Induction of a novel class of diacylglycerol acyltransferases and triacylglycerol accumulation in Mycobacterium tuberculosis as it goes into a dormancy-like state in culture

Lipidome of Acinetobacter baumannii antibiotic persister cells

Persister cells constitute a bacterial subpopulation able to survive to high concentrations of antibiotics. This phenotype is temporary and reversible, and thus could be involved in the recurrence of infections and emergence of antibiotic resistance. To better understand how persister cells survive to such high antibiotic concentration, we examined changes in their lipid composition. We thus compared the lipidome of Acinetobacter baumannii ATCC 19606T persister cells formed under ciprofloxacin treatment with the lipidome of control cells grown without antibiotic. Using matrix assisted laser desorption ionisation-Fourier transform ion cyclotron resonance mass spectrometry, we observed a higher abundance of short chains and secondary chains without hydroxylation for lipid A in persister cells. Using liquid chromatography-tandem mass spectrometry, we found that persister cells produced particular phosphatidylglycerols, as LPAGPE and PAGPE, but also lipids with particular acyl chains containing additional hydroxyl group or uncommon di-unsaturation on C18 and C16 acyl chains. In order to determine the impact of these multiple lipidome modifications on membrane fluidity, fluorescence anisotropy assays were performed. They showed an increase of rigidity for the membrane of persister cells, inducing likely a decrease membrane permeability to protect cells during dormancy. Finally, we highlighted that A. baumannii persister cells also produced particular wax esters, composed of two fatty acids and a fatty diol. These uncommon storage lipids are key metabolites allowing a rapid bacterial regrow when antibiotic pressure disappears. These overall changes in persister lipidome may constitute new therapeutic targets to combat these particular dormant cells.

Host and bacterial lipid metabolism during tuberculosis infections: possibilities to synergise host- and bacteria-directed therapies

Mycobacterium tuberculosis (Mtb) is the causative pathogen of tuberculosis, the most lethal infectious disease resulting in 1.3 million deaths annually. Treatments against Mtb are increasingly impaired by the growing prevalence of antimicrobial drug resistance, which necessitates the development of new antibiotics or alternative therapeutic approaches. Upon infecting host cells, predominantly macrophages, Mtb becomes critically dependent on lipids as a source of nutrients. Additionally, Mtb produces numerous lipid-based virulence factors that contribute to the pathogen's ability to interfere with the host's immune responses and to create a lipid rich environment for itself. As lipids, lipid metabolism and manipulating host lipid metabolism play an important role for the virulence of Mtb, this review provides a state-of-the-art overview of mycobacterial lipid metabolism and concomitant role of host metabolism and host-pathogen interaction therein. While doing so, we will emphasize unexploited bacteria-directed and host-directed drug targets, and highlight potential synergistic drug combinations that hold promise for the development of new therapeutic interventions.

Host and pathogen factors that influence variability of Mycobacterium tuberculosis lipid body content in sputum from patients with tuberculosis: an observational study

BACKGROUND

High proportions of Mycobacterium tuberculosis cells in sputum containing triacylglycerol-rich lipid bodies have been shown to be associated with treatment failure or relapse following antituberculous chemotherapy. Although lipid body determination is a potential biomarker for supporting clinical trial and treatment decisions, factors influencing variability in sputum frequencies of lipid body-positive (%LB+) M tuberculosis in patients are unknown. We aimed to test our hypothesis that exposure to host-generated NO and M tuberculosis strains are factors associated with differences in sputum %LB+.

METHODS

In this observational study, we determined %LB+ frequencies before treatment by microscopy in patients with smear-positive tuberculosis from two separate prospective observational study settings (Gondar, Ethiopia, recruited between May 1, 2010, and April 30, 2011, and Fajara, The Gambia, who provided sputum samples before treatment between May 5, 2010, and Dec 22, 2011). In Ethiopia, fractional exhaled nitric oxide (FeNO) was measured as a biomarker of host NO, and M tuberculosis strain differences were determined by spoligotyping. Treatment response was assessed by percentage weight change after 7 months. In The Gambia, treatment responses were assessed as change in BMI and radiographic burden of disease after 6 months. Sputum M tuberculosis isolates were studied in vitro for their %LB+ and triacylglycerol synthase 1 (tgs1) mRNA responses to NO exposure. Propidium iodide staining was used as a measure of NO strain toxicity. Correlation between in vitro %LB+ frequencies following NO exposure and those of the same strain in sputum was examined with linear regression and Dunnett's multiple comparison test.

FINDINGS

In Ethiopia, 73 patients who were smear positive for pulmonary tuberculosis were recruited (43 [59%] were male and 30 [41%] were female). Of these, the %LB+ in the sputum of 59 patients showed linear correlation with log10 FeNO (r2=0·28; p<0·0001) and an association with strain spoligotype was suggested. Seven M tuberculosis strains from The Gambia showed different dose-responses to NO in vitro, demonstrated by changing lipid body content, tgs1 transcription, and bacterial toxicity. In sputum %LB+ frequencies correlated with in vitro %LB+ responses to NO of the corresponding isolate. In a subset of 34 patients across both cohorts, higher sputum %LB+ frequencies before treatment were associated with weaker responses to treatment than lower sputum %LB+ frequencies.

INTERPRETATION

M tuberculosis strain and exposure to host-generated NO are associated with sputum %LB+. Our results support the use of M tuberculosis strain-dependent sputum %LB+ as a predictive biomarker of treatment response.

FUNDING

The Medical Research Council, the University of Leicester, and the University of Gondar.

Identification of a 1-acyl-glycerol-3-phosphate acyltransferase from Mycobacterium tuberculosis, a key enzyme involved in triacylglycerol biosynthesis

Latent tuberculosis, caused by dormant Mycobacterium tuberculosis (Mtb), poses a threat to global health through the incubation of undiagnosed infections within the community. Dormant Mtb, which is phenotypically tolerant to antibiotics, accumulates triacylglycerol (TAG) utilizing fatty acids obtained from macrophage lipid droplets. TAG is vital to mycobacteria, serving as a cell envelope component and energy reservoir during latency. TAG synthesis occurs by sequential acylation of glycerol-3-phosphate, wherein the second acylation step is catalyzed by acylglycerol-3-phosphate acyltransferase (AGPAT), resulting in the production of phosphatidic acid (PA), a precursor for the synthesis of TAG and various phospholipids. Here, we have characterized a putative acyltransferase of Mtb encoded by Rv3816c. We found that Rv3816c has all four characteristic motifs of AGPAT, exists as a membrane-bound enzyme, and functions as 1-acylglycerol-3-phosphate acyltransferase. The enzyme could transfer the acyl group to acylglycerol-3-phosphate (LPA) from monounsaturated fatty acyl-coenzyme A of chain length 16 or 18 to produce PA. Complementation of Escherichia coli PlsC mutant in vivo by Rv3816c confirmed that it functions as AGPAT. Its active site mutants, H43A and D48A, were incapable of transferring the acyl group to LPA in vitro and were not able to rescue the growth defect of E. coli PlsC mutant in vivo. Identifying Rv3816c as AGPAT and comparing its properties with other AGPAT homologs is not only a step toward understanding the TAG biosynthesis in mycobacteria but has the potential to explore it as a drug target.

Intrabacterial lipid inclusion-associated proteins: a core machinery conserved from saprophyte Actinobacteria to the human pathogen Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb), the aetiologic agent of tuberculosis (TB), stores triacylglycerol (TAG) in the form of intrabacterial lipid inclusions (ILI) to survive and chronically persist within its host. These highly energetic molecules represent a major source of carbon to support bacterial persistence and reactivation, thus playing a leading role in TB pathogenesis. However, despite its physiological and clinical relevance, ILI metabolism in Mtb remains poorly understood. Recent discoveries have suggested that several ILI-associated proteins might be widely conserved across TAG-producing prokaryotes, but still very little is known regarding the nature and the biological functions of these proteins. Herein, we performed an in silico analysis of three independent ILI-associated proteomes previously reported to computationally define a potential core ILI-associated proteome, referred to as ILIome. Our investigation revealed the presence of 70 orthologous proteins that were strictly conserved, thereby defining a minimal ILIome core. We further narrowed our analysis to proteins involved in lipid metabolism and discuss here their putative biological functions, along with their molecular interactions and dynamics at the surface of these bacterial organelles. We also highlight the experimental limitations of the original proteomic investigations and of the present bioinformatic analysis, while describing new technological approaches and presenting biological perspectives in the field. The in silico investigation presented here aims at providing useful datasets that could constitute a scientific resource of broad interest for the mycobacterial community, with the ultimate goal of enlightening ILI metabolism in prokaryotes with a special emphasis on Mtb pathogenesis.

Wax Ester and Triacylglycerol Production in Acinetobacter baumannii: Role in Osmostress Protection, Reactive Oxygen Species, and Antibiotic Sensitivity

Wax esters (WEs) are neutral lipids that are produced by many different bacteria as potential carbon and energy storage compounds. Comparatively little is known about the role of WE in pathogenic bacteria. The opportunistic pathogen Acinetobacter baumannii is a major cause of hospital-acquired infections worldwide. Salt and desiccation resistance foster A. baumannii infections such as urinary tract infections and allow for reinfection when bacteria are taken up from dry surfaces in the hospital environment. Here we report on WE and triacylglycerol (TAG) production in A. baumannii as a response to nitrogen limitation and high salt stress. Fatty acids and fatty alcohols with chain lengths of C16 and C18 were identified as the most prominent WE constituents. We identified the terminal key enzyme of WE biosynthesis, the bifunctional wax ester synthase/acylCoA:diacylglycerol acyltransferase (WS/DGAT) encoded by the wax/dgat gene, and demonstrated that transcription of wax/dgat and production of WS/DGAT are independent of the nitrogen concentration. A Δwax/dgat mutant was impaired in growth in the presence of high salt concentration and was more sensitive to imipenem and reactive oxygen species.

Structural basis for triacylglyceride extraction from mycobacterial inner membrane by MFS transporter Rv1410

Mycobacterium tuberculosis is protected from antibiotic therapy by a multi-layered hydrophobic cell envelope. Major facilitator superfamily (MFS) transporter Rv1410 and the periplasmic lipoprotein LprG are involved in transport of triacylglycerides (TAGs) that seal the mycomembrane. Here, we report a 2.7 Å structure of a mycobacterial Rv1410 homologue, which adopts an outward-facing conformation and exhibits unusual transmembrane helix 11 and 12 extensions that protrude ~20 Å into the periplasm. A small, very hydrophobic cavity suitable for lipid transport is constricted by a functionally important ion-lock likely involved in proton coupling. Combining mutational analyses and MD simulations, we propose that TAGs are extracted from the core of the inner membrane into the central cavity via lateral clefts present in the inward-facing conformation. The functional role of the periplasmic helix extensions is to channel the extracted TAG into the lipid binding pocket of LprG.

Differences in responses to the intracellular macrophage environment between Mycobacterium bovis BCG vaccine strains Moreau and Pasteur

BACKGROUND

The Bacille Calmette-Guérin (BCG) vaccine comprises a family of strains with variable protective efficacy against pulmonary tuberculosis (TB) and leprosy, partly due to genetic differences between strains.

OBJECTIVES

Previous data highlighting differences between the genomes and proteomic profiles of BCG strains Moreau and Pasteur led us to evaluate their behaviour in the macrophage microenvironment, capable of stimulating molecular responses that can impact the protective effect of the vaccine.

METHODS

Strain infectivity, viability, co-localisation with acidified vesicles, macrophage secretion of IL-1 and MCP-1 and lipid droplet biogenesis were evaluated after infection.

FINDINGS

We found that BCG Moreau is internalised more efficiently, with significantly better intracellular survival up to 96 h p.i., whereas more BCG Pasteur bacilli were found co-localised in acidified vesicles up to 6 h p.i. IL-1β and MCP-1 secretion and lipid droplet biogenesis by infected macrophages were more prominent in response to BCG Pasteur.

MAIN CONCLUSION

Overall, our results show that, compared to Pasteur, BCG Moreau has increased fitness and better endurance in the harsh intracellular environment, also regulating anti-microbial responses (lower IL-1b and MCP-1). These findings contribute to the understanding of the physiology of BCG Moreau and Pasteur in response to the intraphagosomal environment in a THP-1 macrophage model.

Bridging the gaps to overcome major hurdles in the development of next-generation tuberculosis vaccines

Although tuberculosis (TB) remains one of the leading causes of death from an infectious disease worldwide, the development of vaccines more effective than bacille Calmette-Guérin (BCG), the only licensed TB vaccine, has progressed slowly even in the context of the tremendous global impact of TB. Most vaccine candidates have been developed to strongly induce interferon-γ (IFN-γ)-producing T-helper type 1 (Th1) cell responses; however, accumulating evidence has suggested that other immune factors are required for optimal protection against Mycobacterium tuberculosis (Mtb) infection. In this review, we briefly describe the five hurdles that must be overcome to develop more effective TB vaccines, including those with various purposes and tested in recent promising clinical trials. In addition, we discuss the current knowledge gaps between preclinical experiments and clinical studies regarding peripheral versus tissue-specific immune responses, different underlying conditions of individuals, and newly emerging immune correlates of protection. Moreover, we propose how recently discovered TB risk or susceptibility factors can be better utilized as novel biomarkers for the evaluation of vaccine-induced protection to suggest more practical ways to develop advanced TB vaccines. Vaccines are the most effective tools for reducing mortality and morbidity from infectious diseases, and more advanced technologies and a greater understanding of host-pathogen interactions will provide feasibility and rationale for novel vaccine design and development.

Mycobacterial formation of intracellular lipid inclusions is a dynamic process associated with rapid replication

Intracellular lipid inclusions (ILI) are triacylglyceride rich organelles produced by mycobacteria thought to serve as energy reservoirs. It is believed that ILI are formed as a result of a dosR mediated transition from replicative growth to non-replicating persistence (NRP). ILI rich Mycobacterium tuberculosis (Mtb) bacilli have been reported during infection and in sputum, establishing their importance in Mtb pathogenesis. Studies conducted in mycobacteria such as Mycobacterium smegmatis, Mycobacterium abscessus, or lab Mtb strains have demonstrated ILI formation in the presence of hypoxic, nitric oxide, nutrient limitation, or low nitrogen stress, conditions believed to emulate the host environment within which Mtb resides. Here, we show that M. marinum and clinical Mtb isolates make ILI during active replication in axenic culture independent of environmental stressors. By tracking ILI formation dynamics we demonstrate that ILI are quickly formed in the presence of fresh media or exogenous fatty acids but are rapidly depleted while bacteria are still actively replicating. We also show that the cell envelope is an alternate site for neutral lipid accumulation observed during stationary phase. In addition, we screen a panel of 60 clinical isolates and observe variation in ILI production during early log phase growth between and among Mtb lineages. Finally, we show that dosR expression level does not strictly correlate with ILI accumulation in fresh clinical isolates. Taken together, our data provide evidence of an active ILI formation pathway in replicating mycobacteria cultured in the absence of stressors, suggesting a decoupling of ILI formation from NRP.

Pangenome Reconstruction of Mycobacterium tuberculosis as a Guide to Reveal Genomic Features Associated with Strain Clinical Phenotype

Tuberculosis (TB) is one of the leading causes of human deaths worldwide caused by infectious diseases. TB infection by Mycobacterium tuberculosis can occur in the lungs, causing pulmonary tuberculosis (PTB), or in any other organ of the body, resulting in extrapulmonary tuberculosis (EPTB). There is no consensus on the genetic determinants of this pathogen that may contribute to EPTB. In this study, we constructed the M. tuberculosis pangenome and used it as a tool to seek genomic signatures associated with the clinical presentation of TB based on its accessory genome differences. The analysis carried out in the present study includes the raw reads of 490 M. tuberculosis genomes (PTB n = 245, EPTB n = 245) retrieved from public databases that were assembled, as well as ten genomes from Mexican strains (PTB n = 5, EPTB n = 5) that were sequenced and assembled. All genomes were annotated and then used to construct the pangenome with Roary and Panaroo. The pangenome obtained using Roary consisted of 2231 core genes and 3729 accessory genes. On the other hand, the pangenome resulting from Panaroo consisted of 2130 core genes and 5598 accessory genes. Associations between the distribution of accessory genes and the PTB/EPTB phenotypes were examined using the Scoary and Pyseer tools. Both tools found a significant association between the hspR, plcD, Rv2550c, pe_pgrs5, pe_pgrs25, and pe_pgrs57 genes and the PTB genotype. In contrast, the deletion of the aceA, esxR, plcA, and ppe50 genes was significantly associated with the EPTB phenotype. Rv1759c and Rv3740 were found to be associated with the PTB phenotype according to Scoary; however, these associations were not observed when using Pyseer. The robustness of the constructed pangenome and the gene-phenotype associations is supported by several factors, including the analysis of a large number of genomes, the inclusion of the same number of PTB/EPTB genomes, and the reproducibility of results thanks to the different bioinformatic tools used. Such characteristics surpass most of previous M. tuberculosis pangenomes. Thus, it can be inferred that the deletion of these genes can lead to changes in the processes involved in stress response and fatty acid metabolism, conferring phenotypic advantages associated with pulmonary or extrapulmonary presentation of TB. This study represents the first attempt to use the pangenome to seek gene-phenotype associations in M. tuberculosis.

Mycobacterium abscessus DosRS two-component system controls a species-specific regulon required for adaptation to hypoxia

Mycobacterium abscessus (Mab), an emerging opportunistic pathogen, predominantly infects individuals with underlying pulmonary diseases such as cystic fibrosis (CF). Current treatment outcomes for Mab infections are poor due to Mab's inherent antibiotic resistance and unique host interactions that promote phenotypic tolerance and hinder drug access. The hypoxic, mucus-laden airways in the CF lung and antimicrobial phagosome within macrophages represent hostile niches Mab must overcome via alterations in gene expression for survival. Regulatory mechanisms important for the adaptation and long-term persistence of Mab within the host are poorly understood, warranting further genetic and transcriptomics study of this emerging pathogen. DosRS Mab , a two-component signaling system (TCS), is one proposed mechanism utilized to subvert host defenses and counteract environmental stress such as hypoxia. The homologous TCS of Mycobacterium tuberculosis (Mtb), DosRS Mtb , is known to induce a ~50 gene regulon in response to hypoxia, carbon monoxide (CO) and nitric oxide (NO) in vitro and in vivo. Previously, a small DosR Mab regulon was predicted using bioinformatics based on DosR Mtb motifs however, the role and regulon of DosRS Mab in Mab pathogenesis have yet to be characterized in depth. To address this knowledge gap, our lab generated a Mab dosRS knockout strain (MabΔdosRS) to investigate differential gene expression, and phenotype in an in vitro hypoxia model of dormancy. qRT-PCR and lux reporter assays demonstrate Mab_dosR and 6 predicted downstream genes are induced in hypoxia. In addition, RNAseq revealed induction of a much larger hypoxia response comprised of >1000 genes, including 127 differentially expressed genes in a dosRS mutant strain. Deletion of DosRS Mab led to attenuated growth under low oxygen conditions, a shift in morphotype from smooth to rough, and down-regulation of 216 genes. This study provides the first look at the global transcriptomic response of Mab to low oxygen conditions encountered in the airways of CF patients and within macrophage phagosomes. Our data also demonstrate the importance of DosRS Mab for adaptation of Mab to hypoxia, highlighting a distinct regulon (compared to Mtb) that is significantly larger than previously described, including both genes conserved across mycobacteria as well as Mab-specific genes.

Flow cytometry: a tool for understanding the behaviour of polyhydroxyalkanoate accumulators

The use of mixed microbial cultures (MMCs) is seen as an attractive strategy for polyhydroxyalkanoate (PHA) production. In order to optimize the MMC-PHA production process, tools are required to improve our understanding of the physiological state of the PHA-storing microorganisms within the MMC. In the present study, we explored the use of flow cytometry to analyse the metabolic state and polyhydroxybutyrate (PHB) content of the microorganisms from an MMC-PHA production process. A sequencing batch reactor under a feast and famine regime was used to enrich an MMC with PHB-storing microorganisms. Interestingly, once the PHB-storing microorganisms are selected, the level of PHB accumulation depends largely on the metabolic state of these microorganisms and not exclusively on the consortium composition. These results demonstrate that flow cytometry is a powerful tool to help to understand the PHA storage response of an MMC-PHA production process. KEY POINTS: • Flow cytometry allows to measure PHB content and metabolic activity over time. • Microorganisms showing high PHB content also have high metabolic activity. • PHB producers with low metabolic activity show low PHB content.

Immunological aspects of host-pathogen crosstalk in the co-pathogenesis of diabetes and latent tuberculosis

Introduction

Diabetes is a potent risk factor for the activation of latent tuberculosis and worsens the tuberculosis (TB) treatment outcome. The major reason for mortality and morbidity in diabetic patients is due to their increased susceptibility to TB. Thus, the study was conducted to understand the crosstalk between M. tuberculosis and its host upon latent tuberculosis infection and under hyperglycemic conditions or diabetes.

Methods

An animal model was employed to study the relationship between latent tuberculosis and diabetes. BCG immunization was done in mice before infection with M. tuberculosis, and latency was confirmed by bacillary load, histopathological changes in the lungs and gene expression of hspX, tgs1, tgs3 and tgs5. Diabetes was then induced by a single high dose of streptozotocin (150 mg/kg body weight). Host factors, like various cytokines and MMPs (Matrix metalloproteinases), which play an important role in the containment of mycobacterial infection were studied in vivo and in vitro.

Results

A murine model of latent TB was developed, which was confirmed by CFU counts (<10⁴ in the lungs and spleen) and granuloma formation in lungs in the latent TB group. Also, the gene expression of hspX, tgs1, and tgs5 was upregulated, and after diabetes induction, blood glucose levels were >200 mg/dl. An in vitro study employing a THP-1 macrophage model of latent and active tuberculosis under normal and high glucose conditions showed that dormant bacilli were better contained in the presence of 5.5 mM glucose concentration as compared with active bacilli. However, the killing and restriction efficiency of macrophages decreased, and CFU counts increased significantly with an increase in glucose concentration.

Discussion

The decreased levels of MCP-1, decreased expression of mmp-9, and increased expression of mmp-1 in the latent group at high glucose concentrations could explain the failure of granuloma formation at high glucose conditions.

Pan-genome association study of Mycobacterium tuberculosis lineage-4 revealed specific genes related to the high and low prevalence of the disease in patients from the North-Eastern area of Medellín, Colombia

Mycobacterium tuberculosis (Mtb) lineage 4 is responsible for the highest burden of tuberculosis (TB) worldwide. This lineage has been the most prevalent lineage in Colombia, especially in the North-Eastern (NE) area of Medellin, where it has been shown to have a high prevalence of LAM9 SIT42 and Haarlem1 SIT62 sublineages. There is evidence that regardless of environmental factors and host genetics, differences among sublineages of Mtb strains play an important role in the course of infection and disease. Nevertheless, the genetic basis of the success of a sublineage in a specific geographic area remains uncertain. We used a pan-genome-wide association study (pan-GWAS) of 47 Mtb strains isolated from NE Medellin between 2005 and 2008 to identify the genes responsible for the phenotypic differences among high and low prevalence sublineages. Our results allowed the identification of 12 variants in 11 genes, of which 4 genes showed the strongest association to low prevalence (mmpL12, PPE29, Rv1419, and Rv1762c). The first three have been described as necessary for invasion and intracellular survival. Polymorphisms identified in low prevalence isolates may suggest related to a fitness cost of Mtb, which might reflect a decrease in their capacity to be transmitted or to cause an active infection. These results contribute to understanding the success of some sublineages of lineage-4 in a specific geographical area.

slr2103, a homolog of type-2 diacylglycerol acyltransferase genes, for plastoquinone-related neutral lipid synthesis and NaCl-stress acclimatization in a cyanobacterium, Synechocystis sp. PCC 6803

A cyanobacterium, Synechocystis sp. PCC 6803, contains a lipid with triacylglycerol-like TLC mobility but its identity and physiological roles remain unknown. Here, on ESI-positive LC-MS² analysis, it is shown that the triacylglycerol-like lipid (lipid X) is related to plastoquinone and can be grouped into two subclasses, X_a and X_b, the latter of which is esterified by 16:0 and 18:0. This study further shows that a Synechocystis homolog of type-2 diacylglycerol acyltransferase genes, slr2103, is essential for lipid X synthesis: lipid X disappears in a Synechocystis slr2103-disruptant whereas it appears in an slr2103-overexpressing transformant (OE) of Synechococcus elongatus PCC 7942 that intrinsically lacks lipid X. The slr2103 disruption causes Synechocystis cells to accumulate plastoquinone-C at an abnormally high level whereas slr2103 overexpression in Synechococcus causes the cells to almost completely lose it. It is thus deduced that slr2103 encodes a novel acyltransferase that esterifies 16:0 or 18:0 with plastoquinone-C for the synthesis of lipid X_b. Characterization of the slr2103-disruptant in Synechocystis shows that slr2103 contributes to sedimented-cell growth in a static culture, and to bloom-like structure formation and its expansion by promoting cell aggregation and floatation upon imposition of saline stress (0.3-0.6 M NaCl). These observations provide a basis for elucidation of the molecular mechanism of a novel cyanobacterial strategy to acclimatize to saline stress, and one for development of a system of seawater-utilization and economical harvesting of cyanobacterial cells with high-value added compounds, or blooming control of toxic cyanobacteria.

Clinical strains of Mycobacterium tuberculosis exhibit differential lipid metabolism-associated transcriptome changes in in vitro cholesterol and infection models

Many studies have identified host-derived lipids, characterised by the abundance of cholesterol, as a major source of carbon nutrition for Mycobacterium tuberculosis during infection. Members of the Mycobacterium tuberculosis complex are biologically different with regards to degree of disease, host range, pathogenicity and transmission. Therefore, the current study aimed at elucidating transcriptome changes during early infection of pulmonary epithelial cells and on an in vitro cholesterol-rich minimal media, in M. tuberculosis clinical strains F15/LAM4/KZN and Beijing, and the laboratory H37Rv strain. Infection of pulmonary epithelial cells elicited the upregulation of fadD28 and hsaC in both the F15/LAM4/KZN and Beijing strains and the downregulation of several other lipid-associated genes. Growth curve analysis revealed F15/LAM4/KZN and Beijing to be slow growers in 7H9 medium and cholesterol-supplemented media. RNA-seq analysis revealed strain-specific transcriptomic changes, thereby affecting different metabolic processes in an in vitro cholesterol model. The differential expression of these genes suggests that the genetically diverse M. tuberculosis clinical strains exhibit strain-specific behaviour that may influence their ability to metabolise lipids, specifically cholesterol, which may account for phenotypic differences observed during infection.

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.

The High Plasticity of Nonpathogenic Mycobacterium brumae Induces Rapid Changes in Its Lipid Profile during Pellicle Maturation: The Potential of This Bacterium as a Versatile Cell Factory for Lipid Compounds of Therapeutic Interest

The immunomodulatory potential of mycobacteria to be used for therapeutic purposes varies by species and culture conditions and is closely related to mycobacterial lipid composition. Although the lipids present in the mycobacterial cell wall are relevant, lipids are mainly stored in intracellular lipid inclusions (ILIs), which have emerged as a crucial structure in understanding mycobacteria-host interaction. Little is known about ILI ultrastructure, production, and composition in nonpathogenic species. In this study, we compared the lipid profiles of the nonpathogenic immunomodulatory agent Mycobacterium brumae during pellicle maturation under different culture conditions with qualitative and quantitative approaches by using high-resolution imaging and biochemical and composition analyses to understand ILI dynamics. The results showed wax esters, mainly in early stages of development, and acylglycerols in mature ILI composition, revealing changes in dynamics, amount, and morphometry, depending on pellicle maturation and the culture media used. Low-glycerol cultures induced ILIs with lower molecular weights which were smaller in size in comparison with the ILIs produced in glycerol-enriched media. The data also indicate the simple metabolic plasticity of lipid synthesis in M. brumae, as well as its high versatility in generating different lipid profiles. These findings provide an interesting way to enhance the production of key lipid structures via the simple modulation of cell culture conditions.

The role of triacylglycerols and repurposing DGAT1 inhibitors for the treatment of Mycobacterium tuberculosis

Latent tuberculosis poses a significant threat to global health through the incubation of undiagnosed infections within the community, and through its tolerance to antibiotics. This Special Features article explores the mechanisms by which the dormant Mycobacterium tuberculosis pathogen can store energy in the form of lipid inclusion bodies and triacylglycerols, which may be key in the development of novel therapeutics to treat TB.

Drug resistant tuberculosis: Implications for transmission, diagnosis, and disease management

Drug resistant tuberculosis contributes significantly to the global burden of antimicrobial resistance, often consuming a large proportion of the healthcare budget and associated resources in many endemic countries. The rapid emergence of resistance to newer tuberculosis therapies signals the need to ensure appropriate antibiotic stewardship, together with a concerted drive to develop new regimens that are active against currently circulating drug resistant strains. Herein, we highlight that the current burden of drug resistant tuberculosis is driven by a combination of ongoing transmission and the intra-patient evolution of resistance through several mechanisms. Global control of tuberculosis will require interventions that effectively address these and related aspects. Interrupting tuberculosis transmission is dependent on the availability of novel rapid diagnostics which provide accurate results, as near-patient as is possible, together with appropriate linkage to care. Contact tracing, longitudinal follow-up for symptoms and active mapping of social contacts are essential elements to curb further community-wide spread of drug resistant strains. Appropriate prophylaxis for contacts of drug resistant index cases is imperative to limit disease progression and subsequent transmission. Preventing the evolution of drug resistant strains will require the development of shorter regimens that rapidly eliminate all populations of mycobacteria, whilst concurrently limiting bacterial metabolic processes that drive drug tolerance, mutagenesis and the ultimate emergence of resistance. Drug discovery programs that specifically target bacterial genetic determinants associated with these processes will be paramount to tuberculosis eradication. In addition, the development of appropriate clinical endpoints that quantify drug tolerant organisms in sputum, such as differentially culturable/detectable tubercle bacteria is necessary to accurately assess the potential of new therapies to effectively shorten treatment duration. When combined, this holistic approach to addressing the critical problems associated with drug resistance will support delivery of quality care to patients suffering from tuberculosis and bolster efforts to eradicate this disease.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Loss of RNase J leads to multi-drug tolerance and accumulation of highly structured mRNA fragments in Mycobacterium tuberculosis

Despite the existence of well-characterized, canonical mutations that confer high-level drug resistance to Mycobacterium tuberculosis (Mtb), there is evidence that drug resistance mechanisms are more complex than simple acquisition of such mutations. Recent studies have shown that Mtb can acquire non-canonical resistance-associated mutations that confer survival advantages in the presence of certain drugs, likely acting as stepping-stones for acquisition of high-level resistance. Rv2752c/rnj, encoding RNase J, is disproportionately mutated in drug-resistant clinical Mtb isolates. Here we show that deletion of rnj confers increased tolerance to lethal concentrations of several drugs. RNAseq revealed that RNase J affects expression of a subset of genes enriched for PE/PPE genes and stable RNAs and is key for proper 23S rRNA maturation. Gene expression differences implicated two sRNAs and ppe50-ppe51 as important contributors to the drug tolerance phenotype. In addition, we found that in the absence of RNase J, many short RNA fragments accumulate because they are degraded at slower rates. We show that the accumulated transcript fragments are targets of RNase J and are characterized by strong secondary structure and high G+C content, indicating that RNase J has a rate-limiting role in degradation of highly structured RNAs. Taken together, our results demonstrate that RNase J indirectly affects drug tolerance, as well as reveal the endogenous roles of RNase J in mycobacterial RNA metabolism.

Insights Into the Biogenesis and Emerging Functions of Lipid Droplets From Unbiased Molecular Profiling Approaches

Lipid droplets (LDs) are spherical, single sheet phospholipid-bound organelles that store neutral lipids in all eukaryotes and some prokaryotes. Initially conceived as relatively inert depots for energy and lipid precursors, these highly dynamic structures play active roles in homeostatic functions beyond metabolism, such as proteostasis and protein turnover, innate immunity and defense. A major share of the knowledge behind this paradigm shift has been enabled by the use of systematic molecular profiling approaches, capable of revealing and describing these non-intuitive systems-level relationships. Here, we discuss these advances and some of the challenges they entail, and highlight standing questions in the field.

A combination screening to identify enhancers of para-aminosalicylic acid against Mycobacterium tuberculosis

Para-aminosalicylic acid (PAS) is an antibiotic that was largely used for the multi-therapy of tuberculosis in the twentieth century. To try to overcome the inconvenience of its low efficacy and poor tolerance, we searched for novel chemical entities able to synergize with PAS using a combination screening against growing axenic Mycobacterium tuberculosis. The screening was performed at a sub-inhibitory concentration of PAS on a library of about 100,000 small molecules. Selected hit compounds were analyzed by dose–response and further probed with an intracellular macrophage assay. Scaffolds with potential additive effect with PAS are reported, opening interesting prospects for mechanism of action studies. We also report here evidence of a yet unknown bio-activation mechanism, involving activation of pyrido[1,2-a]pyrimidin-4-one (PP) derivatives through the Rv3087 protein.

Mycobacterium tuberculosis Affects Protein and Lipid Content of Circulating Exosomes in Infected Patients Depending on Tuberculosis Disease State

Tuberculosis (TB), which is caused by the bacterium Mycobacterium tuberculosis (Mtb), is still one of the deadliest infectious diseases. Understanding how the host and pathogen interact in active TB will have a significant impact on global TB control efforts. Exosomes are increasingly recognized as a means of cell-to-cell contact and exchange of soluble mediators. In the case of TB, exosomes are released from the bacillus and infected cells. In the present study, a comprehensive lipidomics and proteomics analysis of size exclusion chromatography-isolated plasma-derived exosomes from patients with TB lymphadenitis (TBL) and treated as well as untreated pulmonary TB (PTB) was performed to elucidate the possibility to utilize exosomes in diagnostics and knowledge building. According to our findings, exosome-derived lipids and proteins originate from both the host and Mtb in the plasma of active TB patients. Exosomes from all patients are mostly composed of sphingomyelins (SM), phosphatidylcholines, phosphatidylinositols, free fatty acids, triacylglycerols (TAG), and cholesterylesters. Relative proportions of, e.g., SMs and TAGs, vary depending on the disease or treatment state and could be linked to Mtb pathogenesis and dormancy. We identified three proteins of Mtb origin: DNA-directed RNA polymerase subunit beta (RpoC), Diacyglycerol O-acyltransferase (Rv2285), and Formate hydrogenase (HycE), the latter of which was discovered to be differently expressed in TBL patients. Furthermore, we discovered that Mtb infection alters the host protein composition of circulating exosomes, significantly affecting a total of 37 proteins. All TB patients had low levels of apolipoproteins, as well as the antibacterial proteins cathelicidin, Scavenger Receptor Cysteine Rich Family Member (SSC5D), and Ficolin 3 (FCN3). When compared to healthy controls, the protein profiles of PTB and TBL were substantially linked, with 14 proteins being co-regulated. However, adhesion proteins (integrins, Intercellular adhesion molecule 2 (ICAM2), CD151, Proteoglycan 4 (PRG4)) were shown to be more prevalent in PTB patients, while immunoglobulins, Complement component 1r (C1R), and Glutamate receptor-interacting protein 1 (GRIP1) were found to be more abundant in TBL patients, respectively. This study could confirm findings from previous reports and uncover novel molecular profiles not previously in focus of TB research. However, we applied a minimally invasive sampling and analysis of circulating exosomes in TB patients. Based on the findings given here, future studies into host–pathogen interactions could pave the way for the development of new vaccines and therapies.

Environment dependent expression of mycobacterium hormone sensitive lipases: expression pattern under ex-vivo and individual in-vitro stress conditions in M. tuberculosis H37Ra

BACKGROUND

Hormone-sensitive lipase (HSL) is a neutral lipase capable of hydrolysing various kinds of lipids. In comparison to single human Hormone Sensitive Lipase (hHSL), that is induced under nutritional stress, twelve serine hydrolases are annotated as HSL in Mycobacterium tuberculosis (mHSL). Mycobacterium is exposed to multiple stresses inside the host. Therefore, the present study was carried out to investigate if mHSL are also expressed under stress condition and if there is any correlation between various stress conditions and expression pattern of mHSL.

METHODS AND RESULTS

The expression pattern of mHSL under different environmental conditions (in-vitro and ex-vivo) were studied using qRT-PCR in M. tuberculosis H37Ra strain with 16 S rRNA as internal control. Out of 12, only two genes (lipU and lipY) were expressed at very low level in mid log phase culture under aerobic conditions, while 9 genes were expressed at stationary phase of growth. Ten mHSLs were expressed post-infection under ex-vivo conditions in time dependent manner. LipH and lipQ did not express at any time point under ex-vivo condition. The relative expression of most of the genes under individual stress was much higher than observed in ex-vivo conditions. The expression pattern of genes varied with change in stress condition.

CONCLUSIONS

Different sets of mHSL genes were expressed under different individual stress conditions pointing towards the requirement of different mHSL to combat different stress conditions. Overall, most of the mHSLs have demonstrated stress dependent expression pointing towards their role in intracellular survival of mycobacteria.

Host lipidome and tuberculosis treatment failure

INTRODUCTION

Host lipids play important roles in tuberculosis (TB) pathogenesis. Whether host lipids at TB treatment initiation (baseline) affect subsequent treatment outcomes has not been well characterised. We used unbiased lipidomics to study the prospective association of host lipids with TB treatment failure.

METHODS

A case-control study (n=192), nested within a prospective cohort study, was used to investigate the association of baseline plasma lipids with TB treatment failure among adults with pulmonary TB. Cases (n=46) were defined as TB treatment failure, while controls (n=146) were those without failure. Complex lipids and inflammatory lipid mediators were measured using liquid chromatography mass spectrometry techniques. Adjusted least-square regression was used to assess differences in groups. In addition, machine learning identified lipids with highest area under the curve (AUC) to classify cases and controls.

RESULTS

Baseline levels of 32 lipids differed between controls and those with treatment failure after false discovery rate adjustment. Treatment failure was associated with lower baseline levels of cholesteryl esters and oxylipin, and higher baseline levels of ceramides and triglycerides compared to controls. Two cholesteryl ester lipids combined in a unique classifier model provided an AUC of 0.79 (95% CI 0.65-0.93) in the test dataset for prediction of TB treatment failure.

CONCLUSIONS

We identified lipids, some with known roles in TB pathogenesis, associated with TB treatment failure. In addition, a lipid signature with prognostic accuracy for TB treatment failure was identified. These lipids could be potential targets for risk-stratification, adjunct therapy and treatment monitoring.

Insights into the molecular determinants involved in Mycobacterium tuberculosis persistence and their therapeutic implications

The establishment of persistent infections and the reactivation of persistent bacteria to active bacilli are the two hurdles in effective tuberculosis treatment. Mycobacterium tuberculosis, an etiologic tuberculosis agent, adapts to numerous antibiotics and resists the host immune system causing a disease of public health concern. Extensive research has been employed to combat this disease due to its sheer ability to persist in the host system, undetected, waiting for the opportunity to declare itself. Persisters are a bacterial subpopulation that possesses transient tolerance to high doses of antibiotics. There are certain inherent mechanisms that facilitate the persister cell formation in Mycobacterium tuberculosis, some of those had been characterized in the past namely, stringent response, transcriptional regulators, energy production pathways, lipid metabolism, cell wall remodeling enzymes, phosphate metabolism, and proteasome protein degradation. This article reviews the recent advancements made in various in vitro persistence models that assist to unravel the mechanisms involved in the persister cell formation and to hunt for the possible preventive or treatment measures. To tackle the persister population the immunodominant proteins that express specifically at the latent phase of infection can be used for diagnosis to distinguish between the active and latent tuberculosis, as well as to select potential drug or vaccine candidates. In addition, we discuss the genes engaged in the persistence to get more insights into resuscitation and persister cell formation. The in-depth understanding of persistent cells of mycobacteria can certainly unravel novel ways to target the pathogen and tackle its persistence.

In silico comparisons of lipid-related genes between Mycobacterium tuberculosis and BCG vaccine strains

Despite highly variable efficacy, BCG (Bacillus Calmette-Guérin) is the only vaccine available to prevent the tuberculosis (TB). Genomic heterogeneity between attenuated BCG strains and virulent Mycobacterium tuberculosis might help to explain this vaccine’s impaired capacity to induce long-term protection. Here, we investigate the lipid-related genes absent in attenuated BCG strains in order to correlate changes in both lipid metabolism and cell-wall lipid content to vaccine impairment. Whole genome sequences of M. tuberculosis H37Rv and the six most used BCG strains worldwide were aligned and the absent regions functionally categorized. Genomes of the BCG strains showed a total of 14 non-homologous lipid-related genes, including those belonging to mce3 operon, as well as the gene echaA1, which encodes an enoyl-CoA hydratase, and the genes encoding phospholipases PlcA, PlcB and PlcC. Taken together, the depletion of these M. tuberculosis H37Rv genomic regions were associated with marked alterations in lipid-related genes of BCG strains. Such alterations may indicate a dormant-like state and can be determining factors to the vaccine’s inability to induce long-term protection. These lipids can be further evaluated as an adjuvant to boost the current BCG-based vaccine.

Quantitative proteomics reveals that dormancy-related proteins mediate the attenuation in mycobacterium strains

Although members of the Mycobacterium tuberculosis complex (MTBC) exhibit high similarity, they are characterized by differences with respect to virulence, immune response, and transmissibility. To understand the virulence of these bacteria and identify potential novel therapeutic targets, we systemically investigated the total cell protein contents of virulent H37Rv, attenuated H37Ra, and avirulent M. bovis BCG vaccine strains at the log and stationary phases, based on tandem mass tag (TMT) quantitative proteomics. Data analysis revealed that we obtained deep-coverage protein identification and high quantification. Although 272 genetic variations were reported in H37Ra and H37Rv, they showed very little expression difference in log and stationary phase. Quantitative comparison revealed H37Ra and H37Rv had significantly dysregulation in log phase (227) compared with stationary phase (61). While BCG and H37Rv, and BCG and H37Ra showed notable differences in stationary phase (1171 and 1124) with respect to log phase (381 and 414). In the log phase, similar patterns of protein abundance were observed between H37Ra and BCG, whereas a more similar expression pattern was observed between H37Rv and H37Ra in the stationary phase. Bioinformatic analysis revealed that the upregulated proteins detected for H37Rv and H37Ra in log phase were virulence-related factors. In both log and stationary phases, the dysregulated proteins detected for BCG, which have also been identified as M. tuberculosis response proteins under dormancy conditions. We accordingly describe the proteomic profiles of H37Rv, H37Ra, and BCG, which we believe will potentially provide a better understanding of H37Rv pathogenesis, H37Ra attenuation, and BCG immuno protection.

Understanding Metabolic Remodeling in Mycobacterium smegmatis to Overcome Energy Exigency and Reductive Stress Under Energy-Compromised State

Mycobacteria such as Mycobacterium tuberculosis, the causative agent of tuberculosis that annually kills several million people worldwide, and Mycobacterium smegmatis, the non-pathogenic fast-growing mycobacteria, require oxidative phosphorylation to meet their energy requirements. We have previously shown that deletion of one of the two copies of atpD gene that codes for the ATP synthase β-subunit establishes an energy-compromised state in M. smegmatis. Here we report that upon such deletion, a major routing of electron flux occurs through the less energy-efficient complexes of its respiratory chain. ΔatpD bacterium also shows an increased reduced state which is further confirmed by the overexpression of WhiB3, a major redox sensor. We show a substantial modulation of the biosynthesis of cell wall associated lipids and triacylglycerol (TAG). An accumulation of TAG-containing lipid bodies is further confirmed by using ¹⁴C oleate incorporation. Interestingly, the mutant also shows an overexpression of TAG-degrading lipase genes, and the intracellular lipolytic enzymes mediate TAG hydrolysis for their utilization as energy source. We believe that our in vitro energy-depleted model will allow us to explore the critical link between energy metabolism, redox homeostasis, and lipid biosynthesis during ATP-depleted state, which will enhance our understanding of the bacterial adaptation, and will allow us to identify novel drug targets to counter mycobacterial infections.

Structural Changes in the Cap of Rv0183/mtbMGL Modulate the Shape of the Binding Pocket

Tuberculosis continues to be a major threat to the human population. Global efforts to eradicate the disease are ongoing but are hampered by the increasing occurrence of multidrug-resistant strains of Mycobacterium tuberculosis. Therefore, the development of new treatment, and the exploration of new druggable targets and treatment strategies, are of high importance. Rv0183/mtbMGL, is a monoacylglycerol lipase of M. tuberculosis and it is involved in providing fatty acids and glycerol as building blocks and as an energy source. Since the lipase is expressed during the dormant and active phase of an infection, Rv0183/mtbMGL is an interesting target for inhibition. In this work, we determined the crystal structures of a surface-entropy reduced variant K74A Rv0183/mtbMGL in its free form and in complex with a substrate mimicking inhibitor. The two structures reveal conformational changes in the cap region that forms a major part of the substrate/inhibitor binding region. We present a completely closed conformation in the free form and semi-closed conformation in the ligand-bound form. These conformations differ from the previously published, completely open conformation of Rv0183/mtbMGL. Thus, this work demonstrates the high conformational plasticity of the cap from open to closed conformations and provides useful insights into changes in the substrate-binding pocket, the target of potential small-molecule inhibitors.

Microbial synthesis of wax esters

Microbial synthesis of wax esters (WE) from low-cost renewable and sustainable feedstocks is a promising path to achieve cost-effectiveness in biomanufacturing. WE are industrially high-value molecules, which are widely used for applications in chemical, pharmaceutical, and food industries. Since the natural WE resources are limited, the WE production mostly rely on chemical synthesis from rather expensive starting materials, and therefore solution are sought from development of efficient microbial cell factories. Here we report to engineer the yeast Yarrowia lipolytica and bacterium Escherichia coli to produce WE at the highest level up to date. First, the key genes encoding fatty acyl-CoA reductases and wax ester synthase from different sources were investigated, and the expression system for two different Y. lipolytica hosts were compared and optimized for enhanced WE production and the strain stability. To improve the metabolic pathway efficiency, different carbon sources including glucose, free fatty acid, soybean oil, and waste cooking oil (WCO) were compared, and the corresponding pathway engineering strategies were optimized. It was found that using a lipid substrate such as WCO to replace glucose led to a 60-fold increase in WE production. The engineered yeast was able to produce 7.6 g/L WE with a yield of 0.31 (g/g) from WCO within 120 h and the produced WE contributed to 57% of the yeast DCW. After that, E. coli BL21(DE3), with a faster growth rate than the yeast, was engineered to significantly improve the WE production rate. Optimization of the expression system and the substrate feeding strategies led to production of 3.7-4.0 g/L WE within 40 h in a 1-L bioreactor. The predominant intracellular WE produced by both Y. lipolytica and E. coli in the presence of hydrophobic substrates as sole carbon sources were C₃₆, C₃₄ and C₃₂, in an order of decreasing abundance and with a large proportion being unsaturated. This work paved the way for the biomanufacturing of WE at a large scale.

Role of resuscitation promoting factors in reactivation of latent tuberculosis during high glucose conditions employing human granuloma in vitro model

BACKGROUND

Diabetes aggravates the risk of tuberculosis (TB) through impairment of immunity which may lead to the activation of latent tuberculosis (LTBI). LTBI serves as a homeostatic state where host does not develop any symptoms of the disease as host immune system assist in the containment of infection leading to granuloma formation. However, the compromised immunity imbalances this equilibrium which further leads to reactivation of LTBI. The aim of this study was to assess if hyperglycemia like conditions contribute towards activation of latent tuberculosis.

MATERIAL/METHODS

In vitro granuloma model was developed using peripheral blood monocytic cells (PBMCs) under normal and high glucose conditions and the characteristics of dormancy i.e. tolerance towards rifampicin, loss of acid fastness were monitored. Further, activation was assessed by expression analysis of various resuscitation promoting factors rpfA-E.

RESULTS

Granuloma formation was not observed in the presence of high glucose. The gene expression of hspX was downregulated whereas the expression of rpfA-E genes was upregulated under high glucose conditions after 48 h of glucose treatment. The expression of rpfD gene remained upregulated till 72 h of glucose treatment.

CONCLUSION

High glucose concentrations impede the granuloma formation and may lead to activation of latent tubercle bacilli through resuscitation promoting factors. Thus, rpfs represent an important targets for new interventions that can abate the burden from co-pathogenesis of tuberculosis and diabetes.

Characterization of key enzymes involved in triacylglycerol biosynthesis in mycobacteria

Phosphatidic acid phosphatase (PAP) catalyzes the dephosphorylation of phosphatidic acid (PA) yielding diacylglycerol (DAG), the lipid precursor for triacylglycerol (TAG) biosynthesis. PAP activity has a key role in the regulation of PA flux towards TAG or glycerophospholipid synthesis. In this work we have characterized two Mycobacterium smegmatis genes encoding for functional PAP proteins. Disruption of both genes provoked a sharp reduction in de novo TAG biosynthesis in early growth phase cultures under stress conditions. In vivo labeling experiments demonstrated that TAG biosynthesis was restored in the ∆PAP mutant when bacteria reached exponential growth phase, with a concomitant reduction of phospholipid synthesis. In addition, comparative lipidomic analysis showed that the ∆PAP strain had increased levels of odd chain fatty acids esterified into TAGs, suggesting that the absence of PAP activity triggered other rearrangements of lipid metabolism, like phospholipid recycling, in order to maintain the wild type levels of TAG. Finally, the lipid changes observed in the ∆PAP mutant led to defective biofilm formation. Understanding the interaction between TAG synthesis and the lipid composition of mycobacterial cell envelope is a key step to better understand how lipid homeostasis is regulated during Mycobacterium tuberculosis infection.

Sensing mechanisms of iron-sulfur cluster regulatory proteins elucidated using native mass spectrometry

The ability to sense and respond to various key environmental cues is important for the survival and adaptability of many bacteria, including pathogens. The particular sensitivity of iron-sulfur (Fe-S) clusters is exploited in nature, such that multiple sensor-regulator proteins, which coordinate the detection of analytes with a (in many cases) global transcriptional response, are Fe-S cluster proteins. The fragility and sensitivity of these Fe-S clusters make studying such proteins difficult, and gaining insight of what they sense, and how they sense it and transduce the signal to affect transcription, is a major challenge. While mass spectrometry is very widely used in biological research, it is normally employed under denaturing conditions where non-covalently attached cofactors are lost. However, mass spectrometry under conditions where the protein retains its native structure and, thus, cofactors, is now itself a flourishing field, and the application of such 'native' mass spectrometry to study metalloproteins is now relatively widespread. Here we describe recent advances in using native MS to study Fe-S cluster proteins. Through its ability to accurately measure mass changes that reflect chemistry occurring at the cluster, this approach has yielded a remarkable richness of information that is not accessible by other, more traditional techniques.

Cholesterol-dependent transcriptome remodeling reveals new insight into the contribution of cholesterol to Mycobacterium tuberculosis pathogenesis

Mycobacterium tuberculosis (Mtb) is an obligate human pathogen that can adapt to the various nutrients available during its life cycle. However, in the nutritionally stringent environment of the macrophage phagolysosome, Mtb relies mainly on cholesterol. In previous studies, we demonstrated that Mtb can accumulate and utilize cholesterol as the sole carbon source. However, a growing body of evidence suggests that a lipid-rich environment may have a much broader impact on the pathogenesis of Mtb infection than previously thought. Therefore, we applied high-resolution transcriptome profiling and the construction of various mutants to explore in detail the global effect of cholesterol on the tubercle bacillus metabolism. The results allow re-establishing the complete list of genes potentially involved in cholesterol breakdown. Moreover, we identified the modulatory effect of vitamin B₁₂ on Mtb transcriptome and the novel function of cobalamin in cholesterol metabolite dissipation which explains the probable role of B₁₂ in Mtb virulence. Finally, we demonstrate that a key role of cholesterol in mycobacterial metabolism is not only providing carbon and energy but involves also a transcriptome remodeling program that helps in developing tolerance to the unfavorable host cell environment far before specific stress-inducing phagosomal signals occur.

Elucidating the Antimycobacterial Mechanism of Action of Ciprofloxacin Using Metabolomics

In the interest of developing more effective and safer anti-tuberculosis drugs, we used a GCxGC-TOF-MS metabolomics research approach to investigate and compare the metabolic profiles of Mtb in the presence and absence of ciprofloxacin. The metabolites that best describe the differences between the compared groups were identified as markers characterizing the changes induced by ciprofloxacin. Malic acid was ranked as the most significantly altered metabolite marker induced by ciprofloxacin, indicative of an inhibition of the tricarboxylic acid (TCA) and glyoxylate cycle of Mtb. The altered fatty acid, myo-inositol, and triacylglycerol metabolism seen in this group supports previous observations of ciprofloxacin action on the Mtb cell wall. Furthermore, the altered pentose phosphate intermediates, glycerol metabolism markers, glucose accumulation, as well as the reduction in the glucogenic amino acids specifically, indicate a flux toward DNA (as well as cell wall) repair, also supporting previous findings of DNA damage caused by ciprofloxacin. This study further provides insights useful for designing network whole-system strategies for the identification of possible modes of action of various drugs and possibly adaptations by Mtb resulting in resistance.

Intrabacterial lipid inclusions in mycobacteria: unexpected key players in survival and pathogenesis?

Mycobacterial species, including Mycobacterium tuberculosis, rely on lipids to survive and chronically persist within their hosts. Upon infection, opportunistic and strict pathogenic mycobacteria exploit metabolic pathways to import and process host-derived free fatty acids, subsequently stored as triacylglycerols under the form of intrabacterial lipid inclusions (ILI). Under nutrient-limiting conditions, ILI constitute a critical source of energy that fuels the carbon requirements and maintain redox homeostasis, promoting bacterial survival for extensive periods of time. In addition to their basic metabolic functions, these organelles display multiple other biological properties, emphasizing their central role in the mycobacterial lifecycle. However, despite of their importance, the dynamics of ILI metabolism and their contribution to mycobacterial adaptation/survival in the context of infection has not been thoroughly documented. Herein, we provide an overview of the historical ILI discoveries, their characterization, and current knowledge regarding the micro-environmental stimuli conveying ILI formation, storage and degradation. We also review new biological systems to monitor the dynamics of ILI metabolism in extra- and intracellular mycobacteria and describe major molecular actors in triacylglycerol biosynthesis, maintenance and breakdown. Finally, emerging concepts regarding to the role of ILI in mycobacterial survival, persistence, reactivation, antibiotic susceptibility and inter-individual transmission are also discuss.

Understanding Metabolic Regulation Between Host and Pathogens: New Opportunities for the Development of Improved Therapeutic Strategies Against Mycobacterium tuberculosis Infection

Mycobacterium tuberculosis (Mtb) causes chronic granulomatous lung disease in humans. Recently, novel strategies such as host-directed therapeutics and adjunctive therapies that enhance the effect of existing antibiotics have emerged to better control Mtb infection. Recent advances in understanding the metabolic interplay between host immune cells and pathogens have provided new insights into how their interactions ultimately influence disease outcomes and antibiotic-treatment efficacy. In this review, we describe how metabolic cascades in immune environments and relevant metabolites produced from immune cells during Mtb infection play critical roles in the progression of diseases and induction of anti-Mtb protective immunity. In addition, we introduce how metabolic alterations in Mtb itself can lead to the development of persister cells that are resistant to host immunity and can eventually evade antibiotic attacks. Further understanding of the metabolic link between host cells and Mtb may contribute to not only the prevention of Mtb persister development but also the optimization of host anti-Mtb immunity together with enhanced efficacy of existing antibiotics. Overall, this review highlights novel approaches to improve and develop host-mediated therapeutic strategies against Mtb infection by restoring and switching pathogen-favoring metabolic conditions with host-favoring conditions.

Not too fat to fight: The emerging role of macrophage fatty acid metabolism in immunity to Mycobacterium tuberculosis

While the existence of a special relationship between Mycobacterium tuberculosis (Mtb) and host lipids has long been known, it remains a challenging enigma. It was clearly established that Mtb requires host fatty acids (FAs) and cholesterol to produce energy, build its distinctive lipid-rich cell wall, and produce lipid virulence factors. It was also observed that in infected hosts, Mtb constantly resides in a FA-rich environment that the pathogen contributes to generate by inducing a lipid-laden "foamy" phenotype in host macrophages. These observations and the proximity between lipid droplets and phagosomes containing bacteria within infected macrophages gave rise to the hypothesis that Mtb reprograms host cell lipid metabolism to ensure a continuous supply of essential nutrients and its long-term persistence in vivo. However, recent studies question this principle by indicating that in Mtb-infected macrophages, lipid droplet formation prevents bacterial acquisition of host FAs while supporting the production of FA-derived protective lipid mediators. Further, in vivo investigations reveal discrete macrophage phenotypes linking the FA metabolisms of host cell and intracellular pathogen. Notably, FA storage within lipid droplets characterizes both macrophages controlling Mtb infection and dormant intracellular Mtb. In this review, we integrate findings from immunological and microbiological studies illustrating the new concept that cytoplasmic accumulation of FAs is a metabolic adaptation of macrophages to Mtb infection, which potentiates their antimycobacterial responses and forces the intracellular pathogen to shift into fat-saving, survival mode.

Phylogenetic and Biogeographic Patterns of Vibrio parahaemolyticus Strains from North America Inferred from Whole-Genome Sequence Data

Vibrio parahaemolyticus is the most common cause of seafood-borne illness reported in the United States. The draft genomes of 132 North American clinical and oyster V. parahaemolyticus isolates were sequenced to investigate their phylogenetic and biogeographic relationships. The majority of oyster isolate sequence types (STs) were from a single harvest location; however, four were identified from multiple locations. There was population structure along the Gulf and Atlantic Coasts of North America, with what seemed to be a hub of genetic variability along the Gulf Coast, with some of the same STs occurring along the Atlantic Coast and one shared between the coastal waters of the Gulf and those of Washington State. Phylogenetic analyses found nine well-supported clades. Two clades were composed of isolates from both clinical and oyster sources. Four were composed of isolates entirely from clinical sources, and three were entirely from oyster sources. Each single-source clade consisted of one ST. Some human isolates lack tdh, trh, and some type III secretion system (T3SS) genes, which are established virulence genes of V. parahaemolyticus Thus, these genes are not essential for pathogenicity. However, isolates in the monophyletic groups from clinical sources were enriched in several categories of genes compared to those from monophyletic groups of oyster isolates. These functional categories include cell signaling, transport, and metabolism. The identification of genes in these functional categories provides a basis for future in-depth pathogenicity investigations of V. parahaemolyticusIMPORTANCEVibrio parahaemolyticus is the most common cause of seafood-borne illness reported in the United States and is frequently associated with shellfish consumption. This study contributes to our knowledge of the biogeography and functional genomics of this species around North America. STs shared between the Gulf Coast and the Atlantic seaboard as well as Pacific waters suggest possible transport via oceanic currents or large shipping vessels. STs frequently isolated from humans but rarely, if ever, isolated from the environment are likely more competitive in the human gut than other STs. This could be due to additional functional capabilities in areas such as cell signaling, transport, and metabolism, which may give these isolates an advantage in novel nutrient-replete environments such as the human gut.

Mechanisms of Drug-Induced Tolerance in Mycobacterium tuberculosis

Successful treatment of tuberculosis (TB) can be hampered by Mycobacterium tuberculosis populations that are temporarily able to survive antibiotic pressure in the absence of drug resistance-conferring mutations, a phenomenon termed drug tolerance. We summarize findings on M. tuberculosis tolerance published in the past 20 years. Key M. tuberculosis responses to drug pressure are reduced growth rates, metabolic shifting, and the promotion of efflux pump activity. Metabolic shifts upon drug pressure mainly occur in M. tuberculosis's lipid metabolism and redox homeostasis, with reduced tricarboxylic acid cycle activity in favor of lipid anabolism. Increased lipid anabolism plays a role in cell wall thickening, which reduces sensitivity to most TB drugs. In addition to these general mechanisms, drug-specific mechanisms have been described. Upon isoniazid exposure, M. tuberculosis reprograms several pathways associated with mycolic acid biosynthesis. Upon rifampicin exposure, M. tuberculosis upregulates the expression of its drug target rpoB Upon bedaquiline exposure, ATP synthesis is stimulated, and the transcription factors Rv0324 and Rv0880 are activated. A better understanding of M. tuberculosis's responses to drug pressure will be important for the development of novel agents that prevent the development of drug tolerance following treatment initiation. Such agents could then contribute to novel TB treatment-shortening strategies.

Modulation of the M. tuberculosis cell envelope between replicating and non-replicating persistent bacteria

The success of Mycobacterium tuberculosis as a human pathogen depends on the bacterium's ability to persist in a quiescent form in oxygen and nutrient-poor host environments. In vitro studies have demonstrated that these restricting environments induce a shift from bacterial replication to non-replicating persistence (NRP). Entry into NRP involves changes in bacterial metabolism and remodeling of the cell envelope. Findings consistent with the phenotypes observed in vitro have been observed in patient and animal model samples. This review focuses on the cell envelope differences seen between replicating and NRP M. tuberculosis and summarizes the ways in which serine/threonine protein kinases (STPKs) may mediate this process.

Tolerance and Persistence to Drugs: A Main Challenge in the Fight Against Mycobacterium tuberculosis

The treatment of tuberculosis is extremely long. One of the reasons why Mycobacterium tuberculosis elimination from the organism takes so long is that in particular environmental conditions it can become tolerant to drugs and/or develop persisters able to survive killing even from very high drug concentrations. Tolerance develops in response to a harsh environment exposure encountered by bacteria during infection, mainly due to the action of the immune system, whereas persistence results from the presence of heterogeneous bacterial populations with different degrees of drug sensitivity, and can be induced by exposure to stress conditions. Here, we review the actual knowledge on the stress response mechanisms enacted by M. tuberculosis during infection, which leads to increased drug tolerance or development of a highly drug-resistant subpopulation.

Mycobacterium tuberculosis FasR senses long fatty acyl-CoA through a tunnel and a hydrophobic transmission spine

Mycobacterium tuberculosis is a pathogen with a unique cell envelope including very long fatty acids, implicated in bacterial resistance and host immune modulation. FasR is a TetR-like transcriptional activator that plays a central role in sensing mycobacterial long-chain fatty acids and regulating lipid biosynthesis. Here we disclose crystal structures of M. tuberculosis FasR in complex with acyl effector ligands and with DNA, uncovering its molecular sensory and switching mechanisms. A long tunnel traverses the entire effector-binding domain, enabling long fatty acyl effectors to bind. Only when the tunnel is entirely occupied, the protein dimer adopts a rigid configuration with its DNA-binding domains in an open state, leading to DNA dissociation. The protein-folding hydrophobic core connects the two domains, and is completed into a continuous spine when the effector binds. Such a transmission spine is conserved in a large number of TetR-like regulators, offering insight into effector-triggered allosteric functional control.

FasR is a TetR-like transcriptional activator that plays a central role in sensing mycobacterial long-chain fatty acids and regulating lipid biosynthesis in Mycobacterium tuberculosis. Here authors present crystal structures of M. tuberculosis FasR in complex with acyl effector ligands and with DNA, uncovering its molecular sensory and switching mechanisms.

Formation and function of bacterial organelles

Advances in imaging technologies have revealed that many bacteria possess organelles with a proteomically defined lumen and a macromolecular boundary. Some are bound by a lipid bilayer (such as thylakoids, magnetosomes and anammoxosomes), whereas others are defined by a lipid monolayer (such as lipid bodies), a proteinaceous coat (such as carboxysomes) or have a phase-defined boundary (such as nucleolus-like compartments). These diverse organelles have various metabolic and physiological functions, facilitating adaptation to different environments and driving the evolution of cellular complexity. This Review highlights that, despite the diversity of reported organelles, some unifying concepts underlie their formation, structure and function. Bacteria have fundamental mechanisms of organelle formation, through which conserved processes can form distinct organelles in different species depending on the proteins recruited to the luminal space and the boundary of the organelle. These complex subcellular compartments provide evolutionary advantages as well as enabling metabolic specialization, biogeochemical processes and biotechnological advances. Growing evidence suggests that the presence of organelles is the rule, rather than the exception, in bacterial cells.

Caseum: a Niche for Mycobacterium tuberculosis Drug-Tolerant Persisters

Caseum, the central necrotic material of tuberculous lesions, is a reservoir of drug-recalcitrant persisting mycobacteria. Caseum is found in closed nodules and in open cavities connecting with an airway. Several commonly accepted characteristics of caseum were established during the preantibiotic era, when autopsies of deceased tuberculosis (TB) patients were common but methodologies were limited. These pioneering studies generated concepts such as acidic pH, low oxygen tension, and paucity of nutrients being the drivers of nonreplication and persistence in caseum. Here we review widely accepted beliefs about the caseum-specific stress factors thought to trigger the shift of Mycobacterium tuberculosis to drug tolerance. Our current state of knowledge reveals that M. tuberculosis is faced with a lipid-rich diet rather than nutrient deprivation in caseum. Variable caseum pH is seen across lesions, possibly transiently acidic in young lesions but overall near neutral in most mature lesions. Oxygen tension is low in the avascular caseum of closed nodules and high at the cavity surface, and a gradient of decreasing oxygen tension likely forms toward the cavity wall. Since caseum is largely made of infected and necrotized macrophages filled with lipid droplets, the microenvironmental conditions encountered by M. tuberculosis in foamy macrophages and in caseum bear many similarities. While there remain a few knowledge gaps, these findings constitute a solid starting point to develop high-throughput drug discovery assays that combine the right balance of oxygen tension, pH, lipid abundance, and lipid species to model the profound drug tolerance of M. tuberculosis in caseum.