Baeyer-Villiger Monooxygenases EthA and MymA Are Required for Activation of Replicating and Non-replicating Mycobacterium tuberculosis Inhibitors.

Successful treatment of Mycobacterium tuberculosis infection typically requires a complex regimen administered over at least 6 months. Interestingly, many of the antibiotics used to treat M. tuberculosis are prodrugs that require intracellular activation. Here, we describe three small molecules, active against both replicating and non-replicating M. tuberculosis, that require activation by Baeyer-Villiger monooxygenases (BVMOs). Two molecules require BVMO EthA (Rv3854c) for activation and the third molecule requires the BVMO MymA (Rv3083). While EthA is known to activate the antitubercular drug ethionamide, this is the first description of MymA as an activating enzyme of a prodrug. Furthermore, we found that MymA also plays a role in activating ethionamide, with loss of MymA function resulting in ethionamide-resistant M. tuberculosis. These findings suggest overlap in function and specificity of the BVMOs in M. tuberculosis.

Identification of novel inhibitors of nonreplicating Mycobacterium tuberculosis using a carbon starvation model.

During Mycobacterium tuberculosis infection, a population of bacteria is thought to exist in a nonreplicating state, refractory to antibiotics, which may contribute to the need for prolonged antibiotic therapy. The identification of inhibitors of the nonreplicating state provides tools that can be used to probe this hypothesis and the physiology of this state. The development of such inhibitors also has the potential to shorten the duration of antibiotic therapy required. Here we describe the development of a novel nonreplicating assay amenable to high-throughput chemical screening coupled with secondary assays that use carbon starvation as the in vitro model. Together these assays identify compounds with activity against replicating and nonreplicating M. tuberculosis as well as compounds that inhibit the transition from nonreplicating to replicating stages of growth. Using these assays we successfully screened over 300,000 compounds and identified 786 inhibitors of nonreplicating M. tuberculosis In order to understand the relationship among different nonreplicating models, we tested 52 of these molecules in a hypoxia model, and four different chemical scaffolds in a stochastic persister model, and a streptomycin-dependent model. We found that compounds display varying levels of activity in different models for the nonreplicating state, suggesting important differences in bacterial physiology between models. Therefore, chemical tools identified in this assay may be useful for determining the relevance of different nonreplicating in vitro models to in vivo M. tuberculosis infection. Given our current limited understanding, molecules that are active across multiple models may represent more promising candidates for further development.

Diarylcoumarins inhibit mycolic acid biosynthesis and kill Mycobacterium tuberculosis by targeting FadD32.

Infection with the bacterial pathogen Mycobacterium tuberculosis imposes an enormous burden on global public health. New antibiotics are urgently needed to combat the global tuberculosis pandemic; however, the development of new small molecules is hindered by a lack of validated drug targets. Here, we describe the identification of a 4,6-diaryl-5,7-dimethyl coumarin series that kills M. tuberculosis by inhibiting fatty acid degradation protein D32 (FadD32), an enzyme that is required for biosynthesis of cell-wall mycolic acids. These substituted coumarin inhibitors directly inhibit the acyl-acyl carrier protein synthetase activity of FadD32. They effectively block bacterial replication both in vitro and in animal models of tuberculosis, validating FadD32 as a target for antibiotic development that works in the same pathway as the established antibiotic isoniazid. Targeting new steps in well-validated biosynthetic pathways in antitubercular therapy is a powerful strategy that removes much of the usual uncertainty surrounding new targets and in vivo clinical efficacy, while circumventing existing resistance to established targets.

Persistent bacterial infections, antibiotic tolerance, and the oxidative stress response.

Certain bacterial pathogens are able to evade the host immune system and persist within the human host. The consequences of persistent bacterial infections potentially include increased morbidity and mortality from the infection itself as well as an increased risk of dissemination of disease. Eradication of persistent infections is difficult, often requiring prolonged or repeated courses of antibiotics. During persistent infections, a population or subpopulation of bacteria exists that is refractory to traditional antibiotics, possibly in a non-replicating or metabolically altered state. This review highlights the clinical significance of persistent infections and discusses different in vitro models used to investigate the altered physiology of bacteria during persistent infections. We specifically focus on recent work establishing increased protection against oxidative stress as a key element of the altered physiologic state across different in vitro models and pathogens.

Eradication of bacterial persisters with antibiotic-generated hydroxyl radicals.

During Mycobacterium tuberculosis infection, a population of bacteria likely becomes refractory to antibiotic killing in the absence of genotypic resistance, making treatment challenging. We describe an in vitro model capable of yielding a phenotypically antibiotic-tolerant subpopulation of cells, often called persisters, within populations of Mycobacterium smegmatis and M. tuberculosis. We find that persisters are distinct from the larger antibiotic-susceptible population, as a small drop in dissolved oxygen (DO) saturation (20%) allows for their survival in the face of bactericidal antibiotics. In contrast, if high levels of DO are maintained, all cells succumb, sterilizing the culture. With increasing evidence that bactericidal antibiotics induce cell death through the production of reactive oxygen species (ROS), we hypothesized that the drop in DO decreases the concentration of ROS, thereby facilitating persister survival, and maintenance of high DO yields sufficient ROS to kill persisters. Consistent with this hypothesis, the hydroxyl-radical scavenger thiourea, when added to M. smegmatis cultures maintained at high DO levels, rescues the persister population. Conversely, the antibiotic clofazimine, which increases ROS via an NADH-dependent redox cycling pathway, successfully eradicates the persister population. Recent work suggests that environmentally induced antibiotic tolerance of bulk populations may result from enhanced antioxidant capabilities. We now show that the small persister subpopulation within a larger antibiotic-susceptible population also shows differential susceptibility to antibiotic-induced hydroxyl radicals. Furthermore, we show that stimulating ROS production can eradicate persisters, thus providing a potential strategy to managing persistent infections.

Identification of novel inhibitors of M. tuberculosis growth using whole cell based high-throughput screening.

Despite the urgent need for new antitubercular drugs, few are on the horizon. To combat the problem of emerging drug resistance, structurally unique chemical entities that inhibit new targets will be required. Here we describe our investigations using whole cell screening of a diverse collection of small molecules as a methodology for identifying novel inhibitors that target new pathways for Mycobacterium tuberculosis drug discovery. We find that conducting primary screens using model mycobacterial species may limit the potential for identifying new inhibitors with efficacy against M. tuberculosis. In addition, we confirm the importance of developing in vitro assay conditions that are reflective of in vivo biology for maximizing the proportion of hits from whole cell screening that are likely to have activity in vivo. Finally, we describe the identification and characterization of two novel inhibitors that target steps in M. tuberculosis cell wall biosynthesis. The first is a novel benzimidazole that targets mycobacterial membrane protein large 3 (MmpL3), a proposed transporter for cell wall mycolic acids. The second is a nitro-triazole that inhibits decaprenylphosphoryl-ß-D-ribose 2'-epimerase (DprE1), an epimerase required for cell wall biosynthesis. These proteins are both among the small number of new targets that have been identified by forward chemical genetics using resistance generation coupled with genome sequencing. This suggests that methodologies currently employed for screening and target identification may lead to a bias in target discovery and that alternative methods should be explored.