Potassium availability triggers Mycobacterium tuberculosis transition to, and resuscitation from, non-culturable (dormant) states

Nitric oxide induces the distinct invisibility phenotype of Mycobacterium tuberculosis

During infection Mycobacterium tuberculosis (Mtb) forms physiologically distinct subpopulations that are recalcitrant to treatment and undetectable using standard diagnostics. These difficult to culture or differentially culturable (DC) Mtb are revealed in liquid media, their revival is often stimulated by resuscitation-promoting factors (Rpf) and prevented by Rpf inhibitors. Here, we investigated the role of nitric oxide (NO) in promoting the DC phenotype. Rpf-dependent DC Mtb were detected following infection of interferon-γ-induced macrophages capable of producing NO, but not when inducible NO synthase was inactivated. After exposure of Mtb to a new donor for sustained NO release (named NOD), the majority of viable cells were Rpf-dependent and undetectable on solid media. Gene expression analyses revealed a broad transcriptional response to NOD, including down-regulation of all five rpf genes. The DC phenotype was partially reverted by over-expression of Rpfs which promoted peptidoglycan remodelling. Thus, NO plays a central role in the generation of Rpf-dependent Mtb, with implications for improving tuberculosis diagnostics and treatments.

Regulatory role of Mycobacterium tuberculosis MtrA on dormancy/resuscitation revealed by a novel target gene-mining strategy

Introduction

The unique dormancy of Mycobacterium tuberculosis plays a significant role in the major clinical treatment challenge of tuberculosis, such as its long treatment cycle, antibiotic resistance, immune escape, and high latent infection rate.

Methods

To determine the function of MtrA, the only essential response regulator, one strategy was developed to establish its regulatory network according to high-quality genome-wide binding sites.

Results and discussion

The complex modulation mechanisms were implied by the strong bias distribution of MtrA binding sites in the noncoding regions, and 32.7% of the binding sites were located inside the target genes. The functions of 288 potential MtrA target genes predicted according to 294 confirmed binding sites were highly diverse, and DNA replication and damage repair, lipid metabolism, cell wall component biosynthesis, cell wall assembly, and cell division were the predominant pathways. Among the 53 pathways shared between dormancy/resuscitation and persistence, which accounted for 81.5% and 93.0% of the total number of pathways, respectively, MtrA regulatory genes were identified not only in 73.6% of their mutual pathways, but also in 75.4% of the pathways related to dormancy/resuscitation and persistence respectively. These results suggested the pivotal roles of MtrA in regulating dormancy/resuscitation and the apparent relationship between dormancy/resuscitation and persistence. Furthermore, the finding that 32.6% of the MtrA regulons were essential in vivo and/or in vitro for M. tuberculosis provided new insight into its indispensability. The findings mentioned above indicated that MtrA is a novel promising therapeutic target for tuberculosis treatment since the crucial function of MtrA may be a point of weakness for M. tuberculosis.

RegX3-dependent transcriptional activation of kdpDE and repression of rv0500A are linked to potassium homeostasis in Mycobacterium tuberculosis

Ionic homeostasis is essential for the survival and replication of Mycobacterium tuberculosis within its host. Low potassium ion concentrations trigger a transition of M. tuberculosis into dormancy. Our current knowledge of the transcriptional regulation mechanisms governing genes involved in potassium homeostasis remains limited. Potassium transport is regulated by the constitutive Trk system and the inducible Kdp system in M. tuberculosis. The two-component system KdpDE (also known as KdpD/KdpE) activates expression of the kdpFABC operon, encoding the four protein subunits of the Kdp potassium uptake system (KdpFABC). We show that, under potassium deficiency, expression of the two-component system senX3/regX3 is upregulated, and bacterial survival is compromised in a regX3-inactivated mutant, ΔregX3. Electrophoretic mobility shift assays (EMSAs), promoter reporter assays and chromatin immunoprecipitation (ChIP) show that RegX3 binds to the kdpDE promoter and activates it under potassium deficiency, whereas RegX3 (K204A), a DNA binding-deficient mutant, fails to bind to the promoter. Mutation of the RegX3 binding motifs on the kdpDE promoter abrogates RegX3 binding. In addition, EMSAs and ChIP assays show that RegX3 represses Rv0500A, a repressor of kdpFABC, by binding to consensus RegX3 binding motifs on the rv0500A promoter. Our findings provide important insight into two converging pathways regulated by RegX3; one in which it activates an activator of kdpFABC, and the other in which it represses a repressor of kdpFABC, during potassium insufficiency. This culminates in increased expression of the potassium uptake system encoded by kdpFABC, enabling bacterial survival. These results further expand the growing transcriptional network in which RegX3 serves as a central node to enable bacterial survival under stress.

Cellular dormancy: A widespread phenomenon that perpetuates infectious diseases

Under adverse environmental conditions, microorganisms are able to enter a state of cellular dormancy which consists of cell cycle arrest and interruption of multiplication. This process ensures their perpetuation in the infected host organism and enables the spread of disease. Throughout biological evolution, dormancy allowed microorganisms to persist in a harsh niche until favorable conditions for their reactivation were re-established. Here, we propose to discuss the dormancy of bacteria and protozoa pathogens focusing on the potential mechanisms and components associated with dormancy.

Clinical Strains of Mycobacterium tuberculosis Representing Different Genotype Families Exhibit Distinct Propensities to Adopt the Differentially Culturable State

Growing evidence points to the presence of differentially culturable tubercle bacteria (DCTB) in clinical specimens from individuals with active tuberculosis (TB) disease. These bacteria are unable to grow on solid media but can resuscitate in liquid media. Given the epidemiological success of certain clinical genotype families of Mycobacterium tuberculosis, we hypothesize that different strains may have distinct mechanisms of adaptation and tolerance. We used an in vitro carbon starvation model to determine the propensity of strains from lineages 2 and 4 that included the Beijing and LAM families respectively, to generate DCTB. Beijing strains were associated with a greater propensity to produce DCTB compared to LAM strains. Furthermore, LAM strains required culture filtrate (CF) for resuscitation whilst starved Beijing strains were not dependent on CF. Moreover, Beijing strains showed improved resuscitation with cognate CF, suggesting the presence of unique growth stimulatory molecules in this family. Analysis of starved Beijing and LAM strains showed longer cells, which with resuscitation were restored to a shorter length. Cell wall staining with fluorescent D-amino acids identified strain-specific incorporation patterns, indicating that cell surface remodeling during resuscitation was distinct between clinical strains. Collectively, our data demonstrate that M. tuberculosis clinical strains from different genotype lineages have differential propensities to generate DCTB, which may have implications for TB treatment success.

The use of Mycobacterium tuberculosis H37Ra-infected immunocompetent mice as an in vivo model of persisters

Persistence of Mycobacterium tuberculosis (Mtb) is one of the challenges to successful treatment of tuberculosis (TB). In vitro models of non-replicating Mtb are used to test the efficacy of new molecules against Mtb persisters. The H37Ra strain is attenuated for growth in macrophages and mice. We validated H37Ra-infected immunocompetent mice for testing anti-TB molecules against slow/non-replicating Mtb in vivo. Swiss mice were infected intravenously with H37Ra and monitored for CFU burden and histopathology for a period of 12 weeks. The bacteria multiplied at a slow pace reaching a maximum load of ∼106 in 8-12 weeks depending on the infection dose, accompanied by time and dose-dependent histopathological changes in the lungs. Surprisingly, four-weeks of treatment with isoniazid-rifampicin-ethambutol-pyrazinamide combination caused only 0.4 log10 and 1 log10 reduction in CFUs in lungs and spleen respectively. The results show that ∼40 % of the H37Ra bacilli in lungs are persisters after 4 weeks of anti-TB therapy. Isoniazid/rifampicin monotherapy also showed similar results. A combination of bedaquiline and isoniazid reduced the CFU counts to <200 (limit of detection), compared to ∼5000 CFUs by isoniazid alone. The study demonstrates an in vivo model of Mtb persisters for testing new leads using a BSL-2 strain.

TcrXY is an acid-sensing two-component transcriptional regulator of Mycobacterium tuberculosis required for persistent infection

The ability of Mycobacterium tuberculosis (Mtb) to persist in the host complicates and prolongs tuberculosis (TB) patient chemotherapy. Here we demonstrate that a neglected two-component system (TCS) of Mtb, TcrXY, is an autoregulated acid-sensing TCS that controls a functionally diverse 70-gene regulon required for bacterial persistence. Characterisation of two representatives of this regulon, Rv3706c and Rv3705A, implicate these genes as key determinants for the survival of Mtb in vivo by serving as important effectors to mitigate redox stress at acidic pH. We show that genetic silencing of the response regulator tcrX using CRISPR interference attenuates the persistence of Mtb during chronic mouse infection and improves treatment with the two front-line anti-TB drugs, rifampicin and isoniazid. We propose that targeting TcrXY signal transduction blocks the ability of Mtb to sense and respond to acid stress, resulting in a disordered program of persistence to render the organism vulnerable to existing TB chemotherapy.

Specific Cationic Antimicrobial Peptides Enhance the Recovery of Low-Load Quiescent Mycobacterium tuberculosis in Routine Diagnostics

The culture confirmation of Mycobacterium tuberculosis (MTB) remains the gold standard for the diagnosis of Tuberculosis (TB) with culture conversion representing proof of cure. However, over 40% of TB samples fail to isolate MTB even though many patients remain infectious due to the presence of viable non-culturable forms. Previously, we have shown that two short cationic peptides, T14D and TB08L, induce a hormetic response at low concentrations, leading to a stimulation of growth in MTB and the related animal pathogen Mycobacterium bovis (bTB). Here, we examine these peptides showing they can influence the mycobacterial membrane integrity and function through membrane potential reduction. We also show this disruption is associated with an abnormal reduction in transcriptomic signalling from specific mycobacterial membrane sensors that normally monitor the immediate cellular environment and maintain the non-growing phenotype. We observe that exposing MTB or bTB to these peptides at optimal concentrations rapidly represses signalling mechanisms maintaining dormancy phenotypes, which leads to the promotion of aerobic metabolism and conversion into a replicative phenotype. We further show a practical application of these peptides as reagents able to enhance conventional routine culture methods by stimulating mycobacterial growth. We evaluated the ability of a peptide-supplemented sample preparation and culture protocol to isolate the MTB against a gold standard routine method tested in parallel on 255 samples from 155 patients with suspected TB. The peptide enhancement increased the sample positivity rate by 46% and decreased the average time to sample positivity of respiratory/faecal sampling by seven days. The most significant improvements in isolation rates were from sputum smear-negative low-load samples and faeces. The peptide enhancement increased sampling test sensitivity by 19%, recovery in samples from patients with a previously culture-confirmed TB by 20%, and those empirically treated for TB by 21%. We conclude that sample decontamination and culture enhancement with D-enantiomer peptides offer good potential for the much-needed improvement of the culture confirmation of TB.

Distinct Effects of Moxifloxacin and Bedaquiline on Growing and 'Non-Culturable' Mycobacterium abscessus

Mycobacterium abscessus has recently emerged as the cause of an increasing number of human infections worldwide. Unfortunately, it is highly resistant to existing drugs, and new specific agents to combat M. abscessus have not yet been found. The discovery of antibiotics that are effective not only against replicating but also against dormant and often recalcitrant cells is a daunting challenge. In this study, we developed a model of non-replicating M. abscessus, which represents a valuable screening tool for antibacterial agents. Thus, we demonstrated that, under a deficiency of potassium ions in the growth media and prolonged incubation, M. abscessus entered a 'non-culturable' state with a significant loss of colony-forming ability, but it retained viability, as confirmed using the most-probable-number (MPN) assay. The 'non-culturable' mycobacteria possessed decelerated cellular metabolism and noticeable differences in cell morphology from actively growing mycobacteria. 'Non-culturable' cells were used in a comprehensive screening of the efficacy of antibiotics, along with actively growing cells. Both CFU and MPN tests confirmed the prominent bactericidal effect of moxifloxacin on actively growing and 'non-culturable' M. abscessus, as proven by less than 0.01% of cells surviving after antibiotic treatment and prolonged storage. Bedaquiline exhibited a comparable bactericidal effect only on metabolically inactive non-culturable cells aged for 44 days. There were reductions ranging from 1000 to 10,000-fold in CFU and MPN, but it was not so efficient with respect to active cells, resulting in a bacteriostatic effect. The demonstrated specificity of bedaquiline in relation to inert non-replicating M. abscessus offers a new and unexpected result. Based on the findings of this research, moxifloxacin and bedaquiline can be regarded as potential treatments for infections caused by M. abscessus. In addition, a key outcome is the proposal to include the combination of viability assays for comprehensive testing of drug candidates. Relying on CFU-based assays alone resulted in overestimates of antibacterial efficacy, as demonstrated in our experiments.

A Multimodal Desorption Electrospray Ionisation Workflow Enabling Visualisation of Lipids and Biologically Relevant Elements in a Single Tissue Section

The colocation of elemental species with host biomolecules such as lipids and metabolites may shed new light on the dysregulation of metabolic pathways and how these affect disease pathogeneses. Alkali metals have been the subject of extensive research, are implicated in various neurodegenerative and infectious diseases and are known to disrupt lipid metabolism. Desorption electrospray ionisation (DESI) is a widely used approach for molecular imaging, but previous work has shown that DESI delocalises ions such as potassium (K) and chlorine (Cl), precluding the subsequent elemental analysis of the same section of tissue. The solvent typically used for the DESI electrospray is a combination of methanol and water. Here we show that a novel solvent system, (50:50 (%v/v) MeOH:EtOH) does not delocalise elemental species and thus enables elemental mapping to be performed on the same tissue section post-DESI. Benchmarking the MeOH:EtOH electrospray solvent against the widely used MeOH:H2O electrospray solvent revealed that the MeOH:EtOH solvent yielded increased signal-to-noise ratios for selected lipids. The developed multimodal imaging workflow was applied to a lung tissue section containing a tuberculosis granuloma, showcasing its applicability to elementally rich samples displaying defined structural information.

Hypobiosis of Mycobacteria: Biochemical Aspects

Under suboptimal growth conditions, bacteria can transit to the dormant forms characterized by a significantly reduced metabolic activity, resistance to various stress factors, and absence of cell proliferation. Traditionally, the dormant state is associated with the formation of highly differentiated cysts and spores. However, non-spore-forming bacteria can transfer to the dormant-like hypobiotic state with the generation of less differentiated cyst-like forms (which are different from spores). This review focuses on morphological and biochemical changes occurred during formation of dormant forms of mycobacteria in particular pathogenic M. tuberculosis (Mtb) caused latent forms of tuberculosis. These forms are characterized by the low metabolic activity, the absence of cell division, resistance to some antibiotics, marked morphological changes, and loss of ability to grow on standard solid media ("non-culturable" state). Being produced in vitro, dormant Mtb retained ability to maintain latent infection in mice. After a long period of dormancy, mycobacteria retain a number of stable proteins with a potential enzymatic activity which could participate in maintaining of low-level metabolic activity in period of dormancy. Indeed, the metabolomic analysis showed significant levels of metabolites in the dormant cells even after a long period of dormancy, which may be indicative of residual metabolism in dormant mycobacteria. Special role may play intracellularly accumulated trehalose in dormant mycobacteria. Trehalose appears to stabilize dormant cells, as evidenced by the direct correlation between the trehalose content and cell viability during the long-term dormancy. In addition, trehalose can be considered as a reserve energy substrate consumed during reactivation of dormant mycobacteria due to the ATP-dependent conversion of trehalase from the latent to the active state. Another feature of dormant mycobacteria is a high representation of proteins participating in the enzymatic defense against stress factors and of low-molecular-weight compounds protecting cells in the absence of replication. Dormant mycobacteria contain a large number of hydrolyzing enzymes, which, on the one hand, ensure inactivation of biomolecules damaged by stress. On the other hand, the products of these enzymatic reactions can be used for the maintenance of energy state and vital activity of bacterial cells during their long-term survival in the dormant state, i.e., for creating a situation that we propose to refer to as the "catabolic survival". In general, dormant non-replicating mycobacterial cells can be described as morphologically altered forms that contain principal macromolecules and are stabilized and protected from the damaging factors by an arsenal of proteins and low-molecular-weight compounds. Because of the presumable occurrence of metabolic reactions in such cells, this form of survival should be referred to as hypobiosis.

Tuberculosis: The success tale of less explored dormant Mycobacterium tuberculosis

Mycobacterium tuberculosis (M.tb) is an intracellular pathogen that predominantly affects the alveolar macrophages in the respiratory tract. Upon infection, the activation of TLR2 and TLR4- mediated signaling pathways leads to lysosomal degradation of the bacteria. However, bacterium counteracts the host immune cells and utilizes them as a cellular niche for its survival. One distinctive mechanism of M.tb to limit the host stress responses such as hypoxia and nutrient starvation is induction of dormancy. As the environmental conditions become favorable, the bacteria resuscitate, resulting in a relapse of clinical symptoms. Different bacterial proteins play a critical role in maintaining the state of dormancy and resuscitation, namely, DevR (DosS), Hrp1, DATIN and RpfA-D, RipA, etc., respectively. Existing knowledge regarding the key proteins associated with dormancy and resuscitation can be employed to develop novel therapies. In this review we aim to highlight the current knowledge of bacterial progression from dormancy to resuscitation and the gaps in understanding the transition from dormant to active state. We have also focused on elucidating a few therapeutic strategies employed to prevent M.tb resuscitation.

A New Benzothiazolthiazolidine Derivative, 11726172, Is Active In Vitro, In Vivo, and against Nonreplicating Cells of Mycobacterium tuberculosis

Tuberculosis (TB) still poses a global menace as one of the deadliest infectious diseases. A quarter of the human population is indeed latently infected with Mycobacterium tuberculosis. People with latent infection have a 5 to 10% lifetime risk of becoming ill with TB, representing a reservoir for TB active infection. This is a worrisome problem to overcome in the case of relapse; unfortunately, few drugs are effective against nonreplicating M. tuberculosis cells. Novel strategies to combat TB, including its latent form, are urgently needed. In response to the lack of new effective drugs and after screening about 500 original chemical molecules, we selected a compound, 11726172, that is endowed with potent antitubercular activity against M. tuberculosis both in vitro and in vivo and importantly also against dormant nonculturable bacilli. We also investigated the mechanism of action of 11726172 by applying a multidisciplinary approach, including transcriptomic, labeled metabolomic, biochemical, and microbiological procedures. Our results represent an important step forward in the development of a new antitubercular compound with a novel mechanism of action active against latent bacilli. IMPORTANCE The discontinuation of TB services due to COVID-19 causes concern about a future resurgence of TB, also considering that latent infection affects a high number of people worldwide. To combat this situation, the identification of antitubercular compounds targeting Mycobacterium tuberculosis through novel mechanisms of action is necessary. These compounds should be active against not only replicating bacteria cells but also nonreplicating cells to limit the reservoir of latently infected people on which the bacterium can rely to spread after reactivation.

Transcriptional Biomarkers of Differentially Detectable Mycobacterium tuberculosis in Patient Sputum

Certain populations of Mycobacterium tuberculosis go undetected by standard diagnostics but can be enumerated using limiting dilution assays. These differentially detectable M. tuberculosis (DD M. tuberculosis) populations may have relevance for persistence due to their drug tolerance. It is unclear how well DD M. tuberculosis from patients is modeled by a recently developed in vitro model in which M. tuberculosis starved in phosphate-buffered saline is incubated with rifampin to produce DD M. tuberculosis (the PBS-RIF model). This study attempted to answer this question. We selected 14 genes that displayed differential expression in the PBS-RIF model and evaluated their expression in patient sputa containing various proportions of DD M. tuberculosis. The expression of 12/14 genes correlated with the relative abundance of DD M. tuberculosis in patient sputa. Culture filtrate (CF), which promotes recovery of DD M. tuberculosis from certain patient sputa, improved these correlations in most cases. The gene whose reduced expression relative to M. tuberculosis 16S rRNA showed the greatest association with the presence and relative abundance of DD M. tuberculosis in patient sputa, icl1, was recently shown to play a functional role in restraining DD M. tuberculosis formation in the PBS-RIF model. Expression of icl1, combined with two additional DD M. tuberculosis-related genes, showed strong performance for predicting the presence or absence of DD M. tuberculosis in patient sputa (receiver operating characteristic [ROC] area under the curve [AUC] = 0.88). Thus, the in vitro DD M. tuberculosis model developed by Saito et al. (K. Saito, T. Warrier, S. Somersan-Karakaya, L. Kaminski, et al., Proc Natl Acad Sci U S A 114:E4832-E4840, 2017, https://doi.org/10.1073/pnas.1705385114) bears a resemblance to DD M. tuberculosis found in tuberculosis (TB) patients, and DD M. tuberculosis transcriptional profiles may be useful for monitoring DD M. tuberculosis populations in patient sputum. IMPORTANCE Differentially detectable M. tuberculosis (DD M. tuberculosis), which is detectable by limiting dilution assays but not by CFU, is present and enriched for in TB patient sputum after initiation of first-line therapy. These cryptic cells may play a role in disease persistence due to their phenotypic tolerance to anti-TB drugs. A recently developed in vitro model of DD M. tuberculosis (the PBS-RIF model) has expanded our understanding of these cells, though how well it translates to DD M. tuberculosis in patients is currently unknown. To answer this question, we selected 14 genes that displayed differential expression in the PBS-RIF model and evaluated their expression in TB patient sputa. We found that 12/14 of these genes showed a similar expression profile in patient sputa that correlated with the relative abundance of DD M. tuberculosis. Further, the expression of three of these genes showed strong performance for predicting the presence or absence of DD M. tuberculosis in patient sputa. The use of DD M. tuberculosis transcriptional profiles may allow for easier monitoring of DD M. tuberculosis populations in patient sputum in comparison to limiting dilution assays.

Mycobacterium tuberculosis Dormancy: How to Fight a Hidden Danger

Both latent and active TB infections are caused by a heterogeneous population of mycobacteria, which includes actively replicating and dormant bacilli in different proportions. Dormancy substantially affects M. tuberculosis drug tolerance and TB clinical management due to a significant decrease in the metabolic activity of bacilli, which leads to the complexity of both the diagnosis and the eradication of bacilli. Most diagnostic approaches to latent infection deal with a subpopulation of active M. tuberculosis, underestimating the contribution of dormant bacilli and leading to limited success in the fight against latent TB. Moreover, active TB appears not only as a primary form of infection but can also develop from latent TB, when resuscitation from dormancy is followed by bacterial multiplication, leading to disease progression. To win against latent infection, the identification of the Achilles' heel of dormant M. tuberculosis is urgently needed. Regulatory mechanisms and metabolic adaptation to growth arrest should be studied using in vitro and in vivo models that adequately imitate latent TB infection in macroorganisms. Understanding the mechanisms underlying M. tuberculosis dormancy and resuscitation may provide clues to help control latent infection, reduce disease severity in patients, and prevent pathogen transmission in the population.

Clinically encountered growth phenotypes of tuberculosis-causing bacilli and their in vitro study: A review

The clinical manifestations of tuberculosis (TB) vary widely in severity, site of infection, and outcomes of treatment-leading to simultaneous efforts to individualize therapy safely and to search for shorter regimens that can be successfully used across the clinical spectrum. In these endeavors, clinicians and researchers alike employ mycobacterial culture in rich media. However, even within the same patient, individual bacilli among the population can exhibit substantial variability in their culturability. Bacilli in vitro also demonstrate substantial heterogeneity in replication rate and cultivation requirements, as well as susceptibility to killing by antimicrobials. Understanding parallels in clinical, ex vivo and in vitro growth phenotype diversity may be key to identifying those phenotypes responsible for treatment failure, relapse, and the reactivation of bacilli that progresses TB infection to disease. This review briefly summarizes the current role of mycobacterial culture in the care of patients with TB and the ex vivo evidence of variability in TB culturability. We then discuss current advances in in vitro models that study heterogenous subpopulations within a genetically identical bulk culture, with an emphasis on the effect of oxidative stress on bacillary cultivation requirements. The review highlights the complexity that heterogeneity in mycobacterial growth brings to the interpretation of culture in clinical settings and research. It also underscores the intricacies present in the interplay between growth phenotypes and antimicrobial susceptibility. Better understanding of population dynamics and growth requirements over time and space promises to aid both the attempts to individualize TB treatment and to find uniformly effective therapies.

Phenotypic adaptation of Mycobacterium tuberculosis to host-associated stressors that induce persister formation

Mycobacterium tuberculosis exhibits a remarkable ability to interfere with the host antimicrobial response. The pathogen exploits elaborate strategies to cope with diverse host-induced stressors by modulating its metabolism and physiological state to prolong survival and promote persistence in host tissues. Elucidating the adaptive strategies that M. tuberculosis employs during infection to enhance persistence is crucial to understanding how varying physiological states may differentially drive disease progression for effective management of these populations. To improve our understanding of the phenotypic adaptation of M. tuberculosis, we review the adaptive strategies employed by M. tuberculosis to sense and coordinate a physiological response following exposure to various host-associated stressors. We further highlight the use of animal models that can be exploited to replicate and investigate different aspects of the human response to infection, to elucidate the impact of the host environment and bacterial adaptive strategies contributing to the recalcitrance of infection.

Stalking Mycobacterium bovis in the total environment: FLOW-FISH & FACS to detect, quantify, and sort metabolically active and quiescent cells in complex matrices

Mycobacterium bovis causes tuberculosis (TB) at the human-wildlife-livestock interface. Environmental persistence of M. bovis excreted by infected hosts may cause indirect transmission to other animals. However, methodological constrains hamper assessment of M. bovis viability and molecular signature in environmental matrices. In this work, an innovative, modular, and highly efficient single-cell workflow combining flow cytometry (FLOW), fluorescence in situ hybridization (FISH), and fluorescence-activated cell sorting (FACS) was developed, allowing detection, quantification, and sorting of viable and dormant M. bovis cells from environmental matrices. Validation with spiked water and sediments showed high efficiency (90%) of cell recovery, with high linearity between expected and observed results, both in cell viability evaluation (r² =0.93) and FISH-labelled M. bovis cells quantification (r² ≥0.96). The limit of detection was established at 10⁵ cells/g of soil in the cell viability step and 10² cells/g of soil in the taxonomical labelling stage. Moreover, FACS efficiency attained noteworthy recovery yield (50%) and purity (60% viable cells; 70% taxonomically labelled M. bovis). This new methodology represents a huge step for M. bovis assessment outside the mammal host, offering the rapid quantification of M. bovis cell load and cell viability, including viable but non-culturable cells, and further downstream cell analyses after FACS. Subsequent environmental data integration with the clinical component will expand knowledge on transmission routes, promising new paths in TB research and an intervention tool to mitigate the underlying biohazard.

OXSR1 inhibits inflammasome activation by limiting potassium efflux during mycobacterial infection

Mycobacteria up-regulate host kinase OXSR1 preventing potassium efflux and inflammasome activation. Depletion or inhibition of OXSR1 potentiates inflammasome activation and decreases bacterial burden.

Pathogenic mycobacteria inhibit inflammasome activation to establish infection. Although it is known that potassium efflux is a trigger for inflammasome activation, the interaction between mycobacterial infection, potassium efflux, and inflammasome activation has not been investigated. Here, we use Mycobacterium marinum infection of zebrafish embryos and Mycobacterium tuberculosis infection of THP-1 cells to demonstrate that pathogenic mycobacteria up-regulate the host WNK signalling pathway kinases SPAK and OXSR1 which control intracellular potassium balance. We show that genetic depletion or inhibition of OXSR1 decreases bacterial burden and intracellular potassium levels. The protective effects of OXSR1 depletion are at least partially mediated by NLRP3 inflammasome activation, caspase-mediated release of IL-1β, and downstream activation of protective TNF-α. The elucidation of this druggable pathway to potentiate inflammasome activation provides a new avenue for the development of host-directed therapies against intracellular infections.

Unravelling novel roles of the Mycobacterium tuberculosis transcription factor Rv0081 in regulation of the nucleoid-associated proteins Lsr2 and EspR, cholesterol utilization and subversion of lysosomal trafficking in macrophages

The transcriptional network of Mycobacterium tuberculosis is designed to enable the organism to withstand host-associated stresses and to exploit the host milieu for its own survival and multiplication. Rv0081 (MT0088) is a transcriptional regulator whose interplay with other gene regulatory proteins and role in enabling M. tuberculosis to thrive within its host is incompletely understood. M. tuberculosis utilizes cholesterol within the granuloma. We show that deletion of Rv0081 compromises the ability of M. tuberculosis to utilize cholesterol as sole carbon source, to subvert lysosomal trafficking, and to form granulomas in vitro. Rv0081 downregulates expression of the nucleoid associated repressor Lsr2, leading to increased expression of the cholesterol catabolism-linked gene kshA and genes of the cholesterol importing operon, accounting for the requirement of Rv0081 in cholesterol utilization. Further, Rv0081 activates EspR which is required for secretion of ESX-1 substrates, which in turn are involved in subversion of lysosomal traffickingof M. tuberculosisand granuloma expansion. These results provide new insight into the role of Rv0081 under conditions which resemble the environment encountered by M. tuberculosis within its host. Rv0081 emergesas a central regulator of genes linked to various pathways which are crucial for the survival of the bacterium in vivo.

Oxidative damage and delayed replication allow viable Mycobacterium tuberculosis to go undetected

[Figure: see text].

The Mycobacterium tuberculosis sRNA F6 Modifies Expression of Essential Chaperonins, GroEL2 and GroES

Almost 140 years after the identification of Mycobacterium tuberculosis as the etiological agent of tuberculosis, important aspects of its biology remain poorly described. Little is known about the role of posttranscriptional control of gene expression and RNA biology, including the role of most of the small RNAs (sRNAs) identified to date. We have carried out a detailed investigation of the M. tuberculosis sRNA F6 and shown it to be dependent on SigF for expression and significantly induced in starvation conditions in vitro and in a mouse model of infection. Further exploration of F6 using an in vitro starvation model of infection indicates that F6 affects the expression of the essential chaperonins GroEL2 and GroES. Our results point toward a role for F6 during periods of low metabolic activity typically associated with long-term survival of M. tuberculosis in human granulomas.

IMPORTANCE Control of gene expression via small regulatory RNAs (sRNAs) is poorly understood in one of the most successful pathogens, Mycobacterium tuberculosis. Here, we present an in-depth characterization of the sRNA F6, including its expression in different infection models and the differential gene expression observed upon deletion of the sRNA. Our results demonstrate that deletion of F6 leads to dysregulation of the two essential chaperonins GroEL2 and GroES and, moreover, indicate a role for F6 in the long-term survival and persistence of M. tuberculosis in the human host.

Role of the kdpDE Regulatory Operon of Mycobacterium tuberculosis in Modulating Bacterial Growth in vitro

Bacteria use K⁺-uptake transporters differentially for adaptation in varying growth conditions. In Mycobacterium tuberculosis, two K⁺-uptake systems, the Trk comprising the CeoB and CeoC proteins and the Kdp consisting of the two-component system (TCS), KdpDE and KdpFABC, have been characterized, but their selective utilization during bacterial growth has not been completely explored. In the current study, the roles of the M. tuberculosis KdpDE regulatory system alone and in association with the Trk transporters in bacterial growth were investigated by evaluating the growth of M. tuberculosis KdpDE-deletion and KdpDE/Trk (KT)-double knockout mutant strains in planktonic culture under standard growth conditions. The KT-double knockout mutant strain was first constructed using homologous recombination procedures and was evaluated together with the KdpDE-deletion mutant and the wild-type (WT) strains with respect to their rates of growth, K⁺-uptake efficiencies, and K⁺-transporter gene expression during planktonic growth. During growth at optimal K⁺ concentrations and pH levels, selective deletion of the TCS KdpDE (KdpDE-deletion mutant) led to attenuation of bacterial growth and an increase in bacterial K⁺-uptake efficiency, as well as dysregulated expression of the kdpFABC and trk genes. Deletion of both the KdpDE and the Trk systems (KT-double knockout) also led to severely attenuated bacterial growth, as well as an increase in bacterial K⁺-uptake efficiency. These results demonstrate that the KdpDE regulatory system plays a key role during bacterial growth by regulating K⁺ uptake via modulation of the expression and activities of both the KdpFABC and Trk systems and is important for bacterial growth possibly by preventing cytoplasmic K⁺ overload.

Applications of Transcriptomics and Proteomics for Understanding Dormancy and Resuscitation in Mycobacterium tuberculosis

Mycobacterium tuberculosis can survive within its host for extended periods of time without any clinical symptoms of disease and reactivate when the immune system is weakened. A detailed understanding of how M. tuberculosis enters into and exits out of dormancy, is necessary in order to develop new strategies for tackling tuberculosis. Omics methodologies are unsupervised and unbiased to any hypothesis, making them useful tools for the discovery of new drug targets. This review summarizes the findings of transcriptomic and proteomic approaches toward understanding dormancy and reactivation of M. tuberculosis. Within the granuloma of latently infected individuals, the bacteria are dormant, with a marked slowdown of growth, division and metabolism. In vitro models have attempted to simulate these features by subjecting the bacterium to hypoxia, nutrient starvation, potassium depletion, growth in the presence of vitamin C, or growth in the presence of long-chain fatty acids. The striking feature of all the models is the upregulation of the DosR regulon, which includes the transcriptional regulator Rv0081, one of the central hubs of dormancy. Also upregulated are chaperone proteins, fatty acid and cholesterol degrading enzymes, the sigma factors SigE and SigB, enzymes of the glyoxylate and the methylcitrate cycle, the Clp proteases and the transcriptional regulator ClgR. Further, there is increased expression of genes involved in mycobactin synthesis, fatty acid degradation, the glyoxylate shunt and gluconeogenesis, in granulomas formed in vitro from peripheral blood mononuclear cells from latently infected individuals compared to naïve individuals. Genes linked to aerobic respiration, replication, transcription, translation and cell division, are downregulated during dormancy in vitro, but upregulated during reactivation. Resuscitation in vitro is associated with upregulation of genes linked to the synthesis of mycolic acids, phthiocerol mycocerosate (PDIM) and sulfolipids; ribosome biosynthesis, replication, transcription and translation, cell division, and genes encoding the five resuscitation promoting factors (Rpfs). The expression of proteases, transposases and insertion sequences, suggests genome reorganization during reactivation.

Resilience of methane cycle and microbial functional genes to drought and flood in an alkaline wetland: A metagenomic analysis

Alkaline wetlands distributed in arid or semi-arid areas are hotspots of methane (CH₄) emissions. Periods of drought and flood, although regular, are stressful events encountered by methanogenic anaerobes in alkaline wetlands. To investigate the response of the CH₄ cycle of alkaline wetlands to such stresses, we take Zhalong wetland as an example, then determined the CH₄ flux and soil microbiomes in the wetland during wet, dry, and flooded periods. The in-situ CH₄ flux in the wet period was 9.55-17.29 mg‧m^-2‧h^-1, but sharply degraded to 3.37-6.61 mg‧m^-2‧h^-1 in the dry period. It resumed to 4.51-20.80 mg‧m^-2‧h^-1 when the wetland was flooded again, which indicated that methanogenesis is quite resilient to drought. Syntrophic acetogenesis, and subsequently aceticlastic methanogenesis, were the dominant methanogenic pathways and resisted drought. Members belonging to Syntrophobacterales were the dominant syntrophic acetogens. They enter a viable but non-culturable (VBNC) state to resist drought. The dominant Methanosarcinales have the ability to repair reactive oxygen species damage during dry periods. The community of CH₄ sink was governed by anaerobic methanotrophs, which entered a VBNC state or used repair systems to survive dry periods. This study revealed the responses of the CH₄ cycle and microbial functional genes to drought and flood in alkaline wetlands.

Biochemical changes in tuberculosis

Introduction

tuberculosis (TB) remains a global health issue with high morbidity and mortality rates especially in the developing countries. It is a multi-organ disease and can influence biochemical changes. This study sought to determine the influence of tuberculosis and its drug treatment on serum biochemical parameters in patients in Nigeria.

Methods

it was a descriptive observational cohort study on 150 subjects whose blood samples were analyzed for serum albumin, serum sodium, and serum potassium. The subjects were grouped into 3: TB group= 50 new TB subjects not on treatment, F group= 50 TB subjects on treatment for 2/12 or more and C group= 50 non-TB control subjects. These biochemical variables were compared between the 3 groups.

Results

male/female ratio was 1: 1.5, mean age 37.1±0.92 years, and range 18-65 years. The differences in mean values of serum albumin, calcium and sodium between the three groups were significant (p<0.001), whereas that of serum potassium was not significant (p=0.056). Those patients with new case TB had a significantly lower serum sodium, serum albumin and serum calcium than the control group and those on treatment, p<0.001. There was significant positive correlation between serum albumin and serum calcium (r=0.0.420, p<0.001) as well as serum sodium (r=0.310, p<0.001) in the study population. Similarly, the correlation between serum calcium and serum sodium was positive and significant (r=0.200, p=0.014). In contrast, the correlation between serum potassium and serum albumin and that between serum potassium and serum calcium was not significant.

Conclusion

tuberculosis with or without anti-tuberculous medications was associated with significant reduction in serum albumin, serum sodium and serum calcium in this study.

Distinct functional properties of secretory l-asparaginase Rv1538c involved in phagosomal survival of Mycobacterium tuberculosis

The emergence of drug-resistant Mycobacterium tuberculosis (Mtb) stains has escalated the need for developing more efficient drugs and therapeutic strategies against tuberculosis. Here we functionally annotate a secretory mycobacterial asparaginase Rv1538c (MtA) and describe its biochemical properties. MtA primarily existed as dimer along with a minor population of multimers. Circular dichroism and fluorescence spectroscopy demonstrated a compact structure in Tris HCl buffer at pH 8.0. Under these conditions it also displayed optimum activity. It retained ∼40% activity at pH 5.5, supporting its physiological relevance in acidic phagosomal environment. MtA contravened classical Michaelis-Menten kinetics and exhibited product inhibition profile, yielding a K_cat of 869.4 s^-1 and an apparent K_m of 8.36 mM. We report the presence of several antigenic epitopes and a C-terminal YXXXD/E motif in MtA, hinting towards its potential to interact or influence host immune system. This was supported by our observation of morphological changes in MtA-treated human B lymphoblasts. We propose that MtA is a dual purpose enzyme used by Mtb to survive inside its host by; 1) ammonia-mediated neutralization of the phagosomal acidic pH and 2) inducing stress to primary immune cells and compromising the host immune response. Overall, this study contributes to our understanding of the biological role of mycobacterial asparaginase opening avenues for developing effective TB therapeutics.

Carprofen elicits pleiotropic mechanisms of bactericidal action with the potential to reverse antimicrobial drug resistance in tuberculosis

BACKGROUND

The rise of antimicrobial drug resistance in Mycobacterium tuberculosis coupled with the shortage of new antibiotics has elevated TB to a major global health priority. Repurposing drugs developed or used for other conditions has gained special attention in the current scenario of accelerated drug development for several global infectious diseases. In a similar effort, previous studies revealed that carprofen, a non-steroidal anti-inflammatory drug, selectively inhibited the growth of replicating, non-replicating and MDR clinical isolates of M. tuberculosis.

OBJECTIVES

We aimed to reveal the whole-cell phenotypic and transcriptomic effects of carprofen in mycobacteria.

METHODS

Integrative molecular and microbiological approaches such as resazurin microtitre plate assay, high-throughput spot-culture growth inhibition assay, whole-cell efflux inhibition, biofilm inhibition and microarray analyses were performed. Analogues of carprofen were also synthesized and assessed for their antimycobacterial activity.

RESULTS

Carprofen was found to be a bactericidal drug that inhibited mycobacterial drug efflux mechanisms. It also restricted mycobacterial biofilm growth. Transcriptome profiling revealed that carprofen likely acts by targeting respiration through the disruption of membrane potential. The pleiotropic nature of carprofen's anti-TB action may explain why spontaneous drug-resistant mutants could not be isolated in practice.

CONCLUSIONS

This immunomodulatory drug and its chemical analogues have the potential to reverse TB antimicrobial drug resistance, offering a swift path to clinical trials of novel TB drug combinations.

Multi-Omics Technologies Applied to Tuberculosis Drug Discovery

Multi-omics strategies are indispensable tools in the search for new anti-tuberculosis drugs. Omics methodologies, where the ensemble of a class of biological molecules are measured and evaluated together, enable drug discovery programs to answer two fundamental questions. Firstly, in a discovery biology approach, to find new targets in druggable pathways for target-based investigation, advancing from target to lead compound. Secondly, in a discovery chemistry approach, to identify the mode of action of lead compounds derived from high-throughput screens, progressing from compound to target. The advantage of multi-omics methodologies in both of these settings is that omics approaches are unsupervised and unbiased to a priori hypotheses, making omics useful tools to confirm drug action, reveal new insights into compound activity, and discover new avenues for inquiry. This review summarizes the application of Mycobacterium tuberculosis omics technologies to the early stages of tuberculosis antimicrobial drug discovery.

Advanced Quantification Methods To Improve the 18b Dormancy Model for Assessing the Activity of Tuberculosis Drugs In Vitro

One of the reasons for the lengthy tuberculosis (TB) treatment is the difficulty to treat the nonmultiplying mycobacterial subpopulation. In order to assess the ability of (new) TB drugs to target this subpopulation, we need to incorporate dormancy models in our preclinical drug development pipeline. In most available dormancy models, it takes a long time to create a dormant state, and it is difficult to identify and quantify this nonmultiplying condition. The Mycobacterium tuberculosis 18b strain might overcome some of these problems, because it is dependent on streptomycin for growth and becomes nonmultiplying after 10 days of streptomycin starvation but still can be cultured on streptomycin-supplemented culture plates. We developed our 18b dormancy time-kill kinetics model to assess the difference in the activity of isoniazid, rifampin, moxifloxacin, and bedaquiline against log-phase growth compared to the nonmultiplying M. tuberculosis subpopulation by CFU counting, including a novel area under the curve (AUC)-based approach as well as time-to-positivity (TTP) measurements. We observed that isoniazid and moxifloxacin were relatively more potent against replicating bacteria, while rifampin and high-dose bedaquiline were equally effective against both subpopulations. Moreover, the TTP data suggest that including a liquid culture-based method could be of additional value, as it identifies a specific mycobacterial subpopulation that is nonculturable on solid media. In conclusion, the results of our study underline that the time-kill kinetics 18b dormancy model in its current form is a useful tool to assess TB drug potency and thus has its place in the TB drug development pipeline.

Differential determinants of virulence in two Mycobacterium tuberculosis Colombian clinical isolates of the LAM09 family

The heterogeneity of the clinical outcome of Mycobacterium tuberculosis (Mtb) infection may be due in part to different strategies used by circulating strains to cause disease. This heterogeneity is one of the main limitations to eradicate tuberculosis disease. In this study, we have compared the transcriptional response of two closely related Colombian clinical isolates (UT127 and UT205) of the LAM family under two axenic media conditions. These clinical isolates are phenotypically different at the level of cell death, cytokine production, growth kinetics upon in vitro infection of human tissue macrophages, and membrane vesicle secretion upon culture in synthetic medium. Using RNA-seq, we have identified different pathways that account for two different strategies to cope with the stressful condition of a carbon-poor media such as Sauton’s. We showed that the clinical isolate UT205 focus mainly in the activation of virulence systems such as the ESX-1, synthesis of diacyl-trehalose, polyacyl-trehalose, and sulfolipids, while UT127 concentrates its efforts mainly in the survival mode by the activation of the DNA replication, cell division, and lipid biosynthesis. This is an example of two Mtb isolates that belong to the same family and lineage, and even though they have a very similar genome, its transcriptional regulation showed important differences. This results in summary highlight the necessity to reach a better understanding of the heterogeneity in the behavior of these circulating Mtb strains which may help us to design better treatments and vaccines and to identify new targets for drugs.

Photoinactivation of dormant Mycobacterium smegmatis due to its endogenous porphyrins

Mycobacterium tuberculosis is able to transition into a dormant state, causing a latent state of tuberculosis. Dormant mycobacteria acquire phenotypic resistance to all known antibacterial drugs; they are also able to maintain vitality in the host for decades and become active, causing the active form of the disease. In order to cure latent tuberculosis, new approaches should be developed. Earlier, we discovered accumulation in significant concentrations of porphyrins in dormant Mycobacterium smegmatis, which is a close, fast-growing relative of the causative agent of tuberculosis. In this study, we explore a new possibility to kill dormant mycobacteria by photodynamic inactivation (PDI) using accumulated porphyrins as endogenous photosensitisers. The dormant M. smegmatis were obtained under gradual acidification in Sauton's medium, for 14 days. Cells were exposed to light with different wavelengths emitted by three Spectra X light-emitting diodes (395/25, 470/24, 575/25 nm) and one separated 634-nm LED for 15 min. An increase in the concentration of coproporphyrin in M. smegmatis after 6 days of growth correlated with the beginning of a decrease in metabolic activity and formation of ovoid dormant forms. Dormant bacteria were sensitive to PDI and killed after 15-30 min of illumination, in contrast to active cells. The greatest inactivation of dormant mycobacteria occurred at 395 and 575 nm, which coincides with the main maximum of the absorption spectrum of extracted porphyrins. We, for the first time, demonstrate a successful application of PDI for inactivation of dormant mycobacteria, due to significant accumulation of endogenous photosensitisers-porphyrins.

Hypoxic Non-replicating Persistent Mycobacterium tuberculosis Develops Thickened Outer Layer That Helps in Restricting Rifampicin Entry

Bacteria undergo adaptive morphological changes to survive under stress conditions. The present work documents the morphological changes in Mycobacterium tuberculosis (Mtb) cells cultured under hypoxic condition using Wayne’s in vitro hypoxia model involving non-replicating persistence stages 1 and 2 (NRP stage 1 and NRP stage 2) and reveals their physiological significance. Transmission electron microscopy of the NRP stage 2 cells showed uneven but thick outer layer (TOL), unlike the evenly thin outer layer of the actively growing mid-log phase (MLP) cells. On the contrary, the saprophytic Mycobacterium smegmatis NRP stage 2 cells lacked TOL. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) of the Mtb NRP stage 2 cells confirmed the rough uneven surface unlike the smooth surface of the MLP cells. Zeta potential measurements showed high negative charge on the surface of NRP stage 2 cells and polysaccharide specific calcofluor white (CFW) staining of the cells revealed high content of polysaccharide in the TOL. This observation was supported by the real-time PCR data showing high levels of expression of the genes involved in the synthesis of sugars, such as trehalose, mannose and others, which are implicated in polysaccharide synthesis. Experiments to understand the physiological significance of the TOL revealed restricted entry of the biologically low-active 5-carboxyfluorescein-rifampicin (5-FAM-RIF), at concentrations equivalent to microbicidal concentrations of the unconjugated biologically active rifampicin, into the NRP stage 2 cells, unlike in the MLP cells. Further, as expected, mechanical removal of the TOL by mild bead beating or release of the NRP stage 2 cells from hypoxia into normoxia in fresh growth medium also significantly increased 5-FAM-RIF permeability into the NRP stage 2 cells to an extent comparable to that into the MLP cells. Taken together, these observations revealed that Mtb cells under hypoxia develop TOL that helps in restricting rifampicin entry, thereby conferring rifampicin tolerance.

Protein kinase B controls Mycobacterium tuberculosis growth via phosphorylation of the transcriptional regulator Lsr2 at threonine 112

Mycobacterium tuberculosis (Mtb) is able to persist in the body through months of multi‐drug therapy. Mycobacteria possess a wide range of regulatory proteins, including the protein kinase B (PknB) which controls peptidoglycan biosynthesis during growth. Here, we observed that depletion of PknB resulted in specific transcriptional changes that are likely caused by reduced phosphorylation of the H‐NS‐like regulator Lsr2 at threonine 112. The activity of PknB towards this phosphosite was confirmed with purified proteins, and this site was required for adaptation of Mtb to hypoxic conditions, and growth on solid media. Like H‐NS, Lsr2 binds DNA in sequence‐dependent and non‐specific modes. PknB phosphorylation of Lsr2 reduced DNA binding, measured by fluorescence anisotropy and electrophoretic mobility shift assays, and our NMR structure of phosphomimetic T112D Lsr2 suggests that this may be due to increased dynamics of the DNA‐binding domain. Conversely, the phosphoablative T112A Lsr2 had increased binding to certain DNA sites in ChIP‐sequencing, and Mtb containing this variant showed transcriptional changes that correspond with the change in DNA binding. In summary, PknB controls Mtb growth and adaptations to the changing host environment by phosphorylating the global transcriptional regulator Lsr2.

Mycobacterium tuberculosis sensor kinase DosS modulates the autophagosome in a DosR-independent manner

Dormancy is a key characteristic of the intracellular life-cycle of Mtb. The importance of sensor kinase DosS in mycobacteria are attributed in part to our current findings that DosS is required for both persistence and full virulence of Mtb. Here we show that DosS is also required for optimal replication in macrophages and involved in the suppression of TNF-α and autophagy pathways. Silencing of these pathways during the infection process restored full virulence in MtbΔdosS mutant. Notably, a mutant of the response regulator DosR did not exhibit the attenuation in macrophages, suggesting that DosS can function independently of DosR. We identified four DosS targets in Mtb genome; Rv0440, Rv2859c, Rv0994, and Rv0260c. These genes encode functions related to hypoxia adaptation, which are not directly controlled by DosR, e.g., protein recycling and chaperoning, biosynthesis of molybdenum cofactor and nitrogen metabolism. Our results strongly suggest a DosR-independent role for DosS in Mtb.

Application of model systems to study adaptive responses of Mycobacterium tuberculosis during infection and disease

Tuberculosis (TB) claims more human lives than any other infectious organism. The lethal synergy between TB-HIV infection and the rapid emergence of drug resistant strains has created a global public health threat that requires urgent attention. Mycobacterium tuberculosis, the causative agent of TB is an exquisitely well-adapted human pathogen, displaying the ability to promptly remodel metabolism when encountering stressful environments during pathogenesis. A careful study of the mechanisms that enable this adaptation will enhance the understanding of key aspects related to the microbiology of TB disease. However, these efforts require microbiological model systems that mimic host conditions in the laboratory. Herein, we describe several in vitro model systems that generate non-replicating and differentially culturable mycobacteria. The changes that occur in the metabolism of M. tuberculosis in some of these models and how these relate to those reported for human TB disease are discussed. We describe mechanisms that tubercle bacteria use to resuscitate from these non-replicating conditions, together with phenotypic heterogeneity in terms of culturabiliy of M. tuberculosis in sputum. Transcriptional changes in M. tuberculosis that allow for adaptation of the organism to the lung environment are also summarized. Finally, given the emerging importance of the microbiome in various infectious diseases, we provide a description of how the lung and gut microbiome affect susceptibility to TB infection and response to treatment. Consideration of these collective aspects will enhance the understanding of basic metabolism, physiology, drug tolerance and persistence in M. tuberculosis to enable development of new therapeutic interventions.

Using diphenyleneiodonium to induce a viable but non-culturable phenotype in Mycobacterium tuberculosis and its metabolomics analysis

Depletion of oxygen levels is a well-accepted model for induction of non-replicating, persistent states in mycobacteria. Increasing the stress levels in mycobacterium bacilli facilitates their entry into a non-cultivable, dormant state. In this study, it was shown that diphenyleneiodonium, an inhibitor of NADH oxidase, induced a viable, but non-culturable state in mycobacteria, having similar features to dormant bacilli, like loss of acid-fastness, upregulation of stress-regulated genes and decreased superoxide levels as compared to actively growing bacilli. Comprehensive, untargeted metabolic profiling also confirmed a decrease in biogenesis of amino acids, NAD, unsaturated fatty acids and nucleotides. Additionally, an increase in the level of lactate, fumarate, succinate and pentose phosphate pathways along with increased mycothiol and sulfate metabolites, similar to dormant bacilli, was observed in the granuloma. These non-cultivable bacilli were resuscitated by supplementation of fetal bovine serum, regaining their culturability in liquid as well as on agar medium. This study focused on the effect of diphenyleneiodonium treatment in causing mycobacteria to rapidly transition from an active state into a viable, but non-cultivable state, and comparing their characteristics with dormant phenotypes.

Resuscitation of Dormant "Non-culturable" Mycobacterium tuberculosis Is Characterized by Immediate Transcriptional Burst

Under unfavorable conditions such as host immune responses and environmental stresses, human pathogen Mycobacterium tuberculosis may acquire the dormancy phenotype characterized by "non-culturability" and a substantial decrease of metabolic activity and global transcription rates. Here, we found that the transition of M. tuberculosis from the dormant "non-culturable" (NC) cells to fully replicating population in vitro occurred not earlier than 7 days after the start of the resuscitation process, with predominant resuscitation over this time interval evidenced by shortening apparent generation time up to 2.8 h at the beginning of resuscitation. The early resuscitation phase was characterized by constant, albeit low, incorporation of radioactive uracil, indicating de novo transcription immediately after the removal of the stress factor, which resulted in significant changes of the M. tuberculosis transcriptional profile already after the first 24 h of resuscitation. This early response included transcriptional upregulation of genes encoding enzymes of fatty acid synthase system type I (FASI) and type II (FASII) responsible for fatty acid/mycolic acid biosynthesis, and regulatory genes, including whiB6 encoding a redox-sensing transcription factor. The second resuscitation phase took place 4 days after the resuscitation onset, i.e., still before the start of active cell division, and included activation of central metabolism genes encoding NADH dehydrogenases, ATP-synthases, and ribosomal proteins. Our results demonstrate, for the first time, that the resuscitation of dormant NC M. tuberculosis is characterized by immediate activation of de novo transcription followed by the upregulation of genes controlling key metabolic pathways and then, cell multiplication.

Evidence of nitrite acting as a stable and robust inducer of non-cultivability in Mycobacterium tuberculosis with physiological relevance

Tuberculosis (TB) is the ninth leading cause of death worldwide, ranking above human immunodeficiency virus. Latency is the major obstacle in the eradication of this disease. How the physiology of the pathogen changes in transition to the latent stage needs to be understood. The latent bacteria extracted from animal hosts exist in a nonculturable (NC) phase, whereas bacteria extracted from most in vitro models are culture-positive. In the present study, we observed that nitrite, up to a concentration of 5 mM, shows the growth of Mycobacterium tuberculosis (MTB) in liquid media, but this effect starts reversing at higher concentrations. At a concentration of 10 mM, nitrite induces rapid nonculturability of MTB at the aerobic stage. This noncultivable dormancy was confirmed by analyzing the characteristics of NC bacteria. Further differential gene expression analyses clearly supported the formation of a dormancy phenotype. This study will be helpful for the use of this bacillus as a dormancy model in future studies on TB latency.

Exploiting chitosan and gold nanoparticles for antimycobacterial activity of in silico identified antimicrobial motif of human neutrophil peptide-1

The upsurge of drug resistant tuberculosis is major health threat globally. To counteract, antimicrobial peptides are being explored as possible alternatives. However, certain limitations of peptide-based drugs such as potential toxicity, high cost and relatively low stability need to be addressed to enhance their clinical applicability. Use of computer predicted short active motifs of AMPs along with nanotechnology could not only overcome the limitations of AMPs but also potentiate their antimicrobial activity. Therefore, present study was proposed to in silico identify short antimicrobial motif (Pep-H) of human neutrophil peptide-1 (HNP-1) and explore its antimycobacterial activity in free form and using nanoparticles-based delivery systems. Based on colony forming unit analysis, motif Pep-H led to killing of more than 90% M. tb in vitro at 10 μg/ml, whereas, similar activity against intracellularly growing M. tb was observed at 5 μg/ml only. Thereafter, chitosan (244 nm) and gold nanoparticles (20 nm) were prepared for Pep-H with both the formulations showing minimal effects on the viability of human monocyte derived macrophages (MDMs) and RBC integrity. The antimycobacterial activity of Pep-H against intracellular mycobacteria was enhanced in both the nanoformulations as evident by significant reduction in CFU (>90%) at 5-10 times lower concentrations than that observed for free Pep-H. Thus, Pep-H is an effective antimycobacterial motif of HNP-1 and its activity is further enhanced by chitosan and gold nanoformulations.

Effect of various drugs on differentially detectable persisters of Mycobacterium tuberculosis generated by long-term lipid diet

Persisters of Mycobacterium tuberculosis (Mtb) that fail to form colonies on agar media when de-stressed are termed as differentially detectable (DD) persisters. Since in the host, Mtb primarily survives by utilizing lipids, we used a long-term lipid diet model to induce DD persisters of M. tuberculosis. Persisters were induced by replacing the dextrose-containing medium with one containing fatty acids instead of dextrose (FAM). After 2, 4 or 6 weeks, CFU and most probable number assays were performed; the difference between the two gave an estimate of DD persisters. Since rifampicin has been shown to induce formation of DD persisters in vitro, one set of FAM cultures were also given short-term rifampicin stress after 2, 4 or 6 weeks. Fraction of DD persisters increased with time and rifampicin treatment enhanced the effect of fatty acids, at 2 and 4 weeks. At six weeks, even in the absence of rifampicin, ∼95% population were DD persisters. The DD persisters were vulnerable to drugs interfering with bacterial respiration such as thioridazine, bedaquiline and clofazimine. The study indicates potential formation of DD persisters of Mtb in a lipid-rich microenvironment in the host even before antibiotic therapy.

Metatranscriptomic and comparative genomic insights into resuscitation mechanisms during enrichment culturing

BACKGROUND

The pure culture of prokaryotes remains essential to elucidating the role of these organisms. Scientists have reasoned that hard to cultivate microorganisms might grow in pure culture if provided with the chemical components of their natural environment. However, most microbial species in the biosphere that would otherwise be "culturable" may fail to grow because of their growth state in nature, such as dormancy. That means even if scientist would provide microorganisms with the natural environment, such dormant microorganisms probably still remain in a dormant state.

RESULTS

We constructed an enrichment culture system for high-efficiency isolation of uncultured strains from marine sediment. Degree of enrichment analysis, dormant and active taxa calculation, viable but non-culturable bacteria resuscitation analysis, combined with metatranscriptomic and comparative genomic analyses of the interactions between microbial communications during enrichment culture showed that the so-called enrichment method could culture the "uncultured" not only through enriching the abundance of "uncultured," but also through the resuscitation mechanism. In addition, the enrichment culture was a complicated mixed culture system, which contains the competition, cooperation, or coordination among bacterial communities, compared with pure cultures.

CONCLUSIONS

Considering that cultivation techniques must evolve further-from axenic to mixed cultures-for us to fully understand the microbial world, we should redevelop an understanding of the classic enrichment culture method. Enrichment culture methods can be developed and used to construct a model for analyzing mixed cultures and exploring microbial dark matter. This study provides a new train of thought to mining marine microbial dark matter based on mixed cultures.

Intracellular toxicity exerted by PCBs and role of VBNC bacterial strains in biodegradation

Polychlorinated biphenyls (PCBs) are xenobiotic compounds that persists in the environment for long-term, though its productivity is banned. Abatement of the pollutants have become laborious due to it's recalcitrant nature in the environment leading to toxic effects in humans and other living beings. Biphenyl degrading bacteria co-metabolically degrade low chlorinated PCBs using the active metabolic pathway. bph operon possess different genetic arrangements in gram positive and gram negative bacteria. The binding ability of the genes and the active sites were determined by PCB docking studies. The active site of bphA gene with conserved amino acid residues determines the substrate specificity and biodegradability. Accumulation of toxic intermediates alters cellular behaviour, biomass production and downturn the metabolic activity. Several bacteria in the environment attain unculturable state which is viable and metabolically active but not cultivable (VBNC). Resuscitation-promoting factor (Rpf) and Rpf homologous protein retrieve the culturability of the so far uncultured bacteria. Recovery of this adaptive mechanism against various physical and chemical stressors make a headway in understanding the functionality of both environmental and medically important unculturable bacteria. Thus, this paper review about the general aspects of PCBs, cellular toxicity exerted by PCBs, role of unculturable bacterial strains in biodegradation, genes involved and degradation pathways. It is suggested to extrapolate the research findings on extracellular organic matters produced in culture supernatant of VBNC thus transforming VBNC to culturable state.

Fighting tuberculosis by drugs targeting nonreplicating Mycobacterium tuberculosis bacilli

Current tuberculosis (TB) treatment requires 6 months of combination therapy with isoniazid (INH), rifampin (RIF), pyrazinamide (PZA), and ethambutol for active TB and 9 months of INH or 3 months of rifapentine (RFP) + INH for latent TB. The lungs of patients with active and latent TB contain heterogeneous mixtures of cellular and caseous granulomas harboring Mycobacterium tuberculosis bacilli ranging from actively replicating (AR) to nonreplicating (NR), phenotypically drug-resistant stages. Several in vitro models to obtain NR cells were reported, including exposure to hypoxia, nutrient starvation, acid + nitric oxide, and stationary phase. Overall, these models showed that RIF, RFP, PA-824 (PA), metronidazole (MZ), bedaquiline (BQ), and fluoroquinolones were the most active drugs against NR M. tuberculosis. In hypoxia at pH 5.8, some combinations killed AR plus NR cells, as shown by lack of regrowth in liquid media, whereas in hypoxia at pH 7.3 (the pH of the caseum), only RIF and RFP efficiently killed NR bacilli while several other drugs showed little effect. In conventional mouse models, combinations containing RFP, BQ, PA, PZA, moxifloxacin, sutezolid, linezolid, and clofazimine sterilized animals in ≤2 months, as shown by lack of viable bacilli in lung homogenates after 3 months without therapy. Drugs were less effective in C3HeB/FeJ mice forming caseous granulomas. Overall, in vitro observations and in vivo studies suggest that the search for new TB drugs could be addressed to low lipophilic molecules (e.g., new rpoB inhibitors with clogP < 3) killing NR M. tuberculosis in hypoxia at neutral pH and reaching high rates of unbound drug in the caseum.

A Novel TetR-Like Transcriptional Regulator Is Induced in Acid-Nitrosative Stress and Controls Expression of an Efflux Pump in Mycobacteria

Mycobacterium tuberculosis has the ability to survive inside macrophages under acid-nitrosative stress. M. tuberculosis Rv1685c and its ortholog in M. smegmatis, MSMEG_3765, are induced on exposure to acid-nitrosative stress. Both genes are annotated as TetR transcriptional regulators, a family of proteins that regulate a wide range of cellular activities, including multidrug resistance, carbon catabolism and virulence. Here, we demonstrate that MSMEG_3765 is co-transcribed with the upstream genes MSMEG_3762 and MSMEG_3763, encoding efflux pump components. RTq-PCR and GFP-reporter assays showed that the MSMEG_3762/63/65 gene cluster, and the orthologous region in M. tuberculosis (Rv1687c/86c/85c), was up-regulated in a MSMEG_3765 null mutant, suggesting that MSMEG_3765 acts as a repressor, typical of this family of regulators. We further defined the MSMEG_3765 regulon using genome-wide transcriptional profiling and used reporter assays to confirm that the MSMEG_3762/63/65 promoter was induced under acid-nitrosative stress. A putative 36 bp regulatory motif was identified upstream of the gene clusters in both M. smegmatis and M. tuberculosis and purified recombinant MSMEG_3765 protein was found to bind to DNA fragments containing this motif from both M. smegmatis and M. tuberculosis upstream regulatory regions. These results suggest that the TetR repressor MSMEG_3765/Rv1685c controls expression of an efflux pump with an, as yet, undefined role in the mycobacterial response to acid-nitrosative stress.

Activity of human beta defensin-1 and its motif against active and dormant Mycobacterium tuberculosis

The ineffectiveness of anti-tuberculous therapy against dormant and drug-resistant mycobacteria demands scrutiny of alternative candidates like antimicrobial peptides having different mechanisms of action. The present study was designed to explore the activity of human beta defensin-1 (HBD-1) and its in silico identified short motif Pep-B against active and dormant Mycobacterium tuberculosis (M. tb) H37Rv. Activity of HBD-1 and Pep-B was determined against actively growing M. tb in vitro, inside monocyte-derived macrophages (MDMs) and dormant bacilli in in vitro potassium deficiency and human peripheral blood mononuclear cell (PBMC) granuloma models using colony-forming unit enumeration. The minimum inhibitory concentrations (MIC) of HBD-1 and Pep-B were found to be 2 and 20 μg/ml, respectively. These peptides also inhibited intracellular mycobacterial growth at concentrations lower than in vitro MICs along with increased IFN-γ levels. Although at higher concentration, HBD-1 (× 2 MIC) and Pep-B (× 2 MIC) led to decrease in in vitro dormant mycobacterial load as compared to rifampicin (× 25 MIC) and isoniazid (× 16 MIC). Similarly, both peptides showed higher killing efficacy against dormant mycobacteria inside granuloma as compared to rifampicin. Thus, the present study indicates that HBD-1 and its motif are effective antimicrobial players against both actively growing and dormant mycobacteria.

Rifamycin action on RNA polymerase in antibiotic-tolerant Mycobacterium tuberculosis results in differentially detectable populations

Mycobacterium tuberculosis (Mtb) encounters stresses during the pathogenesis and treatment of tuberculosis (TB) that can suppress replication of the bacteria and render them phenotypically tolerant to most available drugs. Where studied, the majority of Mtb in the sputum of most untreated subjects with active TB have been found to be nonreplicating by the criterion that they do not grow as colony-forming units (cfus) when plated on agar. However, these cells are viable because they grow when diluted in liquid media. A method for generating such "differentially detectable" (DD) Mtb in vitro would aid studies of the biology and drug susceptibility of this population, but lack of independent confirmation of reported methods has contributed to skepticism about their existence. Here, we identified confounding artifacts that, when avoided, allowed development of a reliable method of producing cultures of ≥90% DD Mtb in starved cells. We then characterized several drugs according to whether they contribute to the generation of DD Mtb or kill them. Of the agents tested, rifamycins led to DD Mtb generation, an effect lacking in a rifampin-resistant strain with a mutation in rpoB, which encodes the canonical rifampin target, the β subunit of RNA polymerase. In contrast, thioridazine did not generate DD Mtb from starved cells but killed those generated by rifampin.

An attenuated Mycobacterium tuberculosis clinical strain with a defect in ESX-1 secretion induces minimal host immune responses and pathology

Although Mycobacterium tuberculosis (M.tb) DK9897 is an attenuated strain, it was isolated from a patient with extrapulmonary tuberculosis and vaccination with a subunit vaccine (H56) induced poor protection against it. Both attenuation and lack of protection are because M.tb DK9897 cannot secrete the EsxA virulence factor nor induce a host response against it. Genome sequencing identified a frameshift mutation in the eccCa1 gene. Since the encoded EccCa1 protein provides energy for ESX-1 secretion, it suggested a defect in the ESX-1 type VII secretion system. Genetic complementation with a plasmid carrying the M.tb H37Rv sequence of eccCa1-eccCb1-pe35 re-established EsxA secretion, host specific EsxA T-cell responses, and increased strain virulence. The ESX-1 secretion defect prevents several virulence factors from being functional during infection and therefore attenuates M.tb. It precludes specific T-cell responses against strong antigens and we found very little in vivo cytokine production, gross pathology or granuloma formation in lungs from M.tb DK9897 infected animals. This coincides with M.tb DK9897 being unable to disrupt the phagosome membrane and make contact to the cytosol.

Regulation of Inducible Potassium Transporter KdpFABC by the KdpD/KdpE Two-Component System in Mycobacterium smegmatis

Kdp-ATPase is an inducible high affinity potassium uptake system that is widely distributed in bacteria, and is generally regulated by the KdpD/KdpE two-component system (TCS). In this study, conducted on Mycobacterium smegmatis, the kdpFABC (encoding Kdp-ATPase) expression was found to be affected by low concentration of K⁺, high concentrations of Na⁺, and/or [Formula: see text] of the medium. The KdpE was found to be a transcriptional regulator that bound to a specific 22-bp sequence in the promoter region of kdpFABC operon to positively regulate kdpFABC expression. The KdpE binding motif was highly conserved in the promoters of kdpFABC among the mycobacterial species. 5'-RACE data indicated a transcriptional start site (TSS) of the kdpFABC operon within the coding sequence of MSMEG_5391, which comprised a 120-bp long 5'-UTR and an open reading frame of the 87-bp kdpF gene. The kdpE deletion resulted in altered growth rate under normal and low K⁺ conditions. Furthermore, under K⁺ limiting conditions, a single transcript (kdpFABCDE) spanning kdpFABC and kdpDE operons was observed. This study provided the first insight into the regulation of kdpFABC operon by the KdpD/KdpE TCS in M. smegmatis.