Moxifloxacin-mediated killing of Mycobacterium tuberculosis involves respiratory downshift, reductive stress, and ROS accumulation

Abstract

Moxifloxacin is central to treatment of multidrug-resistant tuberculosis. Effects of moxifloxacin on Mycobacterium tuberculosis redox state were explored to identify strategies for increasing lethality and reducing the prevalence of extensively resistant tuberculosis. A non-invasive redox biosensor and an ROS-sensitive dye revealed that moxifloxacin induces oxidative stress correlated with M. tuberculosis death. Moxifloxacin lethality was mitigated by supplementing bacterial cultures with an ROS scavenger (thiourea), an iron chelator (bipyridyl), and, after drug removal, an antioxidant enzyme (catalase). Lethality was also reduced by hypoxia and nutrient starvation. Moxifloxacin increased the expression of genes involved in the oxidative stress response, iron-sulfur cluster biogenesis, and DNA repair. Surprisingly, and in contrast with Escherichia coli studies, moxifloxacin decreased expression of genes involved in respiration, suppressed oxygen consumption, increased the NADH/NAD+ ratio, and increased the labile iron pool in M. tuberculosis. Lowering the NADH/NAD+ ratio in M. tuberculosis revealed that NADH-reductive stress facilitates an iron-mediated ROS surge and moxifloxacin lethality. Treatment with N-acetyl cysteine (NAC) accelerated respiration and ROS production, increased moxifloxacin lethality, and lowered the mutant prevention concentration. Moxifloxacin induced redox stress in M. tuberculosis inside macrophages, and co-treatment with NAC potentiated the anti-mycobacterial efficacy of moxifloxacin during nutrient starvation, inside macrophages, and in mice where NAC restricted the emergence of resistance. Thus, oxidative stress, generated in a novel way, contributes to moxifloxacin-mediated killing of M. tuberculosis. The results open a way to make fluoroquinolones more effective anti-tuberculosis agents and provide a mechanistic basis for NAC-mediated enhancement of fluoroquinolone lethality in vitro and in vivo.Author SummaryA new paradigm was revealed for stress-mediated bacterial death in which moxifloxacin treatment of M. tuberculosis decreases respiration rate (respiration increases in E. coli). Although moxifloxacin-induced, ROS-mediated bacterial death was observed, it derived from elevated levels of NADH and iron, a phenomenon not seen with antibiotic-treated E. coli. Nevertheless, stimulation of respiration and ROS by N-acetyl cysteine (NAC) enhanced moxifloxacin-mediated killing of M. tuberculosis, thereby reinforcing involvement of ROS in killing. NAC stimulation of moxifloxacin-mediated killing of M. tuberculosis and restriction of the emergence of resistance in a murine model of infection emphasize the importance of lethal action against pathogens. The work, plus published benefits of NAC to TB patients, encourage studies of NAC-based enhancement of fluoroquinolones.

Fluoroquinolone heteroresistance, antimicrobial tolerance, and lethality enhancement

“…As with E. coli , we expect ROS to play a central role in fluoroquinolone lethality with M. tuberculosis . Indeed, moxifloxacin increases ROS with cultured M. tuberculosis , as detected by an oxidation-sensitive fluorescent dye and by a redox-sensitive biosensor ( Shee et al., 2022 ). Involvement of hydrogen peroxide is supported by a moxifloxacin-mediated increase in peroxide and the suppression of lethality by adding catalase to the growth medium (hydrogen peroxide diffuses between the cell interior and exterior, making it vulnerable to exogenous catalase-mediated degradation).…”

“…Involvement of hydrogen peroxide is supported by a moxifloxacin-mediated increase in peroxide and the suppression of lethality by adding catalase to the growth medium (hydrogen peroxide diffuses between the cell interior and exterior, making it vulnerable to exogenous catalase-mediated degradation). Moreover, agents that interfere with ROS accumulation (bipyridyl and thiourea) lower ROS and increase survival ( Shee et al., 2022 ). Moxifloxacin also increases the expression of genes involved in the oxidative stress response, iron-sulfur cluster biogenesis, and DNA repair ( Shee et al., 2022 ).…”

“…Moreover, agents that interfere with ROS accumulation (bipyridyl and thiourea) lower ROS and increase survival ( Shee et al., 2022 ). Moxifloxacin also increases the expression of genes involved in the oxidative stress response, iron-sulfur cluster biogenesis, and DNA repair ( Shee et al., 2022 ). In addition, the idea that fluoroquinolones have two ways to kill M. tuberculosis is supported by ROS appearing to act at low drug concentration but killing continuing at high concentration independent of ROS ( Shee et al., 2022 ), as observed with E. coli ( Keren et al., 2013 ).…”

“…Moxifloxacin also increases the expression of genes involved in the oxidative stress response, iron-sulfur cluster biogenesis, and DNA repair ( Shee et al., 2022 ). In addition, the idea that fluoroquinolones have two ways to kill M. tuberculosis is supported by ROS appearing to act at low drug concentration but killing continuing at high concentration independent of ROS ( Shee et al., 2022 ), as observed with E. coli ( Keren et al., 2013 ). Thus, in many ways fluoroquinolone action in M. tuberculosis is similar to that reported for E. coli .…”

“…In principle, tolerance is expected to make clearing infection difficult and contribute to tuberculosis relapse. Tolerance also contributes to elevated frequency of resistance ( Levin-Reisman et al., 2017 ; Shee et al., 2022 ), probably by allowing bacterial numbers to remain high and by reducing the killing of resistant mutant subpopulations. We note that quinolones also kill bacteria by chromosome fragmentation − examples exist in which interference with ROS accumulation fails to block killing completely ( Malik et al., 2007 ; Keren et al., 2013 ).…”