Driving Mycobacterium Tuberculosis to Its Death by Keeping the Accelerator on Cell Replication

Tuberculosis (TB) is an infectious disease characterized by chronic coughing and blood-containing phlegm that is caused by the bacteria Mycobacterium tuberculosis (Mtb). While curable, TB patients are subject to long treatment regimens that are often difficult to adhere to. TB has also become increasingly challenging to treat with the rise of multidrug- and extensively drug-resistant strains of the bacteria. One of the ways that Mtb resists being killed is by entering into a state of quiescence under growth-limiting conditions, known as the stringent response. In this metabolically inactive state, Mtb is able to persist and survive through treatment with antibiotics, like isoniazid (INH), rifampin, and pyrazinamide, which more effectively target actively dividing cells.

In a recent study, Dutta et al. (2019) investigate the role that the enzyme Rel_Mtb plays in facilitating Mtb’s entry into this state of bacterial persistence. Rel_Mtb mediates the synthesis of tetra- and penta-phosphorylated guanosine, referred to as (p)ppGpp, which are growth arrest signals for Mtb. They discovered that under nutrient starvation (NS), Mtb cells deleted for rel_Mtb would continue replicating past the point of nutrient exhaustion and die. The researchers also observed that inhibition of (p)ppGpp synthesis increased cellular sensitivity to killing by INH. In an in vivo model, INH-treated immunocompetent mice infected with ∆rel Mtb showed significantly reduced bacterial counts in their lungs than mice infected with wildtype isogenic Mtb. Additionally, in mice that develop hypoxic and necrotic lung granulomas similar to human TB, stringent response-impaired ∆rel Mtb-infected mice survived longer than mice infected with wildtype Mtb. Seeing Rel_Mtb as a potential therapeutic target for inhibition, Dutta et al. then developed a novel, high throughput fluorescence polarization assay to screen an inhibitor library of over 29,000 compounds for binding to purified, recombinant Rel_Mtb. They identified and tested a promising inhibitor, X9, which had an IC₅₀ of 2 μM against wildtype Mtb in whole-cell assays, and found that treatment with X9 sensitized cells to INH treatment under NS conditions.

By keeping the accelerator on Mtb replication by inhibiting Rel_Mtb under NS conditions, researchers found a way to prevent Mtb from entering into a state of persistence and sensitized cells to killing when exposed to INH. These advances in knowledge allow for the further research and development of new, complementary, shortened therapies to battle the global TB epidemic.

Dutta, N. K. et al. Inhibiting the stringent response blocks Mycobacterium tuberculosis entry into quiescence and reduces persistence. Sci. Adv. (2019).

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