Exploiting the synthetic lethality between terminal respiratory oxidases to kill Mycobacterium tuberculosis and clear host infection

The recent discovery of small molecules targeting the cytochrome bc₁:aa₃ in Mycobacterium tuberculosis triggered interest in the terminal respiratory oxidases for antituberculosis drug development. The mycobacterial cytochrome bc₁:aa₃ consists of a menaquinone: cytochrome c reductase (bc₁) and a cytochrome aa3-type oxidase. The clinical-stage drug candidate Q203 interferes with the function of the subunit b of the menaquinone:cytochrome c reductase. Despite the affinity of Q203 for the bc₁:aa₃ complex, the drug is only bacteriostatic and does not kill drug-tolerant persisters. This raises the possibility that the alternate terminal bd-type oxidase (cytochrome bd oxidase) is capable of maintaining a membrane potential and menaquinol oxidation in the presence of Q203. Here, we show that the electron flow through the cytochrome bd oxidase is sufficient to maintain respiration and ATP synthesis at a level high enough to protect M. tuberculosis from Q203-induced bacterial death. Upon genetic deletion of the cytochrome bd oxidase-encoding genes cydAB, Q203 inhibited mycobacterial respiration completely, became bactericidal, killed drugtolerant mycobacterial persisters, and rapidly cleared M. tuberculosis infection in vivo. These results indicate a synthetic lethal interaction between the two terminal respiratory oxidases that can be exploited for anti-TB drug development. Our findings should be considered in the clinical development of drugs targeting the cytochrome bc₁:aa₃, as well as for the development of a drug combination targeting oxidative phosphorylation in M. tuberculosis.