Iron, mycobacteria and tuberculosis

Liposome-siderophore conjugates loaded with moxifloxacin serve as a model for drug delivery against Mycobacterium tuberculosis

Tuberculosis (TB) is an infectious disease that annually affects millions of people, and resistance to available antibiotics has exacerbated this situation. Another notable characteristic of Mycobacterium tuberculosis, the primary causative agent of TB, is its ability to survive inside macrophages, a key component of the immune system. In our quest for an effective and safe treatment that facilitates the targeted delivery of antibiotics to the site of infection, we have proposed a nanotechnology approach based on an iron chelator. Iron chelators are the primary mechanism by which bacteria acquire iron, a metal essential for their metabolism. Four liposomes were synthesized and characterized using the dynamic light scattering technique (DLS), nanoparticle tracking analysis (NTA), and transmission electron microscopy (TEM). All of these methods revealed the presence of spherical particles, approximately 200 nm in size. NTA indicated a concentration of around 1011 particles/mL. We also developed and validated a high-performance liquid chromatography method for quantifying Moxifloxacin to determine encapsulation efficiency (EE) and release profiles (RF). The EE was 51.31 % for LipMox and 45.76 % for LipIchMox. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed the phagocytosis of liposomal vesicles by macrophages. Functionalizing liposomes with iron chelators can offer significant benefits for TB treatment, such as targeted drug delivery to intracellular bacilli through the phagocytosis of liposomal particles by cells like macrophages.

The role of transcriptional regulators in metal ion homeostasis of Mycobacterium tuberculosis

Metal ions are essential trace elements for all living organisms and play critical catalytic, structural, and allosteric roles in many enzymes and transcription factors. Mycobacterium tuberculosis (MTB), as an intracellular pathogen, is usually found in host macrophages, where the bacterium can survive and replicate. One of the reasons why Tuberculosis (TB) is so difficult to eradicate is the continuous adaptation of its pathogen. It is capable of adapting to a wide range of harsh environmental stresses, including metal ion toxicity in the host macrophages. Altering the concentration of metal ions is the common host strategy to limit MTB replication and persistence. This review mainly focuses on transcriptional regulatory proteins in MTB that are involved in the regulation of metal ions such as iron, copper and zinc. The aim is to offer novel insights and strategies for screening targets for TB treatment, as well as for the development and design of new therapeutic interventions.

Efflux pump Rv1258c activates novel functions of the oxidative stress and via the VII secretion system ESX-3-mediated iron metabolic pathway in Mycobacterium tuberculosis

Oxidative stress and iron metabolism are essential for Mycobacterium tuberculosis (M.tb) survival in host cells. The efflux pump Rv1258c belongs to the major facilitator superfamily (MFS) and can actively pump drugs to promote certain drug resistance in M.tb. In this study, we compared H37RvΔRv1258c with wild-type (WT) M.tb H37Rv. The qRT-PCR results suggested that Rv1258c is potentially involved in the iron metabolic pathway by regulating the expression of ESX-3, which is required for iron uptake. Protein-Protein Affinity Predictor (PPA-Pred2) and the artificial intelligence program AlphaFold 2 were used for prediction and showed that Rv1258c has direct interactions with PPE4 and EccD3 but weak interactions with EccB3. This was further determined via protein-protein interaction analysis of the yeast two-hybrid expression system. By comparing mutant H37RvΔRv1258c strains with WT strains, we discovered that the absence of Rv1258c led to elevated intracellular H+ potential and NAD+/NADH ratios in M.tb, thereby resulting in oxidative stress. We hypothesize that the efflux pump Rv1258c not only has the function of regulating drug resistance in M.tb but also has a novel function in activating oxidative stress and regulating ESX-3-associated iron metabolism in M.tb.

Rationally Designed Novel Phenyloxazoline Synthase Inhibitors: Chemical Synthesis and Biological Evaluation to Accelerate the Discovery of New Antimycobacterial Antibiotics

The uncontrolled spread of drug-resistant tuberculosis (DR-TB) clinical cases necessitates the urgent discovery of newer chemotypes with novel mechanisms of action. Here, we report the chemical synthesis of rationally designed novel transition-state analogues (TSAs) by targeting the cyclization (Cy) domain of phenyloxazoline synthase (MbtB), a key enzyme of the conditionally essential siderophore biosynthesis pathway. Following bio-assay-guided evaluation of TSA analogues preferentially in iron-deprived and iron-rich media to understand target preferentiality against a panel of pathogenic and non-pathogenic mycobacteria strains, we identified a hit, i.e., TSA-5. Molecular docking, dynamics, and MMPBSA calculations enabled us to comprehend TSA-5's stable binding at the active site pocket of MbtB_Cy and the results imply that the MbtB_Cy binding pocket has a strong affinity for electron-withdrawing functional groups and contributes to stable polar interactions between enzyme and ligand. Furthermore, enhanced intracellular killing efficacy (8 μg/mL) of TSA-5 against Mycobacterium aurum in infected macrophages is noted in comparison to moderate in vitro antimycobacterial efficacy (64 μg/mL) against M. aurum. TSA-5 also demonstrates whole-cell efflux pump inhibitory activity against Mycobacterium smegmatis. Identification of TSA-5 by focusing on the modular MbtB_Cy domain paves the way for accelerating novel anti-TB antibiotic discoveries.

Immunological effects of the PE/PPE family proteins of Mycobacterium tuberculosis and related vaccines

Tuberculosis (TB) is a chronic infectious disease caused by Mycobacterium tuberculosis (Mtb), and its incidence and mortality are increasing. The BCG vaccine was developed in the early 20th century. As the most widely administered vaccine in the world, approximately 100 million newborns are vaccinated with BCG every year, which has saved tens of millions of lives. However, due to differences in region and race, the average protective rate of BCG in preventing tuberculosis in children is still not high in some areas. Moreover, because the immune memory induced by BCG will weaken with the increase of age, it is slightly inferior in preventing adult tuberculosis, and BCG revaccination cannot reduce the incidence of tuberculosis again. Research on the mechanism of Mtb and the development of new vaccines against TB are the main strategies for preventing and treating TB. In recent years, Pro-Glu motif-containing (PE) and Pro-Pro-Glu motif-containing (PPE) family proteins have been found to have an increasingly important role in the pathogenesis and chronic protracted infection observed in TB. The development and clinical trials of vaccines based on Mtb antigens are in progress. Herein, we review the immunological effects of PE/PPE proteins and the development of common PE/PPE vaccines.

Differentiating the roles of Mycobacterium tuberculosis substrate binding proteins, FecB and FecB2, in iron uptake

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, poses a great threat to human health. With the emergence of drug resistant Mtb strains, new therapeutics are desperately needed. As iron is critical to the growth and survival of Mtb, mechanisms through which Mtb acquires host iron represent attractive therapeutic targets. Mtb scavenges host iron via Mtb siderophore-dependent and heme iron uptake pathways. While multiple studies describe the import of heme and ferric-siderophores and the export of apo-siderophores across the inner membrane, little is known about their transport across the periplasm and cell-wall environments. Mtb FecB and FecB2 are predicted periplasmic binding proteins implicated in host iron acquisition; however, their precise roles are not well understood. This study sought to differentiate the roles FecB and FecB2 play in Mtb iron acquisition. The crystallographic structures of Mtb FecB and FecB2 were determined to 2.0 Å and 2.2 Å resolution, respectively, and show distinct ligand binding pockets. In vitro ligand binding experiments for FecB and FecB2 were performed with heme and bacterial siderophores from Mtb and other species, revealing that both FecB and FecB2 bind heme, while only FecB binds the Mtb sideophore ferric-carboxymycobactin (Fe-cMB). Subsequent structure-guided mutagenesis of FecB identified a single glutamate residue-Glu339-that significantly contributes to Fe-cMB binding. A role for FecB in the Mtb siderophore-mediated iron acquisition pathway was corroborated by Mycobacterium smegmatis and Mtb pull-down assays, which revealed interactions between FecB and members of the mycobacterial siderophore export and import machinery. Similarly, pull-down assays with FecB2 confirms its role in heme uptake revealing interactions with a potential inner membrane heme importer. Due to ligand preference and protein partners, our data suggest that Mtb FecB plays a role in siderophore-dependent iron and heme acquisition pathways; in addition, we confirm that Mtb FecB2 is involved in heme uptake.

Deferiprone-gallium-protoporphyrin (IX): A promising treatment modality against Mycobacterium abscessus

Non-Tuberculous Mycobacterial Pulmonary Disease (NTM-PD) caused by Mycobacterium abscessus is a frequent complication in patients with cystic fibrosis (CF) that worsens lung function over time. Currently, there is no cure for NTM-PD, hence new therapies are urgently required. Disrupting bacterial iron uptake pathways using gallium-protoporphyrin (IX) (GaPP), a heme analog, has been proposed as a novel antibacterial approach to tackle multi-drug resistant M. abscessus. However, the antibacterial activity of GaPP has been tested only in iron-deficient media, which cannot accurately mirror the potential activity in vivo. Herein, we investigated the potential synergistic activity between GaPP and the iron-chelating agent deferiprone (Def) in regular media against M. abscessus-infected macrophages. The safety of the treatment was assessed in vitro using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay in Nuli-1 and THP-1 cell lines. Def-GaPP had synergistic activity against M. abscessus-infected macrophages where 10 mM-12.5 mg/L of Def-GaPP reduced the viability by up to 0.9 log10. Furthermore, Def-GaPP showed no cytotoxicity to Nuli-1 and THP-1 cell lines at the effective antibacterial concentrations (10 mM-12.5 mg/L) of Def- GaPP. These data encourage future investigation of Def-GaPP as a novel antimicrobial against NTM-PD.

Microbial siderophores as molecular shuttles for metal cations: sources, sinks and application perspectives

Iron is one of the highly abundant elements on the earth's crust, an essential micronutrient for a majority of life forms, and exists in two frequent oxidation states such as ferrous (Fe2+) and ferric (Fe3+). These two oxidation states are interconvertible by redox reactions and form complexes with a wide range of siderophores. At neutral pH in soil, Fe2+ is highly soluble upto 100 mM but have less biological value, whereas Fe3+ is less soluble upto 10-9 M. This reduced bioavailability of Fe3+ induces competition among microorganisms. As many microorganisms need at least 10-6 M of Fe3+ form of iron for their growth, siderophores from these microbes readily withdraw Fe3+ iron from a variety of habitats for their survival. In this review, we bring into light the several recent investigations related to diverse chemistry of microbial siderophores, mechanisms of siderophore uptake, biosynthetic gene clusters in microbial genomes, various sources of heavy metal cations in soil, siderophore-binding protein receptors and commercialisation perspectives of siderophores. Besides, this review unearths the recent advancements in the characterisation of novel siderophores and its heavy metal complexes alongside the interaction kinetics with receptors.

Infection metallomics for critical care in the post-COVID era

Infection metallomics is a mass spectrometry (MS) platform we established based on the central concept that microbial metallophores are specific, sensitive, noninvasive, and promising biomarkers of invasive infectious diseases. Here we review the in vitro, in vivo, and clinical applications of metallophores from historical and functional perspectives, and identify under-studied and emerging application areas with high diagnostic potential for the post-COVID era. MS with isotope data filtering is fundamental to infection metallomics; it has been used to study the interplay between "frenemies" in hosts and to monitor the dynamic response of the microbiome to antibiotic and antimycotic therapies. During infection in critically ill patients, the hostile environment of the host's body activates secondary bacterial, mycobacterial, and fungal metabolism, leading to the production of metallophores that increase the pathogen's chance of survival in the host. MS can reveal the structures, stability, and threshold concentrations of these metal-containing microbial biomarkers of infection in humans and model organisms, and can discriminate invasive disease from benign colonization based on well-defined thresholds distinguishing proliferation from the colonization steady state.

The Review of Current Knowledge on Neutrophil Gelatinase-Associated Lipocalin (NGAL)

Neutrophil gelatinase-associated lipocalin (NGAL) is a 25-kDa protein that is secreted mostly by immune cells such as neutrophils, macrophages, and dendritic cells. Its production is stimulated in response to inflammation. The concentrations of NGAL can be measured in plasma, urine, and biological fluids such as peritoneal effluent. NGAL is known mainly as a biomarker of acute kidney injury and is released after tubular damage and during renal regeneration processes. NGAL is also elevated in chronic kidney disease and dialysis patients. It may play a role as a predictor of the progression of renal function decreases with complications and mortality due to kidney failure. NGAL is also useful in the diagnostic processes of cardiovascular diseases. It is highly expressed in injured heart tissue and atherosclerostic plaque; its serum concentrations correlate with the severity of heart failure and coronary artery disease. NGAL increases inflammatory states and its levels rise in arterial hypertension, obesity, diabetes, and metabolic complications such as insulin resistance, and is also involved in carcinogenesis. In this review, we present the current knowledge on NGAL and its involvement in different pathologies, especially its role in renal and cardiovascular diseases.

Ferritin from Mycobacterium abscessus is involved in resistance to antibiotics and oxidative stress

Mycobacterium abscessus subsp. massiliense (Mycma) is a rapidly growing Mycobacterium belonging to the M. abscessus complex that is often associated with lung and soft tissue infection outbreaks. Mycma is resistant to many antimicrobials, including those used for treating tuberculosis. Therefore, Mycma infections are difficult to treat and may lead to high infectious complication rates. Iron is essential for bacterial growth and establishment of infection. During infection, the host reduces iron concentrations as a defense mechanism. To counteract the host-induced iron deficiency, Mycma produces siderophores to capture iron. Mycma has two ferritins (encoded by mycma_0076 and mycma_0077) modulated by different iron concentrations, which allow the survival of this pathogen during iron scarcity. In this study, we constructed knockout (Mycma 0076KO) and complemented (Mycma 0076KOc) gene strains for mycma_0076 to understand the function of 0076 ferritin. Deletion of mycma_0076 in Mycma led to the transition in colony morphology from smooth to rough, alteration of the glycopeptidolipids spectra, increased permeability of the envelope, reduction in biofilm formation, increased susceptibility to antimicrobials and hydrogen peroxide-induced oxidative stress, and decreased internalization by macrophages. This study shows that Mycma_0076 ferritin in Mycma is involved in resistance to oxidative stress and antimicrobials, and alteration of cell envelope architecture. KEY POINTS: • Deletion of the mycma_0076 gene altered colony morphology to rough; • Mycma 0076KO changed GPL profile; • Absence of Mycma_0076 ferritin results in increased susceptibility to antimicrobials and oxidative stress in Mycma. Legend: a In wild-type M. abscessus subsp. massiliense strain, iron is captured from the environment by carboxymycobactins and mycobactins (1). Iron-dependent regulator (IdeR) proteins bind to ferrous iron (Fe⁺²) in the bacterial cytoplasm leading to the activation of the IdeR-Fe⁺² complex (2). The activated complex binds to the promoter regions of iron-dependent genes, called iron box, which in turn help in the recruitment of RNA polymerase to promote transcription of genes such as mycma_0076 and mycma_0077 ferritin genes (3). Mycma_0076 and Mycma_0077 ferritins bind to excess iron in the medium and promote Fe²⁺ oxidation into ferric iron (Fe³⁺) and store iron molecules to be released under iron scarcity conditions. (4) Genes related to biosynthesis and transport of glycopeptidolipids (GPL) are expressed normally and the cell envelope is composed of different GPL species (colored squares represented on the cell surface (GPLs). Consequently, WT Mycma present smooth colony phenotype (5). b In Mycma 0076KO strain, the lack of ferritin 0076 causes overexpression of mycma_0077 (6), but does not restore wild-type iron homeostasis and thus may result in free intracellular iron, even in the presence of miniferritins (MaDps). The excess iron potentiates oxidative stress (7) by generating hydroxyl radicals through Fenton Reaction. During this process, through an unknown mechanism, that could involve Lsr2 (8), the expression of GPL synthesis locus is regulated positively and/or negatively, resulting in alteration of GPL composition in the membrane (as represented by different colors of squares on the cell surface), resulting in a rough colony phenotype (9). The changes of GPL can increase cell wall permeability, contributing to antimicrobial susceptibility (10).

Toxicity evaluation of choline ionic liquid-based nanocarriers of pharmaceutical agents for lung treatment

In vitro cytotoxicity evaluation of linear copolymer (LC) containing choline ionic liquid units and its conjugates with an antibacterial drug in anionic form, that is, p-aminosalicylate (LC_PAS), clavulanate (LC_CLV), or piperacillin (LC_PIP) was carried out. These systems were tested against normal: human bronchial epithelial cells (BEAS-2B), and cancers: adenocarcinoma human alveolar basal epithelial cells (A549), and human non-small cell lung carcinoma cell line (H1299). Cells viability, after linear copolymer LC and their conjugates addition for 72 h, was measured at concentration range of 3.125-100 μg/mL. The MTT test allowed the designation of IC₅₀ index, which was higher for BEAS-2B, and significantly lower in the case of cancer cell lines. The cytometric analyzes, that is, Annexin-V FITC apoptosis assay and cell cycle analysis as well as gene expression measurements for interleukins IL6 and IL8 were carried out, and showed pro-inflammatory activity of tested compounds toward cancer cells, while it was not observed against normal cell line.

CRISPR and Antisense RNA Technology: Exploiting Nature's Tool to Restrain Virulence in Tenacious Pathogens

Pathogenic bacteria constitute a significant threat to mankind and at the same time represent a huge reservoir of abeyant therapeutics to prevent and treat various diseases. The concept of virulence determinants has been a compelling tool in driving research in the field of bacterial pathogenesis and infectious diseases. In this review, we highlight a few virulence elements forged by the pathogens from the viewpoint of the damage-response scaffold, vandalizing the susceptible host. Seeking an alternative to target the virulence determinants heads a road map toward the exemplary molecular approach. Hence, here we explore some of the exceptional applications of the clustered regulatory interspaced short palindromic repeat (CRISPR)- based therapy and antisense RNA (asRNA) approach, which could be exploited to selectively dismantle adamant components of the pathogen's virulence machinery. To the best of our knowledge, this is the first review paper involving both CRISPR and antisense RNA technology, as an alternative strategy to evade virulence mechanisms in bacterial pathogens.

A zero-sum game or an interactive frame? Iron competition between bacteria and humans in infection war

ABSTRACT

Iron is an essential trace element for both humans and bacteria. It plays a vital role in life, such as in redox reactions and electron transport. Strict regulatory mechanisms are necessary to maintain iron homeostasis because both excess and insufficient iron are harmful to life. Competition for iron is a war between humans and bacteria. To grow, reproduce, colonize, and successfully cause infection, pathogens have evolved various mechanisms for iron uptake from humans, principally Fe 3+ -siderophore and Fe 2+ -heme transport systems. Humans have many innate immune mechanisms that regulate the distribution of iron and inhibit bacterial iron uptake to help resist bacterial invasion and colonization. Meanwhile, researchers have invented detection test strips and coupled antibiotics with siderophores to create tools that take advantage of this battle for iron, to help eliminate pathogens. In this review, we summarize bacterial and human iron metabolism, competition for iron between humans and bacteria, siderophore sensors, antibiotics coupled with siderophores, and related phenomena. We also discuss how competition for iron can be used for diagnosis and treatment of infection in the future.

c-di-AMP Accumulation Regulates Growth, Metabolism, and Immunogenicity of Mycobacterium smegmatis

Cyclic dimeric adenosine monophosphate (c-di-AMP) is a ubiquitous second messenger of bacteria involved in diverse physiological processes as well as host immune responses. MSMEG_2630 is a c-di-AMP phosphodiesterase (cnpB) of Mycobacterium smegmatis, which is homologous to Mycobacterium tuberculosis Rv2837c. In this study, cnpB-deleted (ΔcnpB), -complemented (ΔcnpB::C), and -overexpressed (ΔcnpB::O) strains of M. smegmatis were constructed to investigate the role of c-di-AMP in regulating mycobacterial physiology and immunogenicity. This study provides more precise evidence that elevated c-di-AMP level resulted in smaller colonies, shorter bacteria length, impaired growth, and inhibition of potassium transporter in M. smegmatis. This is the first study to report that elevated c-di-AMP level could inhibit biofilm formation and induce porphyrin accumulation in M. smegmatis by regulating associated gene expressions, which may have effects on drug resistance and virulence of mycobacterium. Moreover, the cnpB-deleted strain with an elevated c-di-AMP level could induce enhanced Th1 immune responses after M. tuberculosis infection. Further, the pathological changes and the bacteria burden in ΔcnpB group were comparable with the wild-type M. smegmatis group against M. tuberculosis venous infection in the mouse model. Our findings enhanced the understanding of the physiological role of c-di-AMP in mycobacterium, and M. smegmatis cnpB-deleted strain with elevated c-di-AMP level showed the potential for a vaccine against tuberculosis.

Approaches for Targeting the Mycobactin Biosynthesis Pathway for Novel Anti-tubercular Drug Discovery: Where We Stand

INTRODUCTION

Several decades of antitubercular drug discovery efforts have focused on novel antitubercular chemotherapies. However, recent efforts have greatly shifted towards countering extremely/multi/total drug-resistant species. Targeting the conditionally essential elements inside Mycobacterium is a relatively new approach against tuberculosis and has received lackluster attention. The siderophore, Mycobactin, is a conditionally essential molecule expressed by mycobacteria in iron-stress conditions. It helps capture the micronutrient iron, essential for the smooth functioning of cellular processes.

AREAS COVERED

The authors discuss opportunities to target the conditionally essential pathways to help develop newer drugs and prolong the shelf life of existing therapeutics, emphasizing the bottlenecks in fast-tracking antitubercular drug discovery.

EXPERT OPINION

While the lack of iron supply can cripple bacterial growth and multiplication, excess iron can cause oxidative overload. Constant up-regulation can strain the bacterial synthetic machinery, further slowing its growth. Mycobactin synthesis is tightly controlled by a genetically conserved mega enzyme family via up-regulation (HupB) or down-regulation (IdeR) based on iron availability in its microenvironment. Furthermore, the recycling of siderophores by the MmpL-MmpS4/5 orchestra provides endogenous drug targets to beat the bugs with iron-toxicity contrivance. These processes can be exploited as chinks in the armor of Mycobacterium and be used for new drug development.

A periplasmic cinched protein is required for siderophore secretion and virulence of Mycobacterium tuberculosis

Iron is essential for growth of Mycobacterium tuberculosis, the causative agent of tuberculosis. To acquire iron from the host, M. tuberculosis uses the siderophores called mycobactins and carboxymycobactins. Here, we show that the rv0455c gene is essential for M. tuberculosis to grow in low-iron medium and that secretion of both mycobactins and carboxymycobactins is drastically reduced in the rv0455c deletion mutant. Both water-soluble and membrane-anchored Rv0455c are functional in siderophore secretion, supporting an intracellular role. Lack of Rv0455c results in siderophore toxicity, a phenotype observed for other siderophore secretion mutants, and severely impairs replication of M. tuberculosis in mice, demonstrating the importance of Rv0455c and siderophore secretion during disease. The crystal structure of a Rv0455c homolog reveals a novel protein fold consisting of a helical bundle with a 'cinch' formed by an essential intramolecular disulfide bond. These findings advance our understanding of the distinct M. tuberculosis siderophore secretion system.

Soil Properties and Moisture Synergistically Influence Nontuberculous Mycobacterial Prevalence in Natural Environments of Hawai'i

Nontuberculous mycobacteria (NTM) are opportunistic pathogens that cause chronic pulmonary disease (PD). NTM infections are thought to be acquired from the environment; however, the basal environmental factors that drive and sustain NTM prevalence are not well understood. The highest prevalence of NTM PD cases in the United States is reported from Hawai'i, which is unique in its climate and soil composition, providing an opportunity to investigate the environmental drivers of NTM prevalence. We used microbiological sampling and spatial logistic regression complemented with fine-scale soil mineralogy to model the probability of NTM presence across the natural landscape of Hawai'i. Over 7 years, we collected and microbiologically cultured 771 samples from 422 geographic sites in natural areas across the Hawaiian Islands for the presence of NTM. NTM were detected in 210 of these samples (27%), with Mycobacterium abscessus being the most frequently isolated species. The probability of NTM presence was highest in expansive soils (those that swell with water) with a high water balance (>1-m difference between rainfall and evapotranspiration) and rich in Fe-oxides/hydroxides. We observed a positive association between NTM presence and iron in wet soils, supporting past studies, but no such association in dry soils. High soil-water balance may facilitate underground movement of NTM into the aquifer system, potentially compounded by expansive capabilities allowing crack formation under drought conditions, representing further possible avenues for aquifer infiltration. These results suggest both precipitation and soil properties are mechanisms by which surface NTM may reach the human water supply. IMPORTANCE Nontuberculous mycobacteria (NTM) are ubiquitous in the environment, being found commonly in soils and natural bodies of freshwater. However, little is known about the environmental niches of NTM and how they relate to NTM prevalence in homes and other human-dominated areas. To characterize NTM environmental associations, we collected and cultured 771 samples from 422 geographic sites in natural areas across Hawai'i, the U.S. state with the highest prevalence of NTM pulmonary disease. We show that the environmental niches of NTM are most associated with highly expansive, moist soils containing high levels of iron oxides/hydroxides. Understanding the factors associated with NTM presence in the natural environment will be crucial for identifying potential mechanisms and risk factors associated with NTM infiltration into water supplies, which are ultimately piped into homes where most exposure risk is thought to occur.

Synthesis and in vitro analysis of novel gallium tetrakis(4-methoxyphenyl)porphyrin and its long-acting nanoparticle as a potent antimycobacterial agent

Bacterial heme uptake pathways offer a novel target for antimicrobial drug discovery. Recently, gallium (Ga) porphyrin complexes were found to be effective against mycobacterial heme uptake pathways. The goal of the current study is to build on this foundation and develop a new Ga(III) porphyrin and its nanoparticles, formulated by a single emulsion-evaporation technique to inhibit the growth of Mycobacterium avium complex (MAC) with enhanced properties. Gallium 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin chloride (GaMeOTP) was synthesized from 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin and GaCl₃. GaMeOTP showed enhanced antimicrobial activity against MAC104 and some clinical M. avium isolates. The synthesized Ga(III) porphyrin antimicrobial activity resulted in the overproduction of reactive oxygen species. Our study also demonstrated that F127 nanoparticles encapsulating GaMeOTP exhibited a smaller size than GaTP nanoparticles and a better duration of activity in MAC-infected macrophages compared to the free GaMeOTP. The nanoparticles were trafficked to endosomal compartments within MAC-infected macrophages, likely contributing to the antimicrobial activity of the drug.

In Silico Exploration of Mycobacterium tuberculosis Metabolic Networks Shows Host-Associated Convergent Fluxomic Phenotypes

Mycobacterium tuberculosis, the causative agent of tuberculosis, is composed of several lineages characterized by a genome identity higher than 99%. Although the majority of the lineages are associated with humans, at least four lineages are adapted to other mammals, including different M. tuberculosis ecotypes. Host specificity is associated with higher virulence in its preferred host in ecotypes such as M. bovis. Deciphering what determines the preference of the host can reveal host-specific virulence patterns. However, it is not clear which genomic determinants might be influencing host specificity. In this study, we apply a combination of unsupervised and supervised classification methods on genomic data of ~27,000 M. tuberculosis clinical isolates to decipher host-specific genomic determinants. Host-specific genomic signatures are scarce beyond known lineage-specific mutations. Therefore, we integrated lineage-specific mutations into the iEK1011 2.0 genome-scale metabolic model to obtain lineage-specific versions of it. Flux distributions sampled from the solution spaces of these models can be accurately separated according to host association. This separation correlated with differences in cell wall processes, lipid, amino acid and carbon metabolic subsystems. These differences were observable when more than 95% of the samples had a specific growth rate significantly lower than the maximum achievable by the models. This suggests that these differences might manifest at low growth rate settings, such as the restrictive conditions M. tuberculosis suffers during macrophage infection.

Mycobactin Analogues with Excellent Pharmacokinetic Profile Demonstrate Potent Antitubercular Specific Activity and Exceptional Efflux Pump Inhibition

In this study, we have designed and synthesized pyrazoline analogues that partially mimic the structure of mycobactin, to address the requirement of novel therapeutics to tackle the emerging global challenge of antimicrobial resistance (AMR). Our investigation resulted in the identification of novel lead compounds 44 and 49 as potential mycobactin biosynthesis inhibitors against mycobacteria. Moreover, candidates efficiently eradicated intracellularly surviving mycobacteria. Thermofluorimetric analysis and molecular dynamics simulations suggested that compounds 44 and 49 bind to salicyl-AMP ligase (MbtA), a key enzyme in the mycobactin biosynthetic pathway. To the best of our knowledge, these are the first rationally designed mycobactin inhibitors to demonstrate an excellent in vivo pharmacokinetic profile. In addition, these compounds also exhibited more potent whole-cell efflux pump inhibition than known efflux pump inhibitors verapamil and chlorpromazine. Results from this study pave the way for the development of 3-(2-hydroxyphenyl)-5-(aryl)-pyrazolines as a new weapon against superbug-associated AMR challenges.

Host glyceraldehyde-3-phosphate dehydrogenase-mediated iron acquisition is hijacked by intraphagosomal Mycobacterium tuberculosis

Availability of iron is a key factor in the survival and multiplication of Mycobacterium tuberculosis (M.tb) within host macrophage phagosomes. Despite host cell iron regulatory machineries attempts to deny supply of this essential micronutrient, intraphagosomal M.tb continues to access extracellular iron. In the current study, we report that intracellular M.tb exploits mammalian secreted Glyceraldehyde 3-phosphate dehydrogenase (sGAPDH) for the delivery of host iron carrier proteins lactoferrin (Lf) and transferrin (Tf). Studying the trafficking of iron carriers in infected cells we observed that sGAPDH along with the iron carrier proteins are preferentially internalized into infected cells and trafficked to M.tb containing phagosomes where they are internalized by resident mycobacteria resulting in iron delivery. Collectively our findings provide a new mechanism of iron acquisition by M.tb involving the hijack of host sGAPDH. This may contribute to its successful pathogenesis and provide an option for targeted therapeutic intervention.

Toxicity of the iron siderophore mycobactin J in mouse macrophages: Evidence for a hypoxia response

Mycobacterium tuberculosis, the causative agent of tuberculosis, is an obligate intracellular pathogen that lives within the phagosome of macrophages. Here we demonstrate that the siderophore mycobactin J, produced by the closely related intracellular pathogen Mycobacterium paratuberculosis, is toxic to murine macrophage cells. Its median lethal dose, 10 μM, is lower than that of the iron chelators desferrioxamine B and TrenCAM, an enterobactin analog. To determine the source of this toxicity, we conducted microarray, ELISA, and metabolite profiling experiments. The primary response is hypoxia-like, which implies iron starvation as the underlying cause of the toxicity. This observation is consistent with our recent finding that mycobactin J is a stronger iron chelator than had been inferred from previous studies. Mycobactin J is known to partition into cell membranes and hydrophobic organelles indicating that enhanced membrane penetration is also a likely factor. Thus, mycobactin J is shown to be toxic, eliciting a hypoxia-like response under physiological conditions.

Host and Bacterial Iron Homeostasis, an Underexplored Area in Tuberculosis Biomarker Research

Mycobacterium tuberculosis (Mtb) "a human adapted pathogen" has found multiple ways to manipulate the host immune response during infection. The human immune response to Mtb infection is a highly complex cascade of reactions, with macrophages as preferred intracellular location. Interaction with the host through infection gives rise to expression of specific gene products for survival and multiplication within the host. The signals that the pathogens encounter during infection cause them to selectively express genes in response to signals. One strategy to identify Mtb antigens with diagnostic potential is to identify genes that are specifically induced during infection or in specific disease stages. The shortcomings of current immunodiagnostics include the failure to detect progression from latent infection to active tuberculosis disease, and the inability to monitor treatment efficacy. This highlights the need for new tuberculosis biomarkers. These biomarkers should be highly sensitive and specific diagnosing TB infection, specifically distinguishing between latent infection and active disease. The regulation of iron levels by the host plays a crucial role in the susceptibility and outcome of Mtb infection. Of interest are the siderophore biosynthetic genes, encoded by the mbt-1 and mbt-2 loci and the SUF (mobilization of sulphur) operon (sufR-sufB-sufD-sufC-csd-nifU-sufT), which encodes the primary iron-sulphur cluster biogenesis system. These genes are induced during iron limitation and intracellular growth of Mtb, pointing to their importance during infection.

Chemical Synthesis of Cell Wall Constituents of Mycobacterium tuberculosis

The pathogen Mycobacterium tuberculosis (Mtb), causing tuberculosis disease, features an extraordinary thick cell envelope, rich in Mtb-specific lipids, glycolipids, and glycans. These cell wall components are often directly involved in host–pathogen interaction and recognition, intracellular survival, and virulence. For decades, these mycobacterial natural products have been of great interest for immunology and synthetic chemistry alike, due to their complex molecular structure and the biological functions arising from it. The synthesis of many of these constituents has been achieved and aided the elucidation of their function by utilizing the synthetic material to study Mtb immunology. This review summarizes the synthetic efforts of a quarter century of total synthesis and highlights how the synthesis layed the foundation for immunological studies as well as drove the field of organic synthesis and catalysis to efficiently access these complex natural products.

Effect of ambient air pollution on tuberculosis risks and mortality in Shandong, China: a multi-city modeling study of the short- and long-term effects of pollutants

Few studies conducted in China have assessed the effects of ambient air pollution exposure on tuberculosis (TB) risk and mortality, especially with a multicity setting. We evaluated the effect of short- and long-term ambient sulfur dioxide (SO₂), nitrogen dioxide (NO₂), carbon monoxide (CO), ozone (O₃), and particulate matter≤2.5 μm in aerodynamic diameter (PM_2.5) exposures on development and mortality of active TB in 7 Chinese cities in Shandong province from January 1, 2013, to December 31, 2017. We estimated the pollution-associated risk to new infection TB, recurrent TB, and mortality in relation to 1-μg/m³ increases in air pollutants using the penalized multivariate Poisson regression models. A total of 83,555 new infective TB and 3060 recurrent TB including 997 deaths were recorded. Short- and long-term exposures to outdoor air pollutants (SO₂, NO₂, CO, O₃, and PM_2.5) were significantly associated with new infection TB, recurrent TB risk, and mortality. The dominant positive effects of SO₂, NO₂, CO, and PM_2.5 for new infection and recurrent TB risk were observed at long-term (>30 days) exposure, whereas the dominant effects of SO₂, CO, and PM_2.5 for mortality were observed at short-term (≤30 days) exposures. Of the 5 air pollutants we assessed, SO₂ and PM_2.5 exhibited more consistent and strong associations with TB-related outcomes. We estimated an increase of 1.33% (95% CI 1.29%, 1.37%) and 3.04% (95% CI 2.98%, 3.11%) in new infection TB count for each 1-μg/m³ increase of SO₂ at lag 0-180 days and PM_2.5 at lag 0-365 days, respectively. This epidemiologic study in China shows that air pollution exposure is associated with increased risk of active TB development and mortality. The control of ambient air pollution may benefit the control and decrease the mortality of TB disease.

Transporters Involved in the Biogenesis and Functionalization of the Mycobacterial Cell Envelope

The biology of mycobacteria is dominated by a complex cell envelope of unique composition and structure and of exceptionally low permeability. This cell envelope is the basis of many of the pathogenic features of mycobacteria and the site of susceptibility and resistance to many antibiotics and host defense mechanisms. This review is focused on the transporters that assemble and functionalize this complex structure. It highlights both the progress and the limits of our understanding of how (lipo)polysaccharides, (glyco)lipids, and other bacterial secretion products are translocated across the different layers of the cell envelope to their final extra-cytoplasmic location. It further describes some of the unique strategies evolved by mycobacteria to import nutrients and other products through this highly impermeable barrier.

Nutritional immunity: the impact of metals on lung immune cells and the airway microbiome during chronic respiratory disease

Nutritional immunity is the sequestration of bioavailable trace metals such as iron, zinc and copper by the host to limit pathogenicity by invading microorganisms. As one of the most conserved activities of the innate immune system, limiting the availability of free trace metals by cells of the immune system serves not only to conceal these vital nutrients from invading bacteria but also operates to tightly regulate host immune cell responses and function. In the setting of chronic lung disease, the regulation of trace metals by the host is often disrupted, leading to the altered availability of these nutrients to commensal and invading opportunistic pathogenic microbes. Similarly, alterations in the uptake, secretion, turnover and redox activity of these vitally important metals has significant repercussions for immune cell function including the response to and resolution of infection. This review will discuss the intricate role of nutritional immunity in host immune cells of the lung and how changes in this fundamental process as a result of chronic lung disease may alter the airway microbiome, disease progression and the response to infection.

Dissecting druggability of ABC transporter proteins in Mycobacterium species through network modeling

Mycobacterium tuberculosis (Mtb) is an infectious disease that affects nearly 9.6 million people every year. Metals are important determinants of growth and pathogenicity of mycobacterium. In the present study, we have analyzed protein-protein interaction networks belonging to the iron, sulfur and molybdenum metabolism of Mycobacterium. Our analysis has identified some of the important target proteins one among them being irtA. Iron taken up by siderophores from the host is transported to irtA through which iron enters Mycobacterium. Thus, irtA plays a major role as an iron transporter in Mycobacterium. As irtA protein structure was not solved experimentally, we have predicted 3D structure of irtA. After successful model evaluation, we have identified thiosemicarbazones as possible drug candidates for irtA. Henceforth, we have designed five analogues of thiosemicarbazones and tested in silico for their efficacy against irtA using molecular docking, among them analogue 1 showed a very good efficacy.Communicated by Ramaswamy H. Sarma.

Iron homeostasis during anemia of inflammation: a prospective study in patients with tuberculosis

Anemia of inflammation is a hallmark of tuberculosis. Factors controlling iron metabolism during anemia of inflammation and its resolution are uncertain. Whether iron supplements should be given during anti-tuberculosis treatment to support Hb recovery is unclear. Before and during treatment of tuberculosis, we assessed iron kinetics, and changes in inflammation and iron metabolism indices. In a 26-wk prospective study, Tanzanian adults with tuberculosis (n=18) were studied before treatment and then every two weeks during treatment; oral and intravenous iron tracers were administered before treatment, after intensive phase (8/12 wk) and complete treatment (24 wk); no iron supplements were given. Before treatment, hepcidin and erythroferrone (ERFE) were greatly elevated, erythrocyte iron utilization was high (~80%) and iron absorption was negligible (<1%). During treatment, hepcidin and IL-6 decreased ~70% after only 2 wk (p<0.001); in contrast, ERFE did not significantly decrease until 8 wk (p<0.01). ERFE and IL-6 were the main opposing determinants of hepcidin (p<0.05) and greater ERFE was associated with reticulocytosis and hemoglobin (Hb) repletion (p<0.01). Dilution of baseline tracer concentration was 2.6-fold higher during intensive phase treatment (p<0.01) indicating enhanced erythropoiesis. After treatment completion, iron absorption increased ~20-fold (p<0.001); Hb increased ~25% (p<0.001). In tuberculosis-associated anemia of inflammation, our findings suggest elevated ERFE is unable to suppress hepcidin and iron absorption is negligible. During treatment, as inflammation resolves, ERFE may remain elevated, contributing to hepcidin suppression and Hb repletion. Iron is well-absorbed only after tuberculosis treatment and supplementation should be reserved for patients remaining anemic after treatment. (ClinicalTrials.gov Identifier:NCT02176772).

Immunoinflammatory effects of dietary bioactive compounds

Inflammation is a key mechanism of the immune system that can be elicited by several factors, among them several chemical, physical and biological agents. Once stimulated, the inflammatory response activates a series of signaling pathways and a number of immune cells which promote, in a very coordinated manner, the neutralization of the infectious agent. However, if uncontrolled, the inflammatory status may become chronic leading, potentially, to tissue damage and disease onset. Several risk factors are associated with the development of chronic inflammation and, among these factors, diet plays an essential role. In this chapter the effects of some dietary bioactive compounds, including micronutrients, omega-3 fatty acids, nucleotides and polyphenols, on the immunoinflammatory responses in different cellular, animal and human studies have been summarized.

Iron uptake and transport by the carboxymycobactin-mycobactin siderophore machinery of Mycobacterium tuberculosis is dependent on the iron-regulated protein HupB

Iron-starved Mycobacterium tuberculosis utilises the carboxymycobactin-mycobactin siderophore machinery to acquire iron. These two siderophores have high affinity for ferric iron and can withdraw the metal ion from insoluble iron hydroxides and iron-binding proteins. We first reported HupB, a multi-functional mycobacterial protein to be associated with iron acquisition in M. tuberculosis. This 28 kDa cell wall protein, up regulated upon iron limitation functions as a transcriptional activator of mycobactin biosynthesis and is essential for the pathogen to survive inside macrophages. The focus of this study is to understand the role of HupB in iron uptake and transport by the carboxmycobactin-mycobactin siderophore machinery in M. tuberculosis. Experimental approaches included radiolabelled iron uptake studies by viable organisms and protein-ligand binding studies using the purified HupB and the two siderophores. Uptake of ⁵⁵Fe-carboxymycobactin by wild type M. tuberculosis (WT M.tb.H37Rv) and not by the hupB KO mutant (M.tb.ΔhupB) showed that HupB is necessary for the uptake of ferri-carboxymycobactin. Additionally, the radiolabel recovery was high in HupB-incorporated liposomes upon addition of the labelled siderophore. Bioinformatic and experimental studies using spectrofluorimetry, CD analysis and surface plasmon resonance not only confirmed the binding of HupB with ferri-carboxymycobactin and ferri-mycobactin but also with free iron. In conclusion, HupB is established as a ferri- carboxymycobactin receptor and by virtue of its property to bind ferric iron, functions as a transporter of the ferric iron from the extracellular siderophore to mycobactin within the cell envelope.

The Mycobactin Biosynthesis Pathway: A Prospective Therapeutic Target in the Battle against Tuberculosis

The alarming rise in drug-resistant clinical cases of tuberculosis (TB) has necessitated the rapid development of newer chemotherapeutic agents with novel mechanisms of action. The mycobactin biosynthesis pathway, conserved only among the mycolata family of actinobacteria, a group of intracellularly surviving bacterial pathogens that includes Mycobacterium tuberculosis, generates a salicyl-capped peptide mycobactin under iron-stress conditions in host macrophages to support the iron demands of the pathogen. This in vivo essentiality makes this less explored mycobactin biosynthesis pathway a promising endogenous target for novel lead-compounds discovery. In this Perspective, we have provided an up-to-date account of drug discovery efforts targeting selected enzymes (MbtI, MbtA, MbtM, and PPTase) from the mbt gene cluster (mbtA-mbtN). Furthermore, a succinct discussion on non-specific mycobactin biosynthesis inhibitors and the Trojan horse approach adopted to impair iron metabolism in mycobacteria has also been included in this Perspective.

Use of Calcium Channel Blockers and Risk of Active Tuberculosis Disease: A Population-Based Analysis

Calcium channel blockers (CCBs) are known to reduce the availability of iron-an important mineral for intracellular pathogens. Nonetheless, whether the use of CCBs modifies the risk of active tuberculosis in the clinical setting remains unclear. To determine whether CCBs may modify the risk of active tuberculosis disease, we conducted a nested case-control study using the National Health Insurance Research Database of Taiwan between January 1999 and December 2011. Conditional logistic regression and disease risk score adjustment were used to calculate the risk of active tuberculosis disease associated with CCB use. Subgroup analyses investigated the effect of different types of CCBs and potential effect modification in different subpopulations. A total of 8164 new active tuberculosis cases and 816 400 controls were examined. Use of CCBs was associated with a 32% decrease in the risk of active tuberculosis (relative risk [RR], 0.68 [95% CI, 0.58-0.78]) after adjustment with disease risk score. Compared with nonuse of CCBs, the use of dihydropyridine CCBs was associated with a lower risk of tuberculosis (RR, 0.63 [95% CI, 0.53-0.79]) than nondihydropyridine CCBs (RR, 0.73 [95% CI, 0.57-0.94]). In contrast, use of β-blockers (RR, 0.99 [95% CI, 0.83-1.12]) or loop diuretics (RR, 0.88 [95% CI, 0.62-1.26]) was not associated with lower risk of tuberculosis. In subgroup analyses, the risk of tuberculosis associated with the use of CCBs was similar among patients with heart failure or cerebrovascular diseases. Our study confirms that use of dihydropyridine CCBs decreases the risk of active tuberculosis.

ESAT-6 Protein of Mycobacterium tuberculosis Increases Holotransferrin-Mediated Iron Uptake in Macrophages by Downregulating Surface Hemochromatosis Protein HFE

Iron is an essential element for Mycobacterium tuberculosis; it has at least 40 enzymes that require iron as a cofactor. Accessibility of iron at the phagosomal surface inside macrophage is crucial for survival and virulence of M. tuberculosis ESAT-6, a 6-kDa-secreted protein of region of difference 1, is known to play a crucial role in virulence and pathogenesis of M. tuberculosis In our earlier study, we demonstrated that ESAT-6 protein interacts with β-2-microglobulin (β2M) and affects class I Ag presentation through sequestration of β2M inside endoplasmic reticulum, which contributes toward inhibition of MHC class I:β2M:peptide complex formation. The 6 aa at C-terminal region of ESAT-6 are essential for ESAT6:β2M interaction. β2M is essential for proper folding of HFE, CD1, and MHC class I and their surface expression. It is known that M. tuberculosis recruit holotransferrin at the surface of the phagosome. But the upstream mechanism by which it modulates holotransferrin-mediated iron uptake at the surface of macrophage is not well understood. In the current study, we report that interaction of the ESAT-6 protein with β2M causes downregulation of surface HFE, a protein regulating iron homeostasis via interacting with transferrin receptor 1 (TFR1). We found that ESAT-6:β2M interaction leads to sequestration of HFE in endoplasmic reticulum, causing poorer surface expression of HFE and HFE:TFR1 complex (nonfunctional TFR1) in peritoneal macrophages from C57BL/6 mice, resulting in increased holotransferrin-mediated iron uptake in these macrophages. These studies suggest that M. tuberculosis probably targets the ESAT-6 protein to increase iron uptake.

Anti-Mycobacterial Peroxides: A New Class of Agents for Development Against Tuberculosis

BACKGROUND

With few exceptions, existing tuberculosis drugs were developed many years ago and resistance profiles have emerged. This has created a need for new drugs with discrete modes of action. There is evidence that tuberculosis (like other bacteria) is susceptible to oxidative pressure and this has yet to be properly utilised as a therapeutic approach in a manner similar to that which has proven highly successful in malaria therapy.

OBJECTIVE

To develop an alternative approach to the incorporation of bacterial siderophores that results in the creation of antitubercular peroxidic leads for subsequent development as novel agents against tuberculosis.

METHODS

Eight novel peroxides were prepared and the antitubercular activity (H37Rv) was compared to existing artemisinin derivatives in vitro. The potential for toxicity was evaluated against the L6 rat skeletal myoblast and HeLa cervical cancer lines in vitro.

RESULTS

The addition of a pyrimidinyl residue to an artemisinin or, preferably, a tetraoxane peroxidic structure results in antitubercular activity in vitro. The same effect is not observed in the absence of the pyrimidine or with other heteroaromatic substituents.

CONCLUSION

The incorporation of a pyrimidinyl residue adjacent to the peroxidic function in an organic peroxide results in anti-tubercular activity in an otherwise inactive peroxidic compound. This will be a useful approach for creating oxidative drugs to target tuberculosis.

Isoniazid Preventive Therapy in Contacts of Multidrug-Resistant Tuberculosis

Rationale: The World Health Organization recommends the use of isoniazid (INH) alone or in combination with rifapentine to treat latent tuberculosis infections. The recent rise of drug-resistant tuberculosis has complicated the choice of treatment regimen for latent tuberculosis infection.Objectives: To evaluate the effects of INH preventive therapy on the contacts of patients with multidrug-resistant tuberculosis.Methods: In a prospective cohort study conducted between September 2009 and August 2012, we identified 4,500 index patients with tuberculosis and 14,044 tuberculosis-exposed household contacts who we followed for 1 year for the occurrence of incident tuberculosis disease. Although Peruvian national guidelines specify that INH preventive therapy should be provided to contacts aged 19 years old or younger, only half this group received INH preventive therapy.Measurements and Main Results: Among 4,216 contacts under 19 years of age, 2,106 contacts (50%) initiated INH preventive therapy at enrollment. The protective effect of INH was more extreme in contacts exposed to drug-sensitive tuberculosis (adjusted hazard ratio, 0.30; 95% confidence interval, 0.18-0.48) and to multidrug-resistant tuberculosis (adjusted hazard ratio, 0.19; 95% confidence interval, 0.05-0.66) compared with those exposed to mono-INH-resistant tuberculosis (adjusted hazard ratio, 0.80; 95% confidence interval, 0.23-2.80). In the second independent study, tuberculosis occurred in none of the 76 household contacts who received INH preventive therapy compared with 3% (8 of 273) of those who did not.Conclusions: Household contacts who received INH preventive therapy had a lower incidence of tuberculosis disease even when they had been exposed to an index patient with multidrug-resistant tuberculosis. INH may have a role in the management of latent multidrug-resistant tuberculosis infection.

Vitamin D - A host directed autophagy mediated therapy for tuberculosis

According to the WHO report 2019, Tuberculosis (TB) is an ancient disease of humanity that is curable. TB has caused significant morbidity and mortality even in 2018. The etiological agent of TB, Mycobacterium tuberculosis (MTB) exploits its virulence factors to escape from host immunity and therapeutic drugs. Host Directed Therapy (HDT) is an adjunctive therapy where repurposed drugs, small molecules, vitamins, cytokines, and monoclonal antibodies are used to overcome the pathogen exploited pathways in the host. One of the HDTs, i.e. induction of autophagy is a highly regulated intracellular self-degradative process in which pathogens are sequestered in double-layered autophagosomes and targeted to the lysosome for degradation. Apart from the pathogen clearance, autophagy involves the release of nutrients during starvation, removal of damaged organelles and aggregated proteins, antigen presentation, tumor suppression, and anti-aging mechanisms. Xenophagy is a type of selective autophagy against microbes induced by ubiquitin receptors (p62/SQSTM1, NDP52, NBR1, OPTN, Parkin and Smurf proteins) after pathogen recognition. ULK1/2, Beclin-1, ATG5-ATG12-ATG16 L and LC-II-PE complexes along with two nutrient-sensing protein complexes, mTOR and AMPK activate autophagy mechanisms to limit infection. Pattern Recognition Receptors (PRRs) such as TLR2, recognize lipopolysaccharide (LPS) of MTB and triggers vitamin D₃ activating enzymes. Activated vitamin D₃ induces the synthesis of antimicrobial peptide, LL-37, which further enhances xenophagy. Apart from vitamin D, few micronutrients such as zinc and iron also regulate autophagy. In this review, we discuss current knowledge, advances and perspectives of autophagy against TB.

[Iron metabolism and indicators reflecting its changes in pulmonary tuberculosis (literature review).]

Iron is an essential element for human and bacteria, including mycobacterium tuberculosis. Over review includesthe literature on the problem of iron metabolism in patients with tuberculosis and with comorbid pathology HIV infection and tuberculosis. The literature was searched for when writing this review using the RSCI, CyberLeninka, Scopus, Web of Science, MedLine, PubMed databases using the following keywords: iron, ferritin, hepsidin, lactoferrin, tuberculosis, pneumonia, HIV infection. Iron compounds are involved in many redox reactions: oxygen transport, cellular respiration, the trichloroacetic acid cycle, DNA biosynthesis, and others. The ratio of intracellular and extracellular iron in the body is regulated by the low molecular weight hormone hepcidin, the mechanism of action of which is to block the function of ferroportin, an exporter of iron from cells, which leads to the accumulation of the intracellular iron pool and the prevention of the toxic effect of free iron. The role of iron in the interaction of the human body with ferro-dependent bacteria has been established. Iron is necessary for the growth and development of bacterial cells, and the methods for its production from the host are different. Information on the effect of iron metabolism on pulmonary tuberculosis is scarce and contradictory: some authors have identified a decrease in hemoglobin and transferrin in combination with elevated levels of ferritin in patients with tuberculosis; according to other sources, hyperferritinemia in tuberculosis cannot be predictive, but is a marker inflammation. At the same time, there are studies that indicate a significant increase in ferritin in patients with disseminated tuberculosis relative to other clinical forms. Currently, the incidence of tuberculosis in patients with HIV infection is increasing, while diagnostic tests are not very informative. The search for diagnostic markers in terms of iron metabolism may open up new possibilities for the diagnosis of pulmonary tuberculosis.

Role of a 21-kDa iron-regulated protein IrpA in the uptake of ferri-exochelin by Mycobacterium smegmatis

AIMS

To characterize the 21-kDa iron-regulated cell wall protein in Mycobacterium smegmatis co-expressed with the siderophores mycobactin, exochelin and carboxymycobactin upon iron limitation.

METHODS AND RESULTS

Mycobacterium smegmatis, grown in the presence of 0·02 μg Fe ml^-1 (low iron) produced high levels of all the three siderophores, which were repressed in bacteria supplemented with 8 μg Fe ml^-1 (high iron). Exochelin, the major extracellular siderophore was the first to rise and was expressed at high levels during log phase of growth. Carboxymycobactin, a minor component in log phase iron-starved M. smegmatis continued to rise when cultured for longer periods, reaching levels greater than exochelin. Iron-starved bacteria expressed a 21-kDa iron-regulated protein (IrpA) that was identified as Clp protease subunit (MSMEG_3671) and characterized as a receptor for ferri-exochelin.

CONCLUSIONS

Ferri-exochelin is the preferred siderophore in M. smegmatis and this ferri-exochelin: IrpA machinery is absent in Mycobacterium tuberculosis.

SIGNIFICANCE AND IMPACT OF THE STUDY

Exochelin machinery is functional in M. smegmatis and the carboxymycobactin-mycobactin machinery is the sole iron uptake system in M. tuberculosis. The absence of the ferri-exochelin: IrpA system in the pathogen signifies the importance of the carboxymycobactin-mycobactin system machinery in M. tuberculosis.

Gram-scale preparation of the antibiotic lead compound salicyl-AMS, a potent inhibitor of bacterial salicylate adenylation enzymes

Salicyl-AMS (1) is a potent inhibitor of salicylate adenylation enzymes used in bacterial siderophore biosynthesis and a promising lead compound for the treatment of tuberculosis. An optimized, multigram synthesis is presented, which provides salicyl-AMS as its sodium salt (1·Na) in three synthetic steps followed by a two-step salt formation process. The synthesis proceeds in 11.6% overall yield from commercially available adenosine 2',3'-acetonide and provides highly purified material.

Comprehensive analysis of iron utilization by Mycobacterium tuberculosis

Iron is essential for nearly all bacterial pathogens, including Mycobacterium tuberculosis (Mtb), but is severely limited in the human host. To meet its iron needs, Mtb secretes siderophores, small molecules with high affinity for iron, and takes up iron-loaded mycobactins (MBT) and carboxymycobactins (cMBT), from the environment. Mtb is also capable of utilizing heme and hemoglobin which contain more than 70% of the iron in the human body. However, many components of these iron acquisition pathways are still unknown. In this study, a high-density transposon mutagenesis coupled with deep sequencing (TnSeq) showed that Mtb exhibits nearly opposite requirements for 165 genes in the presence of heme and hemoglobin versus MBT and cMBT as iron sources. The ESX-3 secretion system was assessed as essential for siderophore-mediated iron uptake and, surprisingly, also for heme utilization by Mtb. Predictions derived from the TnSeq analysis were validated by growth experiments with isogenic Mtb mutants. These results showed that (i) the efflux pump MmpL5 plays a dominant role in siderophore secretion, (ii) the Rv2047c protein is essential for growth of Mtb in the presence of mycobactin, and (iii) the transcriptional repressor Zur is required for heme utilization by Mtb. The novel genetic determinants of iron utilization revealed in this study will stimulate further experiments in this important area of Mtb physiology.

Integrated Target-Based and Phenotypic Screening Approaches for the Identification of Anti-Tubercular Agents That Bind to the Mycobacterial Adenylating Enzyme MbtA

Iron is essential for the pathogenicity and virulence of Mycobacterium tuberculosis, which synthesises salicyl-capped siderophores (mycobactins) to acquire this element from the host. MbtA is the adenylating enzyme that catalyses the initial reaction of mycobactin biosynthesis and is solely expressed by mycobacteria. A 3200-member library comprised of lead-like, structurally diverse compounds was screened against M. tuberculosis for whole-cell inhibitory activity. A set of 846 compounds that inhibited the tubercle bacilli growth were then tested for their ability to bind to MbtA using a fluorescence-based thermal shift assay and NMR-based Water-LOGSY and saturation transfer difference (STD) experiments. We identified an attractive hit molecule, 5-hydroxyindol-3-ethylamino-(2-nitro-4-trifluoromethyl)benzene (5), that bound with high affinity to MbtA and produced a MIC90 value of 13 μm. The ligand was docked into the MbtA crystal structure and displayed an excellent fit within the MbtA active pocket, adopting a binding mode different from that of the established MbtA inhibitor Sal-AMS.

Present and future of siderophore-based therapeutic and diagnostic approaches in infectious diseases

Iron is an essential micronutrient required for the growth of almost all aerobic organisms; the iron uptake pathway in bacteria therefore represents a possible target for novel antimicrobials, including hybrids between antimicrobials and siderophores. Siderophores are low molecular weight iron chelators that bind to iron and are actively transported inside the cell through specific binding protein complexes. These binding protein complexes are present both in Gram negative bacteria, in their outer and inner membrane, and in Gram positive bacteria in their cytoplasmic membrane. Most bacteria have the ability to produce siderophores in order to survive in environments with limited concentrations of free iron, however some bacteria synthetize natural siderophore-antibiotic conjugates that exploit the siderophore-iron uptake pathway to deliver antibiotics into competing bacterial cells and gain a competitive advantage. This approach has been referred to as a Trojan Horse Strategy. To overcome the increasing global problem of antibiotic resistance in Gram negative bacteria, which often have reduced outer membrane permeability, siderophore-antibiotic hybrid conjugates have been synthetized in vitro. Cefiderocol is the first siderophore-antibiotic conjugate that progressed to late stage clinical development so far. In studies on murine models the iron-siderophore uptake pathway has been also exploited for diagnostic imaging of infectious diseases, in which labelled siderophores have been used as specific probes. The aim of this review is to describe the research progress in the field of siderophore-based therapeutic and diagnostic approaches in infectious diseases.

Heme and hemoglobin utilization by Mycobacterium tuberculosis

Iron is essential for growth of Mycobacterium tuberculosis (Mtb), but most iron in the human body is stored in heme within hemoglobin. Here, we demonstrate that the substrate-binding protein DppA of the inner membrane Dpp transporter is required for heme and hemoglobin utilization by Mtb. The 1.27 Å crystal structure of DppA shows a tetrapeptide bound in the protein core and a large solvent-exposed crevice for heme binding. Mutation of arginine 179 in this cleft eliminates heme binding to DppA and prevents heme utilization by Mtb. The outer membrane proteins PPE36 and PPE62 are also required for heme and hemoglobin utilization, indicating that these pathways converge at the cell surface of Mtb. Albumin, the most abundant blood protein, binds heme specifically and bypasses the requirements for PPE36, PPE62 and Dpp. Thus, our study reveals albumin-dependent and -independent heme uptake pathways, highlighting the importance of iron acquisition from heme for Mtb.

A review on anti-tuberculosis peptides: Impact of peptide structure on anti-tuberculosis activity

Antibiotic resistance is a major public health problem globally. Particularly concerning amongst drug-resistant human pathogens is Mycobacterium tuberculosis that causes the deadly infectious tuberculosis (TB) disease. Significant issues associated with current treatment options for drug-resistant TB and the high rate of mortality from the disease makes the development of novel treatment options against this pathogen an urgent need. Antimicrobial peptides are part of innate immunity in all forms of life and could provide a potential solution against drug-resistant TB. This review is a critical analysis of antimicrobial peptides that are reported to be active against the M tuberculosis complex exclusively. However, activity on non-TB strains such as Mycobacterium avium and Mycobacterium intracellulare, whenever available, have been included at appropriate sections for these anti-TB peptides. Natural and synthetic antimicrobial peptides of diverse sequences, along with their chemical structures, are presented, discussed, and correlated to their observed antimycobacterial activities. Critical analyses of the structure allied to the anti-mycobacterial activity have allowed us to draw important conclusions and ideas for research and development on these promising molecules to realise their full potential. Even though the review is focussed on peptides, we have briefly summarised the structures and potency of the various small molecule drugs that are available and under development, for TB treatment.

Cross-resistance of isoniazid, para-aminosalicylic acid and pasiniazid against isoniazid-resistant Mycobacterium tuberculosis isolates in China

OBJECTIVES

Pasiniazid is a chemical complex of isoniazid (INH) and para-aminosalicylic acid (PAS). The aim of this study was to explore the cross-resistance of INH, PAS and pasiniazid against INH-resistant Mycobacterium tuberculosis isolates in China.

METHODS

A Microplate alamarBlue® Assay was performed to determine the minimum inhibitory concentrations (MICs) of INH, PAS and pasiniazid against 109 INH-resistant M. tuberculosis isolates. A statistical analysis of the relationship between different genotypes, gene mutations, and INH, PAS or pasiniazid susceptibility was then performed.

RESULTS

Among the 109 INH-resistant isolates, 13 (11.9%) and 21 (19.3%) showed resistance to PAS and pasiniazid, respectively. Among the 13 PAS-resistant M. tuberculosis isolates, 11 remained susceptible to pasiniazid. Of 63 INH-resistant isolates harbouring mutations in katG, the inhA promoter or the oxyR-ahpC intergenic region, 52 remained susceptible to pasiniazid. Moreover, 11 of 13 pasiniazid-resistant isolates carried mutations in katG, the inhA promoter or the oxyR-ahpC intergenic region.

CONCLUSION

Taken together, these results demonstrate that PAS resistance and mutations in thekatG gene, inhA promoter or oxyR-ahpC intergenic region in INH-resistant M. tuberculosis have little effect on pasiniazid susceptibility.