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 streptomycinaddicted 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.

Recent updates on drug resistance in Mycobacterium tuberculosis

“…The environmental stresses which induce latency in vivo have also been used in vitro to form dormant‐like Mtb cells and these include hypoxia (oxygen depletion), nutrient starvation (e.g. phosphate/nitrogen limitation), antibiotics ( d ‐cycloserine) and acidification of medium (low pH) (Gengenbacher and Kaufmann ; Gold and Nathan ). All these models share common features such as reduced respiration, reduced metabolism, reduced ATP levels, increased lipid metabolism and enhanced multidrug tolerance; however, certain features may differ depending on inducers (Lipworth et al ; Trastoy et al .…”

Tuberculosis: Just the FAQs

“…The underlying biology of the slow response to treatment is still unclear. Most current studies focus on inherent properties of the bacteria—their ability to enter nonreplicating states and their ability to display drug tolerance (the ability to generate subpopulations of cells that are not genetically resistant but are more slowly killed by antibiotics) ( 33 – 35 ). Recent work, however, suggests that many of the drugs used to treat TB might not penetrate effectively into some granulomas ( 36 ).…”

Kunkel Lecture: Fundamental immunodeficiency and its correction

“…Discovery of mechanisms of AMR has profoundly affected both basic science and clinical care. In basic science, studies of heritable AMR had a prominent role in introducing the concept that small chemical compounds can have specific macromolecular targets in biological systems and can serve as tools to identify the targets’ functions ( Gold and Nathan, 2017 ). Clinically, mechanistic understanding of heritable AMR allowed the design of combination chemotherapy with agents that thwart resistance.…”

“…Subsequently, >100 compounds have been reported to kill nonreplicating Mtb selectively, including novel cephalosporins ( Gold et al, 2016 ). However, in only a few cases did the investigators exclude the possibility that carryover of compound into the replicative phase of the assay may have led to a false impression of activity in the preceding, nonreplicative phase of the assay ( Gold and Nathan, 2017 ).…”

“…With respect to TB, this has been reported with 8-hydoxyquinolines ( Darby and Nathan, 2010 ; Shah et al, 2016 ) and nitazoxanide, an antibiotic approved for other indications ( de Carvalho et al, 2009 ). In vitro, the nitroimidazole PA-824 (pretomanid) kills both replicating and nonreplicating Mtb to comparable extents and at comparable concentrations ( Singh et al, 2008 ; Gold and Nathan, 2017 ). Under nonreplicating conditions, the mechanism involves generation of RNS ( Singh et al, 2008 ), a striking example of a synthetic antibiotic mimicking host immunity ( Nathan, 2008 ).…”