency, M. tuberculosis divides infrequently and is likely in a minimal metabolic state.One approach to study the M. tuberculosis metabolic state during latent infection is to use in vitro models that mimic conditions thought to exist i...

c Mycobacterium tuberculosis persists in the tissues of mammalian hosts despite inducing a robust immune response dominated by the macrophage-activating cytokine gamma interferon (IFN-␥). We identified the M. tuberculosis phosphate-specific transport (Pst) system component PstA1 as a factor required to resist IFN-␥-dependent immunity. A ⌬pstA1 mutant was fully virulent in IFN-␥ ؊/؊ mice but attenuated in wild-type (WT) mice and mice lacking specific IFN-␥-inducible immune mechanisms: nitric oxide synthase (NOS2), phagosome-associated p47 GTPase (Irgm1), or phagocyte oxidase (phox). These phenotypes suggest that ⌬pstA1 bacteria are sensitized to an IFN-␥-dependent immune mechanism(s) other than NOS2, Irgm1, or phox. In other species, the Pst system has a secondary role as a negative regulator of phosphate starvation-responsive gene expression through an interaction with a two-component signal transduction system. In M. tuberculosis, we found that ⌬pstA1 bacteria exhibited dysregulated gene expression during growth in phosphate-rich medium that was mediated by the two-component sensor kinase/response regulator system SenX3-RegX3. Remarkably, deletion of the regX3 gene suppressed the replication and virulence defects of ⌬pstA1 bacteria in NOS2 ؊/؊ mice, suggesting that M. tuberculosis requires the Pst system to negatively regulate activity of RegX3 in response to available phosphate in vivo. We therefore speculate that inorganic phosphate is readily available during replication in the lung and is an important signal controlling M. tuberculosis gene expression via the Pst-SenX3-RegX3 signal transduction system. Inability to sense this environmental signal, due to Pst deficiency, results in dysregulation of gene expression and sensitization of the bacteria to the host immune response.

In Mycobacterium tuberculosis, the sensor kinases DosT and DosS activate the transcriptional regulator DosR, resulting in the induction of the DosR regulon, which is important for anaerobic survival and perhaps latent infection. The individual and collective roles of these sensors have been postulated biochemically, but their roles in vivo have remained unclear. This work demonstrates distinct and additive roles for each sensor during anaerobic dormancy. Both sensors are necessary for wild-type levels of DosR regulon induction, and concomitantly, full induction of the regulon is required for wild-type anaerobic survival. In the anaerobic model, DosT plays an early role, responding to hypoxia. DosT then induces the regulon and with it DosS, which sustains and further induces the regulon. DosT then loses its functionality as oxygen becomes limited, and DosS alone maintains induction of the genes from that point forward. Thus, M. tuberculosis has evolved a system whereby it responds to hypoxic conditions in a stepwise fashion as it enters an anaerobic state.

New drugs and drugs with a novel mechanism of action are desperately needed to shorten the duration of tuberculosis treatment, to prevent the emergence of drug resistance, and to treat multiple-drug-resistant strains of Mycobacterium tuberculosis. Recently, there has been renewed interest in clofazimine (CFZ). In this study, we utilized the C3HeB/FeJ mouse model, possessing highly organized, hypoxic pulmonary granulomas with caseous necrosis, to evaluate CFZ monotherapy in comparison to results with BALB/c mice, which form only multifocal, coalescing cellular aggregates devoid of caseous necrosis. While CFZ treatment was highly effective in BALB/c mice, its activity was attenuated in the lungs of C3HeB/FeJ mice. This lack of efficacy was directly related to the pathological progression of disease in these mice, since administration of CFZ prior to the formation of hypoxic, necrotic granulomas reconstituted bactericidal activity in this mouse strain. These results support the continued use of mouse models of tuberculosis infection which exhibit a granulomatous response in the lungs that more closely resembles the pathology found in human disease.

Since the discovery of antibiotics, mortality due to bacterial infection has decreased dramatically. However, infections from difficult to treat bacteria such as Mycobacterium tuberculosis and multidrug-resistant pathogens have been on the rise. An understanding of the cascade of events that leads to cell death downstream of specific drug-target interactions is not well understood. We have discovered that killing by several classes of antibiotics was stopped by maintaining pH balance within the bacterial cell, consistent with a shared mechanism of antibiotic killing. Our findings suggest a mechanism of antibiotic killing that stems from the antibiotic’s ability to increase the pH within bacterial cells by disrupting proton entry without affecting proton pumping out of cells. Knowledge of the core mechanism necessary for antibiotic killing could have a significant impact on the development of new lethal antibiotics and for the treatment of recalcitrant and drug-resistant pathogens.