Mycolactone: a polyketide toxin from Mycobacterium ulcerans required for virulence

3-Hydroxy-3-(2-oxopropyl)indolin-2-one, a product of a human-derived Enterocloster strain, is an inhibitor of nitric oxide production

When cultured anaerobically, Enterocloster sp. RD014215 was found to produce 1. Using nuclear magnetic resonance and mass spectroscopy, the planar structure of 1 was determined to be 3-hydroxy-3-(2-oxopropyl)indolin-2-one. The chirality of 1 was implied as S by comparing the optical rotation value of 1 with literature reports of the synthesized compounds. To our knowledge, this work represents the first discovery of the metabolite produced by Enterocloster strain. 1 exhibited inhibition of nitric oxide (NO) production, demonstrating a 50% inhibitory activity (IC50) of 34 µm for NO production by murine macrophage cells subjected to lipopolysaccharide stimulation.

Unlocking Opportunities for Mycobacterium leprae and Mycobacterium ulcerans

In the recent decade, scientific communities have toiled to tackle the emerging burden of drug-resistant tuberculosis (DR-TB) and rapidly growing opportunistic nontuberculous mycobacteria (NTM). Among these, two neglected mycobacteria species of the Acinetobacter family, Mycobacterium leprae and Mycobacterium ulcerans, are the etiological agents of leprosy and Buruli ulcer infections, respectively, and fall under the broad umbrella of neglected tropical diseases (NTDs). Unfortunately, lackluster drug discovery efforts have been made against these pathogenic bacteria in the recent decade, resulting in the discovery of only a few countable hits and majorly repurposing anti-TB drug candidates such as telacebec (Q203), P218, and TB47 for current therapeutic interventions. Major ignorance in drug candidate identification might aggravate the dramatic consequences of rapidly spreading mycobacterial NTDs in the coming days. Therefore, this Review focuses on an up-to-date account of drug discovery efforts targeting selected druggable targets from both bacilli, including the accompanying challenges that have been identified and are responsible for the slow drug discovery. Furthermore, a succinct discussion of the all-new possibilities that could be alternative solutions to mitigate the neglected mycobacterial NTD burden and subsequently accelerate the drug discovery effort is also included. We anticipate that the state-of-the-art strategies discussed here may attract major attention from the scientific community to navigate and expand the roadmap for the discovery of next-generation therapeutics against these NTDs.

Pseudogenomic insights into the evolution of Mycobacterium ulcerans

BACKGROUND

Buruli ulcer (BU) disease, caused by Mycobacterium ulcerans (MU), and characterized by necrotic ulcers is still a health problem in Africa and Australia. The genome of the bacterium has several pseudogenes due to recent evolutionary events and environmental pressures. Pseudogenes are genetic elements regarded as nonessential in bacteria, however, they are less studied due to limited available tools to provide understanding of their evolution and roles in MU pathogenicity.

RESULTS

This study developed a bioinformatic pipeline to profile the pseudogenomes of sequenced MU clinical isolates from different countries. One hundred and seventy-two MU genomes analyzed revealed that pseudogenomes of African strains corresponded to the two African lineages 1 and 2. Pseudogenomes were lineage and location specific and African lineage 1 was further divided into A and B. Lineage 2 had less relaxation in positive selection than lineage 1 which may signify different evolutionary points. Based on the Gil-Latorre model, African MU strains may be in the latter stages of evolutionary adaption and are adapting to an environment rich in metabolic resources with a lower temperature and decreased UV radiation. The environment fosters oxidative metabolism and MU may be less reliant on some secondary metabolites. In-house pseudogenomes from Ghana and Cote d'Ivoire were different from other African strains, however, they were identified as African strains.

CONCLUSION

Our bioinformatic pipeline provides pseudogenomic insights to complement other whole genome analyses, providing a better view of the evolution of the genome of MU and suggest an adaptation model which is important in understanding transmission. MU pseudogene profiles vary based on lineage and country, and an apparent reduction in insertion sequences used for the detection of MU which may adversely affect the sensitivity of diagnosis.

Inflammasome-triggered IL-18 controls skin inflammation in the progression of Buruli ulcer

Buruli ulcer is an emerging chronic infectious skin disease caused by Mycobacterium ulcerans. Mycolactone, an exotoxin produced by the bacterium, is the only identified virulence factor so far, but the functions of this toxin and the mechanisms of disease progression remain unclear. By interfering Sec61 translocon, mycolactone inhibits the Sec61-dependent co-translational translocation of newly synthesized proteins, such as induced cytokines and immune cell receptors, into the endoplasmic reticulum. However, in regard to IL-1β, which is secreted by a Sec61-independent mechanism, mycolactone has been shown to induce IL-1β secretion via activation of inflammasomes. In this study, we clarified that cytokine induction, including that of IL-1β, in infected macrophages was suppressed by mycolactone produced by M. ulcerans subsp. shinshuense, despite the activation of caspase-1 through the inflammasome activation triggered in a manner independent of mycolactone. Intriguingly, mycolactone suppressed the expression of proIL-1β as well as TNF-α at the transcriptional level, suggesting that mycolactone of M. ulcerans subsp. shinshuense may exert additional inhibitory effect on proIL-1β expression. Remarkably, constitutively produced IL-18 was cleaved and mature IL-18 was actually released from macrophages infected with the causative mycobacterium. IL-18-deficient mice infected subcutaneously with M. ulcerans exhibited exacerbated skin inflammation during the course of disease progression. On the other hand, IL-1β controls bacterial multiplication in skin tissues. These results provide information regarding the mechanisms and functions of the induced cytokines in the pathology of Buruli ulcer.

What about Current Diversity of Mycolactone-Producing Mycobacteria? Implication for the Diagnosis and Treatment of Buruli Ulcer

The identification of an emerging pathogen in humans can remain difficult by conventional methods such as enrichment culture assays that remain highly selective, require appropriate medium and cannot avoid misidentifications, or serological tests that use surrogate antigens and are often hampered by the level of detectable antibodies. Although not originally designed for this purpose, the implementation of polymerase-chain-reaction (PCR) has resulted in an increasing number of diagnostic tests for many diseases. However, the design of specific molecular assays relies on the availability and reliability of published genetic sequences for the target pathogens as well as enough knowledge on the genetic diversity of species and/or variants giving rise to the same disease symptoms. Usually designed for clinical isolates, molecular tests are often not suitable for environmental samples in which the target DNA is mixed with a mixture of environmental DNA. A key challenge of such molecular assays is thus to ensure high specificity of the target genetic markers when focusing on clinical and environmental samples in order to follow the dynamics of disease transmission and emergence in humans. Here we focus on the Buruli ulcer (BU), a human necrotizing skin disease mainly affecting tropical and subtropical areas, commonly admitted to be caused by Mycobacterium ulcerans worldwide although other mycolactone-producing mycobacteria and even mycobacterium species were found associated with BU or BU-like cases. By revisiting the literature, we show that many studies have used non-specific molecular markers (IS2404, IS2606, KR-B) to identify M. ulcerans from clinical and environmental samples and propose that all mycolactone-producing mycobacteria should be definitively considered as variants from the same group rather than different species. Importantly, we provide evidence that the diversity of mycolactone-producing mycobacteria variants as well as mycobacterium species potentially involved in BU or BU-like skin ulcerations might have been underestimated. We also suggest that the specific variants/species involved in each BU or BU-like case should be carefully identified during the diagnosis phase, either via the key to genetic identification proposed here or by broader metabarcoding approaches, in order to guide the medical community in the choice for the most appropriate antibiotic therapy.

Behavioral interplay between mosquito and mycolactone produced by Mycobacterium ulcerans and bacterial gene expression induced by mosquito proximity

Mycolactone is a cytotoxic lipid metabolite produced by Mycobacterium ulcerans, the environmental pathogen responsible for Buruli ulcer, a neglected tropical disease. Mycobacterium ulcerans is prevalent in West Africa, particularly found in lentic environments, where mosquitoes also occur. Researchers hypothesize mosquitoes could serve as a transmission mechanism resulting in infection by M. ulcerans when mosquitoes pierce skin contaminated with M. ulcerans. The interplay between the pathogen, mycolactone, and mosquito is only just beginning to be explored. A triple-choice assay was conducted to determine the host-seeking preference of Aedes aegypti between M. ulcerans wildtype (MU, mycolactone active) and mutant (MUlac-, mycolactone inactive). Both qualitative and quantitative differences in volatile organic compounds' (VOCs) profiles of MU and MUlac- were determined by GC-MS. Additionally, we evaluated the interplay between Ae. aegypti proximity and M. ulcerans mRNA expression. The results showed that mosquito attraction was significantly greater (126.0%) to an artificial host treated with MU than MUlac-. We found that MU and MUlac produced differential profiles of VOCs associated with a wide range of biological importance from quorum sensing (QS) to human odor components. RT-qPCR assays showed that mycolactone upregulation was 24-fold greater for MU exposed to Ae. aegypti in direct proximity. Transcriptome data indicated significant induction of ten chromosomal genes of MU involved in stress responses and membrane protein, compared to MUlac- when directly having access to or in near mosquito proximity. Our study provides evidence of possible interkingdom interactions between unicellular and multicellular species that MU present on human skin is capable of interreacting with unrelated species (i.e., mosquitoes), altering its gene expression when mosquitoes are in direct contact or proximity, potentially impacting the production of its VOCs, and consequently leading to the stronger attraction of mosquitoes toward human hosts. This study elucidates interkingdom interactions between viable M. ulcerans bacteria and Ae. aegypti mosquitoes, which rarely have been explored in the past. Our finding opens new doors for future research in terms of disease ecology, prevalence, and pathogen dispersal outside of the M. ulcerans system.

Mycolactone A vs. B: Multiscale Simulations Reveal the Roles of Localization and Association in Isomer-Specific Toxicity

Mycolactone is an exotoxin produced by Mycobacterium ulcerans that causes the neglected tropical skin disease Buruli ulcer. This toxin inhibits the Sec61 translocon in the endoplasmic reticulum (ER), preventing the host cell from producing several secretory and transmembrane proteins, resulting in cytotoxic and immunomodulatory effects. Interestingly, only one of the two dominant isoforms of mycolactone is cytotoxic. Here, we investigate the origin of this specificity by performing extensive molecular dynamics (MD) simulations with enhanced free energy sampling to query the association trends of the two isoforms with both the Sec61 translocon, using two distinct cryo-electron microscopy (cryo-EM) models as references, and the ER membrane, which serves as a toxin reservoir prior to association. Our results suggest that mycolactone B (the cytotoxic isoform) has a stronger association with the ER membrane than mycolactone A due to more favorable interactions with membrane lipids and water molecules. This could increase the reservoir of toxin proximal to the Sec61 translocon. In one model of Sec61 inhibited by mycolactone, we find that isomer B interacts more closely with residues thought to play a key role in signal peptide recognition and, thus, are essential for subsequent protein translocation. In the other model, we find that isomer B interacts more closely with the lumenal and lateral gates of the translocon, the dynamics of which are essential for protein translocation. These interactions induce a more closed conformation, which has been suggested to block signal peptide insertion and subsequent protein translocation. Collectively, these findings suggest that isomer B's unique cytotoxicity is a consequence of both increased localization to the ER membrane and channel-locking association with the Sec61 translocon, facets that could be targeted in the development of Buruli Ulcer diagnostics and Sec61-targeted therapeutics.

Alternative boronic acids in the detection of Mycolactone A/B using the thin layer chromatography (f-TLC) method for diagnosis of Buruli ulcer

BACKGROUND

Mycobacterium ulcerans is the causative agent of Buruli ulcer. The pathology of M. ulcerans disease has been attributed to the secretion of a potent macrolide cytotoxin known as mycolactone which plays an important role in the virulence of the disease. Mycolactone is a biomarker for the diagnosis of BU that can be detected using the fluorescent-thin layer chromatography (f-TLC) technique. The technique relies on the chemical derivatization of mycolactone A/B with 2-naphthylboronic acid (BA) which acts as a fluorogenic chemosensor. However, background interferences due to co-extracted human tissue lipids, especially with clinical samples coupled with the subjectivity of the method call for an investigation to find an alternative to BA.

METHODS

Twenty-six commercially available arylboronic acids were initially screened as alternatives to BA using the f-TLC experiment. UV-vis measurements were also conducted to determine the absorption maximum spectra of mycolactone A/B and myco-boronic acid adducts followed by an investigation of the fluorescence-enhancing ability of the boronate ester formation between mycolactone A/B and our three most promising boronic acids (BA15, BA18, and BA21). LC-MS technique was employed to confirm the adduct formation between mycolactone and boronic acids. Furthermore, a comparative study was conducted between BA18 and BA using 6 Polymerase Chain Reaction (PCR) confirmed BU patient samples.

RESULTS

Three of the boronic acids (BA15, BA18, and BA21) produced fluorescent band intensities superior to BA. Complexation studies conducted on thin layer chromatography (TLC) using 0.1 M solution of the three boronic acids and various volumes of 10 ng/µL of synthetic mycolactone ranging from 1 µL - 9 µL corresponding to 10 ng - 90 ng gave similar results with myco-BA18 adduct emerging with the most visibly intense fluorescence bands. UV-vis absorption maxima (λ_max) for the free mycolactone A/B was observed at 362 nm, and the values for the adducts myco-BA15, myco-BA18, and myco-BA21 were at 272 nm, 270 nm, and 286 nm respectively. The comparable experimental λ_max of 362 nm for mycolactone A/B to the calculated Woodward-Fieser value of 367 nm for the fatty acid side chain of mycolactone A/B demonstrate that even though 2 cyclic boronates were formed, only the boronate of the southern side chain with the chromophore was excited by irradiation at 365 nm. Fluorescence experiments have demonstrated that coupling BA18 to mycolactone A/B along the 1,3-diols remarkably enhanced the fluorescence intensity at 537 nm. High-Resolution Mass Spectrometer (HR-MS) was used to confirm the formation of the myco-BA15 adduct. Finally, f-TLC analysis of patient samples with BA18 gave improved BA18-adduct intensities compared to the original BA-adduct.

CONCLUSION

Twenty-six commercially available boronic acids were investigated as alternatives to BA, used in the f-TLC analysis for the diagnosis of BU. Three (3) of them BA15, BA18, and BA21 gave superior fluorescence band intensity profiles. They gave profiles that were easier to interpret after the myco-boronic acid adduct formation and in experiments with clinical samples from patients with BA18 the best. BA18, therefore, has been identified as a potential alternative to BA and could provide a solution to the challenge of background interference of co-extracted human tissue lipids from clinical samples currently associated with the use of BA.

Type-I interferons promote innate immune tolerance in macrophages exposed to Mycobacterium ulcerans vesicles

Buruli ulcer is a chronic infectious disease caused by Mycobacterium ulcerans. The pathogen persistence in host skin is associated with the development of ulcerative and necrotic lesions leading to permanent disabilities in most patients. However, few of diagnosed cases are thought to resolve through an unknown self-healing process. Using in vitro and in vivo mouse models and M. ulcerans purified vesicles and mycolactone, we showed that the development of an innate immune tolerance was only specific to macrophages from mice able to heal spontaneously. This tolerance mechanism depends on a type I interferon response and can be induced by interferon beta. A type I interferon signature was further detected during in vivo infection in mice as well as in skin samples from patients under antibiotics regiment. Our results indicate that type I interferon-related genes expressed in macrophages may promote tolerance and healing during infection with skin damaging pathogen.

From Bacterial Toxin to Therapeutic Agent: The Unexpected Fate of Mycolactone

"Recognizing a surprising fact is the first step towards discovery." This famous quote from Louis Pasteur is particularly appropriate to describe what led us to study mycolactone, a lipid toxin produced by the human pathogen Mycobacterium ulcerans. M. ulcerans is the causative agent of Buruli ulcer, a neglected tropical disease manifesting as chronic, necrotic skin lesions with a "surprising" lack of inflammation and pain. Decades after its first description, mycolactone has become much more than a mycobacterial toxin. This uniquely potent inhibitor of the mammalian translocon (Sec61) helped reveal the central importance of Sec61 activity for immune cell functions, the spread of viral particles and, unexpectedly, the viability of certain cancer cells. We report in this review the main discoveries that marked our research into mycolactone, and the medical perspectives they opened up. The story of mycolactone is not over and the applications of Sec61 inhibition may go well beyond immunomodulation, viral infections, and oncology.

The Isolation and Structure Elucidation of Spirotetronate Lobophorins A, B, and H8 from Streptomyces sp. CB09030 and Their Biosynthetic Gene Cluster

Lobophorins (LOBs) are a growing family of spirotetronate natural products with significant cytotoxicity, anti-inflammatory, and antibacterial activities. Herein, we report the transwell-based discovery of Streptomyces sp. CB09030 from a panel of 16 in-house Streptomyces strains, which has significant anti-mycobacterial activity and produces LOB A (1), LOB B (2), and LOB H8 (3). Genome sequencing and bioinformatic analyses revealed the potential biosynthetic gene cluster (BGC) for 1-3, which is highly homologous with the reported BGCs for LOBs. However, the glycosyltransferase LobG1 in S. sp. CB09030 has certain point mutations compared to the reported LobG1. Finally, LOB analogue 4 (O-β-D-kijanosyl-(1→17)-kijanolide) was obtained through an acid-catalyzed hydrolysis of 2. Compounds 1-4 showed different antibacterial activities against Mycobacterium smegmatis and Bacillus subtilis, which revealed the varying roles of different sugars in their antibacterial activities.

Buruli ulcer caused by Mycobacterium ulcerans subsp. shinshuense: A case report

Buruli ulcer is the third most common mycobacterial infection worldwide and is mainly diagnosed in tropical regions. Globally, this progressive disease is caused by Mycobacterium ulcerans; however, Mycobacterium ulcerans subsp. shinshuense, an Asian variant, has been exclusively identified in Japan. Because of insufficient clinical cases, the clinical features of M. ulcerans subsp. shinshuense-associated Buruli ulcer remain unclear. A 70-year-old Japanese woman presented with erythema on her left backhand. The skin lesion deteriorated without an apparent etiology of inflammation, and she was referred to our hospital 3 months after disease onset. A biopsy specimen was incubated in 2% Ogawa medium at 30 °C. After 66 days, we detected small yellow-pigmented colonies, suggesting scotochromogens. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI Biotyper; Bruker Daltonics, Billerica, MA, USA) indicated that the organism was Mycobacterium pseudoshottsii or Mycobacterium marinum. However, additional PCR testing for the insertion sequence 2404 (IS2404) was positive, suggesting that the pathogen was either M. ulcerans or M. ulcerans subsp. shinshuense. Further examination by 16S rRNA sequencing analysis, focusing on nucleotide positions 492, 1247, 1288, and 1449-1451, we finally identified the organism as M. ulcerans subsp. shinshuense. The patient was successfully treated with 12 weeks of clarithromycin and levofloxacin treatment. Mass spectrometry is the latest microbial diagnostic method; however, it cannot be used to identify M. ulcerans subsp. shinshuense. To accurately detect this enigmatic pathogen and uncover its epidemiology and clinical characteristics in Japan, more accumulation of clinical cases with accurate identification of the causative pathogen is essential.

Mycolactone causes catastrophic Sec61-dependent loss of the endothelial glycocalyx and basement membrane: a new indirect mechanism driving tissue necrosis in Mycobacterium ulcerans infection

The drivers of tissue necrosis in Mycobacterium ulcerans infection (Buruli ulcer disease) have historically been ascribed solely to the directly cytotoxic action of the diffusible exotoxin, mycolactone. However, its role in the clinically-evident vascular component of disease aetiology remains poorly explained. We have now dissected mycolactone's effects on primary vascular endothelial cells in vitro and in vivo. We show that mycolactone-induced changes in endothelial morphology, adhesion, migration, and permeability are dependent on its action at the Sec61 translocon. Unbiased quantitative proteomics identified a profound effect on proteoglycans, driven by rapid loss of type II transmembrane proteins of the Golgi, including enzymes required for glycosaminoglycan (GAG) synthesis, combined with a reduction in the core proteins themselves. Loss of the glycocalyx is likely to be of particular mechanistic importance, since knockdown of galactosyltransferase II (beta-1,3-galactotransferase 6; B3Galt6), the GAG linker-building enzyme, phenocopied the permeability and phenotypic changes induced by mycolactone. Additionally, mycolactone depleted many secreted basement membrane components and microvascular basement membranes were disrupted in vivo. Remarkably, exogenous addition of laminin-511 reduced endothelial cell rounding, restored cell attachment and reversed the defective migration caused by mycolactone. Hence supplementing mycolactone-depleted extracellular matrix may be a future therapeutic avenue, to improve wound healing rates.

Mycobacterium abscessus infection results in decrease of oxidative metabolism of lung airways cells and relaxation of the epithelial mucosal tight junctions

Mycobacterium abscessus complex is a group of environmental pathogens that recently have been isolated more from patients with underlying lung diseases, such and COPD, bronchiectasis, and cystic fibrosis. The mechanisms involved in the pathogenesis of these diseases have only recently been investigated. Infection is associated with biofilm formation on the airway mucosa, invasion of the mucosal epithelial cells and a time-dependent impairment of the integrity of the monolayer. Using electron microscopy, it was shown that Mycobacterium abscessus induced lesions of the cell surface structures. Tight junction proteins claudin-1 and occludin-1 have increased transcription in cells exposed to Mycobacterium abscessus, in contrast to cells exposed to Mycobacterium avium. Infection of A549 alveolar epithelial cells by Mycobacterium abscessus reduced the oxidative metabolism of the cell, without inducing necrosis. A transposon library screen identified mutants that do not alter the metabolism of the A549 cells.Once the bacterium crosses the epithelial barrier, it may encounter sub-epithelial macrophages. Select mutants were used for infection assays to determine their effects on membrane integrity. Translocated select mutants were attenuated in macrophages compared to wild type Mycobacterium abscessus. In summary, the dynamics of Mycobacterium abscessus infection appears to be different from other non-tuberculous mycobacteria (NTMs). Future studies will attempt to address the mechanism involved in airway membrane lesions.

Detection and Semi-quantification of Lipids on High-Performance Thin-Layer Chromatography Plate using Ceric Ammonium Molybdate Staining

It is desirable to quickly check the composition of lipids in small size samples, but achieving this is challenging using the existing staining methods. Herein, we developed a highly sensitive and semi-quantitative method for analysis of lipid samples with ceric ammonium molybdate (CAM) staining. The CAM detection method was systematically evaluated with a wide range of lipid classes including phospholipids, sphingolipids, glycerolipids, fatty acids (FA) and sterols, demonstrating high sensitivity, stability, and overall efficiency. Additionally, CAM staining provides a clean yellow background in high performance thin-layer chromatography (HPTLC) which facilitates quantification of lipids using image processing software. Lipids can be stained with CAM reagent regardless of their head group types, position of the carbon-carbon double bonds, geometric isomerism and the variation in the length of FA chain, but staining is mostly affected by the degree of unsaturation of the FA backbone. The mechanism of the CAM staining of lipids was proposed on principles of the reduction-oxidation reaction, in which Mo(VI) oxidizes the unsaturated lipids into carbonyl compounds on the HPTLC plate upon heating, while itself being reduced to Mo(IV). This method was applied for the separation, identification, and quantification of lipid extracts from porcine brain.

Can membrane composition traffic toxins? Mycolactone and preferential membrane interactions

Mycolactone is a cytotoxic and immunosuppressive macrolide produced by Mycobacterium ulcerans and the sole causative agent of the neglected tropical skin disease Buruli ulcer. The toxin acts by invading host cells and interacting with intracellular targets to disrupt multiple fundamental cellular processes. Mycolactone's amphiphilic nature enables strong interactions with lipophilic environments, including cellular membranes; however, the specificity of these interactions and the role of membranes in the toxin's pathogenicity remain unknown. It is likely that preferential interactions with lipophilic carriers play a key role in the toxin's distribution in the host, which, if understood, could provide insights to aid in the development of needed diagnostics for Buruli ulcer disease. In this work, molecular dynamics simulations were combined with enhanced free-energy sampling to characterize mycolactone's association with and permeation through models of the mammalian endoplasmic reticulum (ER) and plasma membranes (PMs). We find that increased order in the PMs not only leads to a different permeation mechanism compared with that in the ER membrane but also an energetic driving force for ER localization. Increased hydration, membrane deformation, and preferential interactions with unsaturated lipid tails stabilize the toxin in the ER membrane, while disruption of lipid packing is a destabilizing force in the PMs.

Mycobacteriaceae Mineralizes Micropolyethylene in Riverine Ecosystems

Microplastic (MP) contamination is a serious global environmental problem. Plastic contamination has attracted extensive attention during the past decades. While physiochemical weathering may influence the properties of MPs, biodegradation by microorganisms could ultimately mineralize plastics into CO₂. Compared to the well-studied marine ecosystems, the MP biodegradation process in riverine ecosystems, however, is less understood. The current study focuses on the MP biodegradation in one of the world's most plastic contaminated rivers, Pearl River, using micropolyethylene (mPE) as a model substrate. Mineralization of ¹³C-labeled mPE into ¹³CO₂ provided direct evidence of mPE biodegradation by indigenous microorganisms. Several Actinobacteriota genera were identified as putative mPE degraders. Furthermore, two Mycobacteriaceae isolates related to the putative mPE degraders, Mycobacterium sp. mPE3 and Nocardia sp. mPE12, were retrieved, and their ability to mineralize ¹³C-mPE into ¹³CO₂ was confirmed. Pangenomic analysis reveals that the genes related to the proposed mPE biodegradation pathway are shared by members of Mycobacteriaceae. While both Mycobacterium and Nocardia are known for their pathogenicity, these populations on the plastisphere in this study were likely nonpathogenic as they lacked virulence factors. The current study provided direct evidence for MP mineralization by indigenous biodegraders and predicted their biodegradation pathway, which may be harnessed to improve bioremediation of MPs in urban rivers.

Different modalities of host cell death and their impact on Mycobacterium tuberculosis infection

Mycobacterium tuberculosis (Mtb) is the pathogen that causes tuberculosis (TB), a leading infectious disease of humans worldwide. One of the main histopathological hallmarks of TB is the formation of granulomas comprised of elaborately organized aggregates of immune cells containing the pathogen. Dissemination of Mtb from infected cells in the granulomas due to host and mycobacterial factors induces multiple cell death modalities in infected cells. Based on molecular mechanism, morphological characteristics, and signal dependency, there are two main categories of cell death: programmed and nonprogrammed. Programmed cell death (PCD), such as apoptosis and autophagy, is associated with a protective response to Mtb by keeping the bacteria encased within dead macrophages that can be readily phagocytosed by arriving in uninfected or neighboring cells. In contrast, non-PCD necrotic cell death favors the pathogen, resulting in bacterial release into the extracellular environment. Multiple types of cell death in the PCD category, including pyroptosis, necroptosis, ferroptosis, ETosis, parthanatos, and PANoptosis, may be involved in Mtb infection. Since PCD pathways are essential for host immunity to Mtb, therapeutic compounds targeting cell death signaling pathways have been experimentally tested for TB treatment. This review summarizes different modalities of Mtb-mediated host cell deaths, the molecular mechanisms underpinning host cell death during Mtb infection, and its potential implications for host immunity. In addition, targeting host cell death pathways as potential therapeutic and preventive approaches against Mtb infection is also discussed.

Modified aluminosilicates display antibacterial activity against nontuberculous mycobacteria and adsorb mycolactone and Mycobacterium ulcerans in vitro

Mycobacterium ulcerans (MU) infection of skin and soft tissue leads to chronic skin ulceration known as Buruli ulcer. MU releases a lipid-like toxin, mycolactone, that diffuses into the tissue, effecting disease through localized tissue necrosis and immunosuppression. Cutaneous Buruli ulcer wounds slowly advance from a painless pre-ulcerative stage to an ulcerative lesion, leading to disparities in the timing of medical intervention and treatment outcomes. Novel Buruli ulcer wound management solutions could complement and supplement systemically administered antimicrobials and reduce time to healing. Capitalizing on nanopore structure, adsorption, and exchange capacities, aluminosilicate nanozeolites (nZeos) and geopolymers (GPs) were developed and investigated in the context of therapeutics for mycobacterial disease ulcerative wound care. nZeos were ion exchanged with copper or silver to assess the antimicrobial activity against MU and Mycobacterium marinum, a rapid growing, genetic ancestor of MU that also causes skin and soft tissue infections. Silver- and copper-exchanged nZeos were bactericidal against MU, while only silver-exchanged nZeos killed M. marinum. To mediate adsorption at a biological scale, GPs with different pore sizes and altered surface modifications were generated and assessed for the ability to adsorb MU and mycolactone. Macroporous GPs with and without stearic acid modification equivalently adsorbed MU cells, while mesoporous GPs with stearic acid adsorbed mycolactone toxin significantly better than mesoporous GPs or GPs modified with phenyltriethoxysilane (PTES). In cytotoxicity assays, Cu-nZeos lacked toxicity against Detroit 551, U-937, and WM-115 cells. GPs demonstrated limited cytotoxicity in Detroit 551 and WM-115, but produced time-dependent toxicity in U-937 cells. With their large surface area and adsorptive capacities, aluminosilicates nZeos and GPs may be modified and developed to support conventional BU wound care. Topical application of nZeos and GPs could kill MU within the cutaneous wound environment and physically remove MU and mycolactone with wound dressing changes, thereby improving wound healing and overall patient outcomes.

Analysis of key genes in Mycobacterium ulcerans reveals conserved RNA structural motifs and regions with apparent pressure to remain unstructured

Buruli Ulcer is a neglected tropical disease that results in disfiguring and dangerous lesions in affected persons across a wide geographic area, including much of West Africa. The causative agent of Buruli Ulcer is Mycobacterium ulcerans, a relative of the bacterium that causes tuberculosis and leprosy. Few therapeutic options exist for the treatment of this disease beyond antibiotics in the early stages, which are frequently ineffective, and surgical removal in the later stage. In this study we analyze six genes in Mycobacterium ulcerans that have high potential of therapeutic targeting. We focus our analysis on a combined in silico and comparative sequence study of potential RNA secondary structure across these genes. The result of this work was the comprehensive local RNA structural landscape across each of these significant genes. This revealed multiple sites of ordered and evolved RNA structure interspersed between sequences that either have no bias for structure or, indeed, appear to be ordered to be unstructured and (potentially) accessible. In addition to providing data that could be of interest to basic biology, our results provide guides for efforts aimed at targeting this pathogen at the RNA level. We explore this latter possibility through the in silico analysis of antisense oligonucleotides that could potentially be used to target pathogen RNA.

Genome-wide screening identified SEC61A1 as an essential factor for mycolactone-dependent apoptosis in human premonocytic THP-1 cells

Buruli ulcer is a chronic skin disease caused by a toxic lipid mycolactone produced by Mycobacterium ulcerans, which induces local skin tissue destruction and analgesia. However, the cytotoxicity pathway induced by mycolactone remains largely unknown. Here we investigated the mycolactone-induced cell death pathway by screening host factors using a genome-scale lenti-CRISPR mutagenesis assay in human premonocytic THP-1 cells. As a result, 884 genes were identified as candidates causing mycolactone-induced cell death, among which SEC61A1, the α-subunit of the Sec61 translocon complex, was the highest scoring. CRISPR/Cas9 genome editing of SEC61A1 in THP-1 cells suppressed mycolactone-induced endoplasmic reticulum stress, especially eIF2α phosphorylation, and caspase-dependent apoptosis. Although previous studies have reported that mycolactone targets SEC61A1 based on mutation screening and structural analysis in several cell lines, we have reconfirmed that SEC61A1 is a mycolactone target by genome-wide screening in THP-1 cells. These results shed light on the cytotoxicity of mycolactone and suggest that the inhibition of mycolactone activity or SEC61A1 downstream cascades will be a novel therapeutic modality to eliminate the harmful effects of mycolactone in addition to the 8-week antibiotic regimen of rifampicin and clarithromycin.

Buruli ulcer is a chronic skin disease caused by the bacterium Mycobacterium ulcerans. The disease mainly affects children in West Africa, and the skin ulcers are induced by mycolactone, a toxin produced by the bacteria. The mycolactone diffuses through the skin, killing cells, creating irreversible ulceration, and weakening host immune defenses. However, the cytotoxic pathway induced by mycolactone remains largely unknown. We evaluated the mycolactone-induced cell death pathway by screening host factors using a genome-scale knockout assay in human premonocytic THP-1 cells. We identified 884 genes that are potentially involved in mycolactone-induced cell death, of which SEC61A1, the α-subunit of the Sec61 translocon complex, was the highest ranking. Knockout of SEC61A1 in THP-1 cells resulted in suppression of endoplasmic reticulum stress and caspase-dependent apoptosis induced by mycolactone. These results suggest that SEC61A1 is an essential mediator of mycolactone-induced cell death.

Comprehensive Analysis of the Nocardia cyriacigeorgica Complex Reveals Five Species-Level Clades with Different Evolutionary and Pathogenicity Characteristics

Nocardia cyriacigeorgica is a common etiological agent of nocardiosis that has increasingly been implicated in serious pulmonary infections, especially in immunocompromised individuals. However, the evolution, diversity, and pathogenesis of N. cyriacigeorgica have remained unclear. Here, we performed a comparative genomic analysis using 91 N. cyriacigeorgica strains, 45 of which were newly sequenced in this study. Phylogenetic and average nucleotide identity (ANI) analyses revealed that N. cyriacigeorgica contained five species-level clades (8.6 to 14.6% interclade genetic divergence), namely, the N. cyriacigeorgica complex (NCC). Further pan-genome analysis revealed extensive differences among the five clades in nine functional categories, such as energy production, lipid metabolism, secondary metabolites, and signal transduction mechanisms. All 2,935 single-copy core genes undergoing purifying selection were highly conserved across NCC. However, clades D and E exhibited reduced selective constraints, compared to clades A to C. Horizontal gene transfer (HGT) and mobile genetic elements contributed to genomic plasticity, and clades A and B had experienced a higher level of HGT events than other clades. A total of 129 virulence factors were ubiquitous across NCC, such as the mce operon, hemolysin, and type VII secretion system (T7SS). However, different distributions of three toxin-coding genes and two new types of mce operons were detected, which might contribute to pathogenicity differences among the members of the NCC. Overall, our study provides comprehensive insights into the evolution, genetic diversity, and pathogenicity of NCC, facilitating the prevention of infections.

IMPORTANCENocardia species are opportunistic bacterial pathogens that can affect all organ systems, primarily the skin, lungs, and brain. N. cyriacigeorgica is the most prevalent species within the genus, exhibits clinical significance, and can cause severe infections when disseminated throughout the body. However, the evolution, diversity, and pathogenicity of N. cyriacigeorgica remain unclear. Here, we have conducted a comparative genomic analysis of 91 N. cyriacigeorgica strains and revealed that N. cyriacigeorgica is not a single species but is composed of five closely related species. In addition, we discovered that these five species differ in many ways, involving selection pressure, horizontal gene transfer, functional capacity, pathogenicity, and antibiotic resistance. Overall, our work provides important clues in dissecting the evolution, genetic diversity, and pathogenicity of NCC, thereby advancing prevention measures against these infections.

Mycobacterium ulcerans Experimental Dormancy

Whether Mycobacterium ulcerans, the etiological agent of Buruli ulcer in numerous tropical countries, would exist in a dormant state as reported for closely related Mycobacterium species, has not been established. Six M. ulcerans strains were exposed to a progressive depletion in oxygen for 2 months, using the Wayne model of dormancy previously described for M. tuberculosis, and further examined by microscopy after staining of dynamic, dormant, and dead mycobacteria (DDD staining), microcalorimetry and subculture in the presence of dead and replicative M. ulcerans as controls. Mycobacterium ulcerans CU001 strain died during the progressive oxygen depletion and four of five remaining strains exhibited Nile red-stained intracellular lipid droplets and a 14- to 20-day regrowth when exposed to ambient air, consistent with dormancy. A fifth M. ulcerans 19423 strain stained negative in DDD staining and slowly regrew in 27 days. Three tested M. ulcerans strains yielded microcalorimetric pattern similar to that of the negative (dead) homologous controls, differing from that of the homologous positive (replicative) controls. The relevance of these experimental observations, suggesting a previously unreported dormancy state of M. ulcerans, warrants further investigations in the natural ecological niches where M. ulcerans thrive as well as in Buruli ulcer lesions.

The search for a Buruli Ulcer vaccine and the effectiveness of the Bacillus Calmette-Guérin vaccine

Buruli Ulcer is a neglected tropical disease that is caused by Mycobacterium ulcerans. It is not fatal; however, it manifests a range of devastating symptoms on the hosts' bodies. Various drugs and treatments are available for the disease; however, they are often costly and have adverse effects. There is still much uncertainty regarding the mode of transmission, vectors, and reservoir. At present, there are no official vector control methods, prevention methods, or a vaccine licensed to prevent infection. The Bacillus Calmette-Guérin vaccine developed against tuberculosis has some effectiveness against M. ulcerans. However, it is unable to induce long-lasting protection. Various types of vaccines have been developed based specifically against M. ulcerans; however, to date, none has entered clinical trials or has been released for public use. Additional awareness and funding are needed for research in this field and the development of more treatments, diagnostic tools, and vaccines.

Protective Efficacy of BCG Vaccine against Mycobacterium leprae and Non-Tuberculous Mycobacterial Infections

The genus mycobacterium includes several species that are known to cause infections in humans. The microorganisms are classified into tuberculous and non-tuberculous based on their morphological characteristics, defined by the dynamic relationship between the host defenses and the infectious agent. Non-tuberculous mycobacteria (NTM) include all the species of mycobacterium other than the ones that cause tuberculosis (TB). The group of NTM contains almost 200 different species and they are found in soil, water, animals—both domestic and wild—milk and food products, and from plumbed water resources such as sewers and showerhead sprays. A systematic review of Medline between 1946 and 2014 showed an 81% decline in TB incidence rates with a simultaneous 94% increase in infections caused by NTM. Prevalence of infections due to NTM has increased relative to infections caused by TB owing to the stringent prevention and control programs in Western countries such as the USA and Canada. While the spread of typical mycobacterial infections such as TB and leprosy involves human contact, NTM seem to spread easily from the environment without the risk of acquiring from a human contact except in the case of M. abscessus in patients with cystic fibrosis, where human transmission as well as transmission through fomites and aerosols has been recorded. NTM are opportunistic in their infectious processes, making immunocompromised individuals such as those with other systemic infections such as HIV, immunodeficiencies, pulmonary disease, or usage of medications such as long-term corticosteroids/TNF-α inhibitors more susceptible. This review provides insight on pathogenesis, treatment, and BCG vaccine efficacy against M. leprae and some important NTM infections.

The One That Got Away: How Macrophage-Derived IL-1β Escapes the Mycolactone-Dependent Sec61 Blockade in Buruli Ulcer

Buruli ulcer (BU), caused by Mycobacterium ulcerans, is a devastating necrotizing skin disease. Key to its pathogenesis is mycolactone, the exotoxin virulence factor that is both immunosuppressive and cytotoxic. The discovery that the essential Sec61 translocon is the major cellular target of mycolactone explains much of the disease pathology, including the immune blockade. Sec61 inhibition leads to a loss in production of nearly all cytokines from monocytes, macrophages, dendritic cells and T cells, as well as antigen presentation pathway proteins and costimulatory molecules. However, there has long been evidence that the immune system is not completely incapable of responding to M. ulcerans infection. In particular, IL-1β was recently shown to be present in BU lesions, and to be induced from M. ulcerans-exposed macrophages in a mycolactone-dependent manner. This has important implications for our understanding of BU, showing that mycolactone can act as the "second signal" for IL-1β production without inhibiting the pathways of unconventional secretion it uses for cellular release. In this Perspective article, we validate and discuss this recent advance, which is entirely in-line with our understanding of mycolactone's inhibition of the Sec61 translocon. However, we also show that the IL-1 receptor, which uses the conventional secretory pathway, is sensitive to mycolactone blockade at Sec61. Hence, a more complete understanding of the mechanisms regulating IL-1β function in skin tissue, including the transient intra-macrophage stage of M. ulcerans infection, is urgently needed to uncover the double-edged sword of IL-1β in BU pathogenesis, treatment and wound healing.

A need for null models in understanding disease transmission: the example of Mycobacterium ulcerans (Buruli ulcer disease)

Understanding the interactions of ecosystems, humans and pathogens is important for disease risk estimation. This is particularly true for neglected and newly emerging diseases where modes and efficiencies of transmission leading to epidemics are not well understood. Using a model for other emerging diseases, the neglected tropical skin disease Buruli ulcer (BU), we systematically review the literature on transmission of the etiologic agent, Mycobacterium ulcerans (MU), within a One Health/EcoHealth framework and against Hill's nine criteria and Koch's postulates for making strong inference in disease systems. Using this strong inference approach, we advocate a null hypothesis for MU transmission and other understudied disease systems. The null should be tested against alternative vector or host roles in pathogen transmission to better inform disease management. We propose a re-evaluation of what is necessary to identify and confirm hosts, reservoirs and vectors associated with environmental pathogen replication, dispersal and transmission; critically review alternative environmental sources of MU that may be important for transmission, including invertebrate and vertebrate species, plants and biofilms on aquatic substrates; and conclude with placing BU within the context of other neglected and emerging infectious diseases with intricate ecological relationships that lead to disease in humans, wildlife and domestic animals.

Mycolactone Purification from M. ulcerans Cultures and HPLC-Based Approaches for Mycolactone Quantification in Biological Samples

Mycolactones are a family of polyketide synthase products made by the human pathogen Mycobacterium ulcerans that were recently identified as novel inhibitors of the host membrane translocation complex (Sec61). Here, we provide protocols for the purification of mycolactones from bacterial cultures, and for their quantitative assessment in biological samples.

Detection of Mycolactone by Thin Layer Chromatography

By means of thin layer chromatography coupled to a fluorescence enhancer, a highly sensitive and operationally simple method to detect the mycolactones stemming from the human pathogen Mycobacterium ulcerans was developed and applied to various sample sources.

Aberrant stromal tissue factor localisation and mycolactone-driven vascular dysfunction, exacerbated by IL-1β, are linked to fibrin formation in Buruli ulcer lesions

Buruli ulcer (BU) is a neglected tropical disease caused by subcutaneous infection with Mycobacterium ulcerans and its exotoxin mycolactone. BU displays coagulative necrosis and widespread fibrin deposition in affected skin tissues. Despite this, the role of the vasculature in BU pathogenesis remains almost completely unexplored. We hypothesise that fibrin-driven ischemia can be an ‘indirect’ route to mycolactone-dependent tissue necrosis by a mechanism involving vascular dysfunction. Here, we tracked >900 vessels within contiguous tissue sections from eight BU patient biopsies. Our aim was to evaluate their vascular and coagulation biomarker phenotype and explore potential links to fibrin deposition. We also integrated this with our understanding of mycolactone’s mechanism of action at Sec61 and its impact on proteins involved in maintaining normal vascular function. Our findings showed that endothelial cell dysfunction is common in skin tissue adjacent to necrotic regions. There was little evidence of primary haemostasis, perhaps due to mycolactone-dependent depletion of endothelial von Willebrand factor. Instead, fibrin staining appeared to be linked to the extrinsic pathway activator, tissue factor (TF). There was significantly greater than expected fibrin staining around vessels that had TF staining within the stroma, and this correlated with the distance it extended from the vessel basement membrane. TF-induced fibrin deposition in these locations would require plasma proteins outside of vessels, therefore we investigated whether mycolactone could increase vascular permeability in vitro. This was indeed the case, and leakage was further exacerbated by IL-1β. Mycolactone caused the loss of endothelial adherens and tight junctions by the depletion of VE-cadherin, TIE-1, TIE-2 and JAM-C; all Sec61-dependent proteins. Taken together, our findings suggest that both vascular and lymphatic vessels in BU lesions become “leaky” during infection, due to the unique action of mycolactone, allowing TF-containing structures and plasma proteins into skin tissue, ultimately leading to local coagulopathy and tissue ischemia.

To date, the debilitating skin disease Buruli ulcer remains a public health concern and financial burden in low or middle-income countries, especially in tropical regions. Late diagnosis is frequent in remote areas, perhaps due to the painlessness of the disease. Hence patients often present with large, destructive opened ulcers leading to delayed wound closure or even lifelong disability. The infectious agent produces a toxin called mycolactone that drives the disease. We previously found evidence that the vascular system is disrupted by mycolactone in these lesions, and now we have further explored potential explanations for these findings by looking at the expression of vascular markers in BU. In a detailed analysis of patient skin punch biopsies, we identified distinct expression patterns of certain proteins and found that tissue factor, which initiates the so-called extrinsic pathway of blood clotting, is particularly important. Mycolactone is able to disrupt the barrier function of the endothelium, further aggravating the diseased phenotype, which may explain how clotting factors access the tissue. Altogether, such localised hypercoagulation in Buruli ulcer skin lesions may contribute to the development of the lesion.

Molecular and epidemiological characterization of recurrent Mycobacterium ulcerans infections in Benin

BACKGROUND

Buruli ulcer is a neglected tropical disease caused by Mycobacterium ulcerans, an environmental mycobacterium. Although transmission of M. ulcerans remains poorly understood, the main identified risk factor for acquiring Buruli ulcer is living in proximity of potentially contaminated water sources. Knowledge about the clinical features of Buruli ulcer and its physiopathology is increasing, but little is known about recurrence due to reinfection.

METHODOLOGY/PRINCIPAL FINDINGS

We describe two patients with Buruli ulcer recurrence due to reinfection with M. ulcerans, as demonstrated by comparisons of DNA from the strains isolated at the time of the first diagnosis and at recurrence. Based on the spatial distribution of M. ulcerans genotypes in this region and a detailed study of the behavior of these two patients with respect to sources of water as well as water bodies and streams, we formulated hypotheses concerning the sites at which they may have been contaminated.

CONCLUSIONS/SIGNIFICANCE

Second episodes of Buruli ulcer may occur through reinfection, relapse or a paradoxical reaction. We formally demonstrated that the recurrence in these two patients was due to reinfection. Based on the sites at which the patients reported engaging in activities relating to water, we were able to identify possible sites of contamination. Our findings indicate that the non-random distribution of M. ulcerans genotypes in this region may provide useful information about activities at risk.

Next-generation microbial drugs developed from microbiome's natural products

Scientists working in natural products chemistry have been enticed by the current advancements being made in the discovery of novel "magic bullets" from microbes homed to all conceivable environments. Even though researchers continue to face challenges funneling the novel bioactive compounds in the global therapeutic industries, it seems most likely that the discovery of some "hit molecules" with significant biomedical applications is not that far. We applaud novel natural products for their ability to combat the spread of superbugs and aid in the prevention of currently observed antibiotic resistance. This in-depth investigation covers a wide range of microbiomes with a proclivity for synthesizing novel compounds to combat the spread of superbugs. Furthermore, we use this opportunity to explore various groups of secondary metabolites and their biosynthetic pathways in various microbiota found in mammals, insects, and humans. This systematic study, when taken as a whole, offers detail understanding on the biomedical fate of various groups of compounds originated from diverse microbiomes. For gathering all information that has been uncovered and released so far, we have also presented the huge diversity of microbes that are associated with humans and their metabolic products. To conclude, this concrete review suggests novel ideas that will prove immensely helpful in reducing the danger posed by superbugs while also improving the efficacy of antibiotics.

Inhibitors of the Sec61 Complex and Novel High Throughput Screening Strategies to Target the Protein Translocation Pathway

Proteins targeted to the secretory pathway start their intracellular journey by being transported across biological membranes such as the endoplasmic reticulum (ER). A central component in this protein translocation process across the ER is the Sec61 translocon complex, which is only intracellularly expressed and does not have any enzymatic activity. In addition, Sec61 translocon complexes are difficult to purify and to reconstitute. Screening for small molecule inhibitors impairing its function has thus been notoriously difficult. However, such translocation inhibitors may not only be valuable tools for cell biology, but may also represent novel anticancer drugs, given that cancer cells heavily depend on efficient protein translocation into the ER to support their fast growth. In this review, different inhibitors of protein translocation will be discussed, and their specific mode of action will be compared. In addition, recently published screening strategies for small molecule inhibitors targeting the whole SRP-Sec61 targeting/translocation pathway will be summarized. Of note, slightly modified assays may be used in the future to screen for substances affecting SecYEG, the bacterial ortholog of the Sec61 complex, in order to identify novel antibiotic drugs.

Efficacy of an acid-oxidising solution (AOS) against Mycobacterium ulcerans

For the treatment of chronic wounds, acid-oxidizing solutions (AOSs) with broad-spectrum microbicidal activity without disturbing granulation tissue formation have been developed. We found AOSs to efficiently kill Mycobacterium ulcerans, the causative agent of Buruli ulcer, which is able to survive harsh decontamination treatments. Topical AOS treatment of Buruli ulcer lesions may support the recommended antibiotic therapy (oral rifampin and clarithromycin), prevent contamination of the environment by the mycobacteria, and control secondary infections, which are a prevalent wound management problem in resource-poor settings where Buruli ulcer is endemic.

Overview: Mycolactone , the Macrolide Toxin of Mycobacterium ulcerans

The acquisition by a Mycobacterium marinum-like progenitor of a plasmid encoding enzymes for the biosynthesis of the highly potent macrolide toxin mycolactone has set off the evolution of M. ulcerans toward a new mycobacterial species. While the selective advantage of producing mycolactone for survival in environmental niche(s) of the pathogen is unclear, there is no doubt that the cytotoxic, immunomodulatory, and analgesic properties of mycolactone are key for the establishment and progression of M. ulcerans infections in the host. Improved procedures for the isolation, handling, and detection of the amphiphilic and light-sensitive toxin have facilitated studies to unravel molecular mechanisms of mycolactone action on host cells in vitro and on cellular and immune responses in animal models. The pivotal role of mycolactone in the pathology of Buruli ulcer and the fact that the toxin has not been associated with other pathogens make it an ideal target for therapeutics/vaccines aiming at mycolactone neutralization and for the development of assays for the diagnosis of the disease.

Overview: Mycobacterium ulcerans Disease (Buruli Ulcer)

Enhanced international research efforts since the establishment of the Global BU Initiative in 1998 by the WHO have helped to advance our understanding of the epidemiology, and pathogenesis of Mycobacterium ulcerans infections. Improved methods to cultivate the extremely slow-growing pathogen from BU lesions have laid the groundwork for a variety of studies using M. ulcerans isolates, including the analysis of the genome and proteome of the pathogen, as well as drug susceptibility testing and analyses of host-pathogen interactions in vitro and in animal models. The identification of specific, high-copy number target sequences in the genome of M. ulcerans has enabled the development of diagnostic tests and assays to detect the pathogen in the environment. Important research questions remain about the reservoir(s) of M. ulcerans in aquatic environments, factors leading to or promoting transmission to hosts, and host-pathogen interactions resulting in chronic infection versus spontaneous healing.

Molecular Mechanisms Underpinning the Circulation and Cellular Uptake of Mycobacterium ulcerans Toxin Mycolactone

Mycolactone is a diffusible lipid toxin produced by Mycobacterium ulcerans, the causative agent of Buruli ulcer disease. Altough bacterially derived mycolactone has been shown to traffic from cutaneous foci of infection to the bloodstream, the mechanisms underpinning its access to systemic circulation and import by host cells remain largely unknown. Using biophysical and cell-based approaches, we demonstrate that mycolactone specific association to serum albumin and lipoproteins is necessary for its solubilization and is a major mechanism to regulate its bioavailability. We also demonstrate that Scavenger Receptor (SR)-B1 contributes to the cellular uptake of mycolactone. Overall, we suggest a new mechanism of transport and cell entry, challenging the dogma that the toxin enters host cells via passive diffusion.

Inhibition of the SEC61 translocon by mycolactone induces a protective autophagic response controlled by EIF2S1-dependent translation that does not require ULK1 activity

The Mycobacterium ulcerans exotoxin, mycolactone, is responsible for the immunosuppression and tissue necrosis that characterizes Buruli ulcer. Mycolactone inhibits SEC61-dependent co-translational translocation of proteins into the endoplasmic reticulum and the resultant cytosolic translation triggers degradation of mislocalized proteins by the ubiquitin-proteasome system. Inhibition of SEC61 by mycolactone also activates multiple EIF2S1/eIF2α kinases in the integrated stress response (ISR). Here we show mycolactone increased canonical markers of selective macroautophagy/autophagy LC3B-II, ubiquitin and SQSTM1/p62 in diverse disease-relevant primary cells and cell lines. Increased formation of puncta positive for the early autophagy markers WIPI2, RB1CC1/FIP200 and ATG16L1 indicates increased initiation of autophagy. The mycolactone response was SEC61A1-dependent and involved a pathway that required RB1CC1 but not ULK. Deletion of Sqstm1 reduced cell survival in the presence of mycolactone, suggesting this response protects against the increased cytosolic protein burden caused by the toxin. However, reconstitution of baseline SQSTM1 expression in cells lacking all autophagy receptor proteins could not rescue viability. Translational regulation by EIF2S1 in the ISR plays a key role in the autophagic response to mycolactone. Mycolactone-dependent induction of SQSTM1 was reduced in eif2ak3^−/-/perk^−/- cells while the p-EIF2S1 antagonist ISRIB reversed the upregulation of SQSTM1 and reduced RB1CC1, WIPI2 and LC3B puncta formation. Increased SQSTM1 staining could be seen in Buruli ulcer patient skin biopsy samples, reinforcing genetic data that suggests autophagy is relevant to disease pathology. Since selective autophagy and the ISR are both implicated in neurodegeneration, cancer and inflammation, the pathway uncovered here may have a broad relevance to human disease.

Abbreviations: ATF4: activating transcription factor 4; ATG: autophagy related; BAF: bafilomycin A₁; ATG16L1: autophagy related 16 like 1; BU: Buruli ulcer; CQ: chloroquine; EIF2AK3: eukaryotic translation initiation factor 2 alpha kinase 3; CALCOCO2: calcium binding and coiled-coil domain 2; DMSO: dimethyl sulfoxide; EIF2S1: eukaryotic translation initiation factor 2 subunit alpha; ER: endoplasmic reticulum; GFP: green fluorescent protein; HDMEC: human dermal microvascular endothelial cells; HFFF: human fetal foreskin fibroblasts; ISR: integrated stress response; ISRIB: integrated stress response inhibitor; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MEF: mouse embryonic fibroblast; Myco: mycolactone; NBR1: NBR1 autophagy cargo receptor; NFE2L2: nuclear factor, erythroid 2 like 2; OPTN: optineurin; PFA: paraformaldehyde; PtdIns3P: phosphatidylinositol-3-phosphate; RB1CC1: RB1-inducible coiled coil 1; SQSTM1: sequestosome 1; TAX1BP1: Tax1 binding protein 1; ULK: unc-51 like autophagy activating kinase; UPS: ubiquitin-proteasome system; WIPI: WD repeat domain, phosphoinositide interacting; WT: wild type.

Transcriptional adaptation of Mycobacterium ulcerans in an original mouse model: New insights into the regulation of mycolactone

Mycobacterium ulcerans is the causal agent of Buruli ulcer, a chronic infectious disease and the third most common mycobacterial disease worldwide. Without early treatment, M. ulcerans provokes massive skin ulcers, caused by the mycolactone toxin, its main virulence factor. However, spontaneous healing may occur in Buruli ulcer patients several months or years after the disease onset. We have shown, in an original mouse model, that bacterial load remains high and viable in spontaneously healed tissues, with a switch of M. ulcerans to low levels of mycolactone production, adapting its strategy to survive in such a hostile environment. This original model offers the possibility to investigate the regulation of mycolactone production, by using an RNA-seq strategy to study bacterial adaptation during mouse infection. Pathway analysis and characterization of the tissue environment showed that the bacillus adapted to its new environment by modifying its metabolic activity and switching nutrient sources. Thus, M. ulcerans ensures its survival in healing tissues by reducing its secondary metabolism, leading to an inhibition of mycolactone synthesis. These findings shed new light on mycolactone regulation and pave the way for new therapeutic strategies.

Natural antioxidants attenuate mycolactone toxicity to RAW 264.7 macrophages

Mycobacterium ulcerans produces a macrolide exotoxin, mycolactone which suppresses immune cells activity, is toxic to most cells and the key virulence factor in the pathogenesis of Buruli ulcer disease. Mycolactone is reported to mediate the production of reactive oxygen species in keratinocytes; cells that play critical role in wound healing. Increased levels of reactive oxygen species have been shown to disrupt the well-ordered process of wound repair; hence, the function of wound-healing cells such as macrophages, keratinocytes, and fibroblast could be impaired in the presence of the reactive oxygen species mediator, mycolactone. To ensure regeneration of tissues in chronic ulcers, with proper and timely healing of the wounds, natural antioxidants that can combat the effects of induced reactive oxygen species in wound-healing cells ought to be investigated. Reactive oxygen species activity was determined in mycolactone-treated RAW 264.7 macrophages and the scavenging ability of the antioxidants (ascorbic acid, gallic acid, and green tea kombucha) against mycolactone-induced reactive oxygen species (superoxide anions) was assessed using fluorescein probe (DCF-DA) and nitroblue tetrazolium dye. Cytotoxicity of the antioxidants, mycolactone, and the protective effect of the antioxidants on the cells upon treatment with mycolactone were determined using the Alamar blue assay. The expression levels of endogenous antioxidant enzyme genes (superoxide dismutase, catalase, and glutathione peroxidase) in response to mycolactone-mediated reactive oxygen species were determined using RT-qPCR. Mycolactone induced the production of reactive oxygen species in RAW 264.7 macrophages, and the resulting superoxide anions were scavenged by some of the antioxidants. The selected endogenous antioxidant enzyme genes in the macrophages were upregulated in the presence of the antioxidants and mycolactone. The exogenously supplied ascorbic acid and green tea kombucha offered moderate protection to the macrophages against the toxicity of mycolactone. We conclude that the results provide insights into alternate and adjunct therapeutic approaches in Buruli ulcer treatment, which could significantly attenuate the toxicity of the pathogenic factor; mycolactone.

An Antigen Capture Assay for the Detection of Mycolactone, the Polyketide Toxin of Mycobacterium ulcerans

Mycolactone is a cytotoxin responsible for most of the chronic necrotizing pathology of Mycobacterium ulcerans disease (Buruli ulcer). The polyketide toxin consists of a 12-membered lactone ring with a lower O-linked polyunsaturated acyl side chain and an upper C-linked side chain. Mycolactone is unique to M. ulcerans and an immunological Ag capture assay would represent an important tool for the study of Buruli ulcer pathogenesis and for laboratory diagnosis. When testing sets of mycolactone-specific mouse mAbs, we found that Abs against the hydrophobic lower side chain only bind mycolactone immobilized on a solid support but not when present in solution. This observation supports previous findings that mycolactone forms micellar structures in aqueous solution with the hydrophobic region sequestered into the inner core of the aggregates. Although an Ag capture assay typically requires two Abs that recognize nonoverlapping epitopes, our search for matching pairs of mAbs showed that the same mAb could be used both as capture and as detecting reagent for the detection of the mycolactone aggregates. However, the combination of a core-specific and a core/upper side chain-specific mAb constituted the most sensitive ELISA with a sensitivity in the low nanogram range. The results of a pilot experiment showed that the sensitivity of the assay is sufficient to detect mycolactone in swab samples from Buruli ulcer lesions. Although the described capture ELISA can serve as a tool for research on the biology of mycolactone, the assay system will have to be adapted for use as a diagnostic tool.

Ketogenic diet impairs Mycobacterium ulcerans growth and toxin production, enhancing hosts' response to the infection in an experimental mouse model

Ketogenic diets have been used to treat diverse conditions, and there is growing evidence of their benefits for tissue repair and in inflammatory disease treatment. However, their role in infectious diseases has been little studied. Buruli ulcer (Mycobacterium ulcerans infection) is a chronic infectious disease characterized by large skin ulcerations caused by mycolactone, the major virulence factor of the bacillus. Here, we investigated the impact of ketogenic diet on this cutaneous disease in an experimental mouse model. This diet prevented ulceration, by modulating bacterial growth and host inflammatory response. β-hydroxybutyrate, the major ketone body produced during ketogenic diet and diffusing in tissues, impeded M. ulcerans growth and mycolactone production in vitro underlying its potential key role in infection. These results pave the way for the development of new patient management strategies involving shorter courses of treatment and improving wound healing, in line with the major objectives of the World Health Organization.

Case Report: A New Mycobacterium ulcerans Genotype Causing Buruli Ulcer in Côte d'Ivoire

Mycobacterium ulcerans, the opportunistic pathogen causing Buruli ulcer, is reported to affect rural populations in 36 tropical countries. We report one case of Buruli ulcer in a peri-urban area in Côte d'Ivoire, confirmed by whole genome sequencing which indicated a M. ulcerans genotype previously unreported in Côte d'Ivoire.

The emerging role of exosomal miRNAs as a diagnostic and therapeutic biomarker in Mycobacterium tuberculosis infection

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), has been the world's driving fatal bacterial contagious disease globally. It continues a public health emergency, and around one-third of the global community has been affected by latent TB infection (LTBI). This is mostly due to the difficulty in diagnosing and treating patients with TB and LTBI. Exosomes are nanovesicles (40-100 nm) released from different cell types, containing proteins, lipids, mRNA, and miRNA, and they allow the transfer of one's cargo to other cells. The functional and diagnostic potential of exosomal miRNAs has been demonstrated in bacterial infections, including TB. Besides, it has been recognized that cells infected by intracellular pathogens such as Mtb can be secreting an exosome, which is implicated in the infection's fate. Exosomes, therefore, open a unique viewpoint on the investigative process of TB pathogenicity. This study explores the possible function of exosomal miRNAs as a diagnostic biomarker. Moreover, we include the latest data on the pathogenic and therapeutic role of exosomal miRNAs in TB.

Genetics in the Host-Mycobacterium ulcerans interaction

Buruli ulcer is an emerging infectious disease associated with high morbidity and unpredictable outbreaks. It is caused by Mycobacterium ulcerans, a slow-growing pathogen evolutionarily shaped by the acquisition of a plasmid involved in the production of a potent macrolide-like cytotoxin and by genome rearrangements and downsizing. These events culminated in an uncommon infection pattern, whereby M. ulcerans is both able to induce the initiation of the inflammatory cascade and the cell death of its proponents, as well as to survive within the phagosome and in the extracellular milieu. In such extreme conditions, the host is sentenced to rely on a highly orchestrated genetic landscape to be able to control the infection. We here revisit the dynamics of M. ulcerans infection, drawing parallels from other mycobacterioses and integrating the most recent knowledge on its evolution and pathogenicity in its interaction with the host immune response.

Reconstructive surgery for sequellae of Mycobacterium ulcerans infection (Buruli ulcer) of the upper limb

INTRODUCTION

Infection by Mycobacterium ulcerans constitutes a neglected tropical disease whose prevalence seems to have overrun those of cutaneous tuberculosis and leprosy. Its aggressivity depends on a mycolactone toxin. Lesions may involve skin, tendon and bone with a large spectrum of manifestations: non-ulcerative (papules, nodules, plaques), ulcerative and oedematous presentations as well as osteomyelitis with muscular contraction and ankylosis. Upper limbs account for more than two thirds of the infection sites. Surgical treatment may involve tendon transpositions, partial and total skin grafts. Amputation is relegated to extreme cases.

MATERIAL AND METHODS

Selected iconography from patients during the last 15 years is presented. At least 1500 cases had partial skin grafts (anterior thigh). Total skin grafts (inguinal region) were used in about 200 cases. Complex lesions involved 9 ilioinguinal flaps (5 boys, 4 girls, mean age 11.2 years, range 2-16 years), 5 tendon transfers (4 boys, one girl, mean age 15.4 years, range 12-19 years) and 3 resections of the first carpal row (2 girls, 1 boy, mean age 8 years, range 4-15 years).

RESULTS AND DISCUSSION

Out of 9 ilioinguinal flaps mild, marginal necrosis was the only complication in 2 patients without flap loss. Mean hospital stay was 26.44 days (range, 18-41 days), with return to full weight-bearing after a mean of 12 weeks (range 9-25 weeks) after discharge. Functional thumb opposition to allow pencil prehension was achieved in all three cases of resection of first carpal row resection without postoperative complications.

Ultra-short-course and intermittent TB47-containing oral regimens produce stable cure against Buruli ulcer in a murine model and prevent the emergence of resistance for Mycobacterium ulcerans

Buruli ulcer (BU), caused by Mycobacterium ulcerans, is currently treated with rifampin-streptomycin or rifampin-clarithromycin daily for 8 weeks recommended by World Health Organization (WHO). These options are lengthy with severe side effects. A new anti-tuberculosis drug, TB47, targeting QcrB in cytochrome bc1:aa3 complex is being developed in China. TB47-containing regimens were evaluated in a well-established murine model using an autoluminescent M. ulcerans strain. High-level TB47-resistant spontaneous M. ulcerans mutants were selected and their qcrB genes were sequenced. The in vivo activities of TB47 against both low-level and high-level TB47-resistant mutants were tested in BU murine model. Here, we show that TB47-containing oral 3-drug regimens can completely cure BU in ≤2 weeks for daily use or in ≤3 weeks given twice per week (6 doses in total). All high-level TB47-resistant mutants could only be selected using the low-level mutants which were still sensitive to TB47 in mice. This is the first report of double mutations in QcrB in mycobacteria. In summary, TB47-containing regimens have promise to cure BU highly effectively and prevent the emergence of drug resistance. Novel QcrB mutations found here may guide the potential clinical molecular diagnosis of resistance and the discovery of new drugs against the high-level resistant mutants.

The cell surface protein MUL_3720 confers binding of the skin pathogen Mycobacterium ulcerans to sulfated glycans and keratin

Mycobacterium ulcerans is the causative agent of the chronic, necrotizing skin disease Buruli ulcer. Modes of transmission and molecular mechanisms involved in the establishment of M. ulcerans infections are poorly understood. Interactions with host glycans are often crucial in bacterial pathogenesis and the 22 kDa M. ulcerans protein MUL_3720 has a putative role in host cell attachment. It has a predicted N-terminal lectin domain and a C-terminal peptidoglycan-binding domain and is highly expressed on the surface of the bacilli. Here we report the glycan-binding repertoire of whole, fixed M. ulcerans bacteria and of purified, recombinant MUL_3720. On an array comprising 368 diverse biologically relevant glycan structures, M. ulcerans cells showed binding to 64 glycan structures, representing several distinct classes of glycans, including sulfated structures. MUL_3720 bound only to glycans containing sulfated galactose and GalNAc, such as glycans known to be associated with keratins isolated from human skin. Surface plasmon resonance studies demonstrated that both whole, fixed M. ulcerans cells and MUL_3720 show high affinity interactions with both glycans and human skin keratin extracts. This MUL_3720-mediated interaction with glycans associated with human skin keratin may contribute to the pathobiology of Buruli ulcer.

Mycobacterium ulcerans causes a skin-based disease known as Buruli ulcer. How the bacteria are transmitted and what mechanisms they use to establish the infection of the skin is poorly understood. The only well characterized bacterial factor in Buruli ulcer pathogenesis is mycolactone, a toxin produced by the bacteria. Mycolactone causes apoptosis in human cells, leading to destruction of the skin around extracellular clusters of the mycobacteria. Human cells, like cells of all orders of life, are coated in complex sugar structures and these glycans are one of the major targets of bacteria and viruses for the interaction with host cells. Here we describe the glycan binding of whole Mycobacterium ulcerans cells and a mycobacterial protein, MUL_3720, thought to be involved in glycan binding. We show that both the bacterial cells and MUL_3720 bind to glycans known to be associated with human skin keratin and to skin keratin extracts. This binding of keratin extracts may explain initial bacterial attachment and clustering of the bacteria in the skin, ultimately leading to tissue destruction and ulceration caused by a cloud of secreted mycolactone at the site of infection.