8-Hydroxyquinoline Series Exerts Bactericidal Activity against <i>Mycobacterium tuberculosis</i> Via Copper-Mediated Toxicity

New drugs and mechanisms of action targeting <i>Mycobacterium tuberculosis</i> are urgently needed to solve the global pandemic of tuberculosis. We previously demonstrated that the 8-hydroxyquinoline series has rapid bactericidal activity against <i>M. tuberculosis</i>. In this work, we determined that the activity of the 8HQ series is potentiated by copper ions and that the activity is dependent on copper since activity was reduced when copper was depleted from the medium. We determined that exposure to 8HQs led to an increase in intracellular copper. The increase in copper ions was specific since we saw no changes for other metal cations (zinc, iron, magnesium, manganese, or calcium). We observed the transient generation of reactive oxygen species after 8HQ exposure which disappeared by 24 h. Inhibition of growth could be partially relieved by scavenging hydroxyl radicals. We excluded the possibility that 8HQs are toxic by DNA intercalation. We screened a panel of hypomorph strains and identified sensitized strains. The pattern of sensitized strains did not suggest a specific target, but metalloenzymes, proteins with Fe-S clusters, and cell envelope biosynthetic enzymes were highlighted. These data suggest that 8HQs do not have a specific intracellular target, but act as copper ionophores, and that the mode of action is via copper-dependent toxicity.