Implications of Mycobacterium tuberculosis Metabolic Adaptability on Drug Discovery and Development.

Tuberculosis remains a global health threat that is being exacerbated by the increase in infections attributed to drug resistant Mycobacterium tuberculosis. To combat this, there has been a surge in drug discovery programs to develop new, potent compounds and identify promising drug targets in the pathogen. Two areas of M. tuberculosis biology that have emerged as rich sources of potential novel drug targets are cell wall biosynthesis and energy metabolism. Both processes are important for survival of M. tuberculosis under replicating and nonreplicating conditions. However, both processes are also inherently adaptable under different conditions. Furthermore, cell wall biosynthesis is energy intensive and, thus, reliant on an efficiently functioning energy production system. This Perspective focuses on the interplay between cell wall biosynthesis and energy metabolism in M. tuberculosis, how adaptations in one pathway may affect the other, and what consequences this could have for drug discovery and development and the identification of novel drug targets.

Mycobacterium smegmatis does not display functional redundancy in nitrate reductase enzymes.

Reduction of nitrate to nitrite in bacteria is an essential step in the nitrogen cycle, catalysed by a variety of nitrate reductase (NR) enzymes. The soil dweller, Mycobacterium smegmatis is able to assimilate nitrate and herein we set out to confirm the genetic basis for this by probing NR activity in mutants defective for putative nitrate reductase (NR) encoding genes. In addition to the annotated narB and narGHJI, bioinformatics identified three other putative NR-encoding genes: MSMEG_4206, MSMEG_2237 and MSMEG_6816. To assess the relative contribution of each, the corresponding gene loci were deleted using two-step allelic replacement, individually and in combination. The resulting strains were tested for their ability to assimilate nitrate and reduce nitrate under aerobic and anaerobic conditions, using nitrate assimilation and modified Griess assays. We demonstrated that narB, narGHJI, MSMEG_2237 and MSMEG_6816 were individually dispensable for nitrate assimilation and for nitrate reductase activity under aerobic and anaerobic conditions. Only deletion of MSMEG_4206 resulted in significant reduction in nitrate assimilation under aerobic conditions. These data confirm that in M. smegmatis, narB, narGHJI, MSMEG_2237 and MSMEG_6816 are not required for nitrate reduction as MSMEG_4206 serves as the sole assimilatory NR.