Tuberculosis (TB) is the leading infectious cause of death among the adult population worldwide and the fact that no TB-specific drugs have been discovered in the last 40 years has been a challenge for the scientific community. Understanding the physiology of TB bacilli in vitro and engaging the energy metabolism pathway in chemotherapeutic intervention have been proven to be important approaches towards the development of new drugs against TB. Moreover, one of the major obstacles for eradicating TB by vaccination or chemotherapy is the ability of dormant Mycobacterium tuberculosis to persist within granulomatous lesions. Dormant M. tuberculosis is capable of adapting to an intracellular lifestyle of non-replicating persistence, in which it is phenotypically drug-resistant and responsible for latent TB infection.
We have identified a class of Novartis compounds that is highly active against M. tuberculosis in vitro. Interestingly, this anti-TB activity was exclusively observed when glycerol was present in the media, suggesting that these compounds may interfere with glycerol metabolism, leading to toxicity. Indeed, this hypothesis was harbored by the fact that a glycerol kinase knock-out strain of M. tuberculosis showed resistance to the compounds. We have observed that the levels of intracellular ATP were dramatically depleted as early as 24 hours after wild-type M. tuberculosis exposure to the compounds in the presence of glycerol. Additionally, we have also identified the accumulation of glycerol-3-phosphate, a highly toxic intermediate of glycerol metabolism.
To study the physiology of dormant M. tuberculosis, we have used the anaerobic shift model of latent TB. The effect of different carbon sources on the ability of non-replicating bacteria to persist under hypoxia have been addressed. We have demonstrated that dormant M. tuberculosis cannot persist when excessive amounts of glycerol are present in the media. We have also observed that glycerol toxicity can be reversed when the alternative electron acceptor sodium nitrate is present, suggesting that metabolism of glycerol may interfere with the electron transport chain.
Further studies will focus on clarifying the glycerol metabolism intermediates involved on glycerol-specific toxicity under hypoxic conditions and its correlation with the toxicity observed in response to our compounds, with the ultimate goal of identifying potential drug targets against both dormant and active M. tuberculosis.