Free Glucosylglycerate Is a Novel Marker of Nitrogen Stress in Mycobacterium smegmatis

Nitrogen is an essential element for bacterial growth, and as such, bacteria have evolved several pathways to assimilate nitrogen and adapt to situations of nitrogen limitation. However, the adaptation of mycobacteria to nitrogen stress and the regulation of the stress response pathways is unknown. Identification of key metabolites produced by mycobacteria during nitrogen stress could therefore provide important insights into mycobacterial survival strategies. Here we used NMR-based metabolomics to monitor and quantify intracellular and extracellular metabolite levels (metabolic footprinting) in Mycobacterium smegmatis grown under nitrogen-limiting and nitrogen-rich conditions. There were several metabolic differences between the two conditions: following nitrogen run-out, there was an increase in intracellular α-ketoglutarate and a decrease in intracellular glutamine and glutamate levels. In addition, a sugar-derived compound accumulated in nitrogen-starved cells that was subsequently assigned as glucosylglycerate (GGA). Free GGA production was responsive to nitrogen stress in M. smegmatis but not to oxidative or osmotic stress; lack of a functional GGA synthesis pathway slightly reduced growth and decreased ammonium uptake rates under nitrogen-limiting conditions. Hence, GGA could contribute to the fitness of mycobacteria under nitrogen limitation.