The flavonoid natural compound chlorflavonin was isolated from the endophytic fungus Mucor irregularis, which was obtained from the Cameroonian medicinal plant Moringa stenopetala. Chlorflavonin exhibited strong growth inhibitory activity in vitro against Mycobacterium tuberculosis (MIC90 1.56 μM) while exhibiting no cytotoxicity toward the human cell lines MRC-5 and THP-1 up to concentrations of 100 μM. Mapping of resistance-mediating mutations employing whole-genome sequencing, chemical supplementation assays, and molecular docking studies as well as enzymatic characterization revealed that chlorflavonin specifically inhibits the acetohydroxyacid synthase catalytic subunit IlvB1, causing combined auxotrophies to branched-chain amino acids and to pantothenic acid. While exhibiting a bacteriostatic effect in monotreatment, chlorflavonin displayed synergistic effects with the first-line antibiotic isoniazid and particularly with delamanid, leading to a complete sterilization in liquid culture in combination treatment. Using a fluorescent reporter strain, intracellular activity of chlorflavonin against Mycobacterium tuberculosis inside infected macrophages was demonstrated and was superior to streptomycin treatment.
Keywords: Mycobacterium tuberculosis; acetohydroxyacid synthase; auxothrophy; chlorflavonin; synergistic killing effect.