Targeting Phenotypically TolerantMycobacterium tuberculosis

Abstract

While the immune system is credited with averting tuberculosis in billions of individuals exposed to Mycobacterium tuberculosis, the immune system is also culpable for tempering the ability of antibiotics to deliver swift and durable cure of disease. In individuals afflicted with tuberculosis, host immunity produces diverse microenvironmental niches that support suboptimal growth, or complete growth arrest, of M. tuberculosis. The physiological state of nonreplication in bacteria is associated with phenotypic drug tolerance. Many of these host microenvironments, when modeled in vitro by carbon starvation, complete nutrient starvation, stationary phase, acidic pH, reactive nitrogen intermediates, hypoxia, biofilms, and withholding streptomycin from the streptomycin-addicted strain SS18b, render M. tuberculosis profoundly tolerant to many of the antibiotics that are given to tuberculosis patients in a clinical setting. Targeting nonreplicating persisters is anticipated to reduce the duration of antibiotic treatment and rate of post-treatment relapse. Some promising drugs to treat tuberculosis, such as rifampicin and bedaquiline, only kill nonreplicating M. tuberculosis in vitro at concentrations far greater than their minimal inhibitory concentrations against replicating bacilli. There is an urgent demand to identify which of the currently used antibiotics, and which of the molecules in academic and corporate screening collections, have potent bactericidal action on nonreplicating M. tuberculosis. With this goal, we review methods of high throughput screening to target nonreplicating M. tuberculosis and methods to progress candidate molecules. A classification based on structures and putative targets of molecules that have been reported to kill nonreplicating M. tuberculosis revealed a rich diversity in pharmacophores. However, few of these compounds were tested under conditions that would exclude the impact of adsorbed compound acting during the recovery phase of the assay, and few were tested under more than one condition imposing nonreplication. That nonreplicating mycobacteria are metabolically active was corroborated by their susceptibility to several antibiotics and tool compounds that target the synthesis of lipids, RNA, DNA, proteins, and peptidoglycan.

Genetic and metabolic regulation ofMycobacterium tuberculosisacid growth arrest

“…Similar observations of antibiotic tolerance have been made during Mtb growth arrest in response to nitric oxide [11], low iron [12], and in multiple stress models [13]. The relationship between non-replicating persistence and Mtb phenotypic tolerance makes inhibition of Mtb persistence an inviting therapeutic target [14,15].…”

Extreme Drug Tolerance of Mycobacterium abscessus “Persisters”

“…We believe that the described assays are useful additions to the Mab drug discovery tool box and fill an important assay gap (Wu et al, 2018). In the related field of tuberculosis drug discovery similar (and additional) secondary assays are widely used (Dartois and Barry, 2013;Gold and Nathan, 2017). The predictive value of these assays for Mab drug discovery can only be validated once we have efficacy data of new drugs that showed bactericidal activity in an in vitro "persister assay.…”

“…Thus, these bacteria show "drug tolerance" or "phenotypic drug resistance." For a detailed discussion (and various definitions) of these nongenetic resistance phenomena we refer to reviews by Gold and Nathan (Gold and Nathan, 2017) for mycobacteria and Balaban and colleagues (Balaban et al, 2019) for a consensus statement.…”

Moxifloxacin Replacement in Contemporary Tuberculosis Drug Regimens Is Ineffective against Persistent Mycobacterium tuberculosis in the Cornell Mouse Model

“…tuberculosis persistence is the single most important hurdle hampering effective TB disease control (10). M. tuberculosis has the ability to survive in a dormant, nonmultiplying, and persistent state (11)(12)(13)(14). These persistent bacteria do not grow on solid or liquid media and are undetectable using conventional diagnostic methods; however, they can be resuscitated using resuscitation-promoting factors (RPF), which are present in the M. tuberculosis culture supernatant (15).…”