When the metabolic model says NO: untangling the Gordian knot of TB’s intracellular metabolism

The causative agent of TB, Mycobacterium tuberculosis (Mtb) is once again the world’s number one infectious killer. M. tuberculosis resides primarily within macrophages and metabolic reprogramming within this intracellular niche is a crucial determinant of virulence. We previously applied the metabolic modelling-based tool 13C-flux spectral analysis (13C-FSA) to show that intracellular M. tuberculosis co-metabolises multiple gluconeogenic and glycolytic carbon substrates by utilizing the reactions of the phosphoenolpyruvate (PEP)-pyruvate-oxaloacetate (OAA) or anaplerotic (ANA) node. However, predicting the metabolic mode of operation required for intracellular survival is chellenging using a metabolic network as the ANA node consists of several apparently functionally redundant bidirectional reactions. Here we use multiple techniques including 13C isotopomer profiling, lipid analysis and fluorescent reporter strains to dissect the role of the ANA node. We show that this node has unexpected roles in the life cycle of M. tuberculosis including lipid biosynthesis, protection from known toxic intracellular carbon sources and redox regulation. Inhibiting enzymes at this node with novel therapeutic compounds restricts the growth of M. tuberculosis and limits the ability of this formidable pathogen to survive within the human host cell identifying the ANA node as a potential druggable pathway for controlling TB.