Abstract: M. tuberculosis, grows very slowly with a doubling time which ranges from ̴ 20 hours to 70 days depending on whether it grows in vitro, or in macrophages or mice. For mycobacteria, macrophage is the primary host cell where the bacilli survive, grow and also undergo persistence. However, the phagosomal environment is full of hostile environmental conditions including highly acidic environments. Interestingly, despite encountering adverse conditions, M. tuberculosis adapts a long-term residence within the macrophage phagosome. Under hypoxia, M. tuberculosis growth is significantly slowed down or arrested and the bacteria enters a persistent state, but yet remains viable. Importantly, slower growth or growth arrest is considered to play a critical role in establishing chronic infection and resistance to existing drugs. Thus, adaptation of the pathogen in response to varying environmental cues within the macrophage phagosome, which is primarily controlled at the level of transcription of genes, accounts for one of the major reasons for exceptional success of M. tuberculosis as an intracellular pathogen. Together, probing regulated metabolic response under varying environmental cues including acidic pH and variable carbon source availability, is of importance to understand mechanism(s) that makes M. tuberculosis a successful and difficult-to-treat pathogen. Using high throughput assays coupled with traditional biochemical/ microbiology techniques, we wish to probe the origin(s) of specific interaction(s) in proteins complexes, investigate physiological determinants of complex formation, and understand the consequences on mycobacterial physiology, in vivo survival and virulence regulation.