Globally distributed mycobacterial fish pathogens produce a novel plasmid-encoded toxic macrolide, mycolactone F

Exploring Mycolactone-The Unique Causative Toxin of Buruli Ulcer: Biosynthetic, Synthetic Pathways, Biomarker for Diagnosis, and Therapeutic Potential

Mycolactone is a complex macrolide toxin produced by Mycobacterium ulcerans, the causative agent of Buruli ulcer. The aim of this paper is to review the chemistry, biosynthetic, and synthetic pathways of mycolactone A/B to help develop an understanding of the mode of action of these polyketides as well as their therapeutic potential. The synthetic work has largely been driven by the desire to afford researchers enough (≥100 mg) of the pure toxins for systematic biological studies toward understanding their very high biological activities. The review focuses on pioneering studies of Kishi which elaborate first-, second-, and third-generation approaches to the synthesis of mycolactones A/B. The three generations focused on the construction of the key intermediates required for the mycolactone synthesis. Synthesis of the first generation involves assignment of the relative and absolute stereochemistry of the mycolactones A and B. This was accomplished by employing a linear series of 17 chemical steps (1.3% overall yield) using the mycolactone core. The second generation significantly improved the first generation in three ways: (1) by optimizing the selection of protecting groups; (2) by removing needless protecting group adjustments; and (3) by enhancing the stereoselectivity and overall synthetic efficiency. Though the synthetic route to the mycolactone core was longer than the first generation, the overall yield was significantly higher (8.8%). The third-generation total synthesis was specifically aimed at an efficient, scalable, stereoselective, and shorter synthesis of mycolactone. The synthesis of the mycolactone core was achieved in 14 linear chemical steps with 19% overall yield. Furthermore, a modular synthetic approach where diverse analogues of mycolactone A/B were synthesized via a cascade of catalytic and/or asymmetric reactions as well as several Pd-catalyzed key steps coupled with hydroboration reactions were reviewed. In addition, the review discusses how mycolactone is employed in the diagnosis of Buruli ulcer with emphasis on detection methods of mass spectrometry, immunological assays, RNA aptamer techniques, and fluorescent-thin layer chromatography (f-TLC) methods as diagnostic tools. We examined studies of the structure-activity relationship (SAR) of various analogues of mycolactone. The paper highlights the multiple biological consequences associated with mycolactone such as skin ulceration, host immunomodulation, and analgesia. These effects are attributed to various proposed mechanisms of actions including Wiskott-Aldrich Syndrome protein (WASP)/neural Wiskott-Aldrich Syndrome protein (N-WASP) inhibition, Sec61 translocon inhibition, angiotensin II type 2 receptor (AT2R) inhibition, and inhibition of mTOR. The possible application of novel mycolactone analogues produced based on SAR investigations as therapeutic agents for the treatment of inflammatory disorders and inflammatory pain are discussed. Additionally, their therapeutic potential as anti-viral and anti-cancer agents have also been addressed.

Systematic review: Global host range, case fatality and detection rates of Mycobacterium ulcerans in humans and potential environmental sources

Fundamental aspects of the epidemiology and ecology of Mycobacterium ulcerans (MU) infections including disease burden, host range, reservoir, intermediate hosts, vector and mode of transmission are poorly understood. Understanding the global distribution and burden of MU infections is a paramount to fight against Buruli ulcer (BU). Four databases were queried from inception through December 2023. After critical review of published resources on BU, 155 articles (645 records) published between 1987 and 2023 from 16 countries were selected for this review. Investigating BU in from old endemic and new emerging foci has allowed detection of MU in humans, animals, plants and various environmental samples with prevalence from 0 % up to 100 % depending of the study design. A case fatality rate between 0.0 % and 50 % was described from BU patients and deaths occurred in Central African Republic, Gabon, Democratic Republic of the Congo, Burkina Faso and Australia. The prevalence of MU in humans was higher in Africa. Nucleic Acid Amplification Tests (NAAT) and non-NAAT were performed in > 38 animal species. MU has been recovered in culture from possum faeces, aquatic bugs and koala. More than 7 plant species and several environmental samples have been tested positive for MU. This review provided a comprehensive set of data on the updates of geographic distribution, the burden of MU infections in humans, and the host range of MU in non-human organisms. Although MU have been found in a wide range of environmental samples, only few of these have revealed the viability of the mycobacterium and the replicative non-human reservoirs of MU remain to be explored. These findings should serve as a foundation for further research on the reservoirs, intermediate hosts and transmission routes of MU.

High-mortality epizootic Mycobacterium ulcerans ecovar Liflandii in a colony of Zaire Dwarf Clawed Frogs (Hymenochirus boettgeri)

Mycobacterium ulcerans ecovar Liflandii (MuLiflandii) was identified as the causative agent of mycobacteriosis in a research colony of Zaire dwarf clawed frogs (Hymenochirus boettgeri) at the University of Michigan. Clinical presentation included lethargy, generalized septicemia, cutaneous granulomas, coelomic effusion, and acute mortality. Identification of the mycobacterial species was based on molecular, microbiological, and histopathologic characteristics. These findings indicate that MuLiflandii is a primary cause of morbidity and mortality in Zaire dwarf clawed frogs and should be considered in the differential diagnosis of sepsis and coelomic effusion in amphibians. Mycobacterial speciation is important given the variability in pathogenesis within the family Mycobacteriaceae and the implications for both animal and human health as potential zoonoses. The Zaire dwarf clawed frog is a species common in the pet trade, and these findings provide consideration for this pathogen as a potentially important public health concern. This is the first report of MuLiflandii infection in the genus Hymenochirus and illustrates the diagnostic challenges of differentiating among both mycolactone-producing mycobacteria and Mycobacterium marinum. Furthermore, we demonstrate the utility of environmental sampling for this pathogen within the tank system, suggesting this mode of sampling could replace the need for direct frog surveillance.

Mycobacterium ulcerans challenge strain selection for a Buruli ulcer controlled human infection model

Critical scientific questions remain regarding infection with Mycobacterium ulcerans, the organism responsible for the neglected tropical disease, Buruli ulcer (BU). A controlled human infection model has the potential to accelerate our knowledge of the immunological correlates of disease, to test prophylactic interventions and novel therapeutics. Here we present microbiological evidence supporting M. ulcerans JKD8049 as a suitable human challenge strain. This non-genetically modified Australian isolate is susceptible to clinically relevant antibiotics, can be cultured in animal-free and surfactant-free media, can be enumerated for precise dosing, and has stable viability following cryopreservation. Infectious challenge of humans with JKD8049 is anticipated to imitate natural infection, as M. ulcerans JKD8049 is genetically stable following in vitro passage and produces the key virulence factor, mycolactone. Also reported are considerations for the manufacture, storage, and administration of M. ulcerans JKD8049 for controlled human infection.

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.

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.

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.

Actinobacteria from Arctic and Atlantic deep-sea sediments-Biodiversity and bioactive potential

The deep-sea covers over 70% of the Earth's surface and harbors predominantly uncharacterized bacterial communities. Actinobacteria are the major prokaryotic source of bioactive natural products that find their way into drug discovery programs, and the deep-sea is a promising source of biotechnologically relevant actinobacteria. Previous studies on actinobacteria in deep-sea sediments were either regionally restricted or did not combine a community characterization with the analysis of their bioactive potential. Here we characterized the actinobacterial communities of upper layers of deep-sea sediments from the Arctic and the Atlantic (Azores and Madeira) ocean basins, employing 16S rRNA metabarcoding, and studied the biosynthetic potential of cultivable actinobacteria retrieved from those samples. Metabarcoding analysis showed that the actinobacterial composition varied between the sampled regions, with higher abundance in the Arctic samples but higher diversity in the Atlantic ones. Twenty actinobacterial genera were detected using metabarcoding, as a culture-independent method, while culture-dependent methods only allowed the identification of nine genera. Isolation of actinobacteria resulted on the retrieval of 44 isolates, mainly associated with Brachybacterium, Microbacterium, and Brevibacterium genera. Some of these isolates were only identified on a specific sampled region. Chemical extracts of the actinobacterial isolates were subsequently screened for their antimicrobial, anticancer and anti-inflammatory activities. Extracts from two Streptomyces strains demonstrated activity against Candida albicans. Additionally, eight extracts (obtained from Brachybacterium, Brevibacterium, Microbacterium, Rhodococcus, and Streptomyces isolates) showed significant activity against at least one of the tested cancer cell lines (HepG2 and T-47D). Furthermore, 15 actinobacterial extracts showed anti-inflammatory potential in the RAW 264.4 cell model assay, with no concomitant cytotoxic response. Dereplication and molecular networking analysis of the bioactive actinobacterial extracts showed the presence of some metabolites associated with known natural products, but one of the analyzed clusters did not show any match with the natural products described as responsible for these bioactivities. Overall, we were able to recover taxonomically diverse actinobacteria with different bioactivities from the studied deep-sea samples. The conjugation of culture-dependent and -independent methods allows a better understanding of the actinobacterial diversity of deep-sea environments, which is important for the optimization of approaches to obtain novel chemically-rich isolates.

Complete Genome and Partial Megaplasmid Sequences of Mycobacterium pseudoshottsii Strain NJB1907-Z4, Isolated from an Aquarium-Reared Japanese Sardine (Sardinops melanostictus) in Japan

Mycobacterium pseudoshottsii , a slow-growing nontuberculous mycobacterium, has been isolated from wild and cultured fish. We report here the complete genome and partial megaplasmid sequences of a strain isolated from an aquarium-reared Japanese sardine ( Sardinops melanostictus ) in Japan, M. pseudoshottsii NJB1907-Z4.

Host–Pathogen Interactions of Marine Gram-Positive Bacteria

Simple Summary

Complex interactions between marine Gram-positive pathogens and fish hosts in the marine environment can result in diseases of economically important finfish, which cause economic losses in the aquaculture industry. Understanding how these pathogens interact with the fish host and generate disease will contribute to efficient prophylactic measures and treatments. To our knowledge, there are no systematic reviews on marine Gram-positive pathogens. Therefore, here we reviewed the host–pathogen interactions of marine Gram-positive pathogens from the pathogen-centric and host-centric points of view.

Abstract

Marine Gram-positive bacterial pathogens, including Renibacterium salmoninarum, Mycobacterium marinum, Nocardia seriolae, Lactococcus garvieae, and Streptococcus spp. cause economic losses in marine fish aquaculture worldwide. Comprehensive information on these pathogens and their dynamic interactions with their respective fish–host systems are critical to developing effective prophylactic measures and treatments. While much is known about bacterial virulence and fish immune response, it is necessary to synthesize the knowledge in terms of host–pathogen interactions as a centerpiece to establish a crucial connection between the intricate details of marine Gram-positive pathogens and their fish hosts. Therefore, this review provides a holistic view and discusses the different stages of the host–pathogen interactions of marine Gram-positive pathogens. Gram-positive pathogens can invade fish tissues, evade the fish defenses, proliferate in the host system, and modulate the fish immune response. Marine Gram-positive pathogens have a unique set of virulence factors that facilitate adhesion (e.g., adhesins, hemagglutination activity, sortase, and capsules), invasion (e.g., toxins, hemolysins/cytolysins, the type VII secretion system, and immune-suppressive proteins), evasion (e.g., free radical quenching, actin-based motility, and the inhibition of phagolysosomal fusion), and proliferation and survival (e.g., heme utilization and siderophore-mediated iron acquisition systems) in the fish host. After infection, the fish host initiates specific innate and adaptive immune responses according to the extracellular or intracellular mechanism of infection. Although efforts have continued to be made in understanding the complex interplay at the host–pathogen interface, integrated omics-based investigations targeting host–pathogen–marine environment interactions hold promise for future research.

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.

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.

Mycobacterium pseudoshottsii in Mediterranean Fish Farms: New Trouble for European Aquaculture?

Mycobacterium pseudoshottsii, a slow-growing mycobacterium closely related to M. marinum, has been isolated only in wild fish in the United States and in Japanese fish farms to date. Here, we report cases of mortality in three farmed fish species (Dicentrarchus labrax, Sparus aurata, and Sciaenops ocellatus) caused by M. pseudoshottsii in Italy. Samples underwent necropsy, histology, and culture with pathogen identification based on PCR and sequencing of housekeeping genes (16S rRNA, hsp65, rpoB). Multifocal to coalescing granulomatous and necrotizing inflammation with acid-fast bacilli were observed in the parenchymatous organs, from which M. pseudoshottsii was isolated and identified. Phylogenetic analysis confirmed the results of gene sequencing and allowed subdivision of the isolates into three distinct groups. M. pseudoshottsii poses a potential threat for Mediterranean aquaculture. Its origin in the area under study needs to be clarified, as well as the threat to the farmed fish species.

The diversity of mycolactone-producing mycobacteria

Mycolactone-producing mycobacteria (MPM) form an intriguing group of environmental opportunistic pathogens of mammals and human patients in whom they cause cutaneous and subcutaneous ulcers known as "Buruli ulcer" when they occur in humans. We reviewed whole genome sequence data and ecological and phenotypic characteristics from 44 MPMs and closely related Mycobacterium marinum. This analysis indicated that all the 24 M. marinum isolates were delineated into seven taxa and our comprehensive, polyphasic taxonomic approach led to the proposal of delineating M. marinum genomospecies, 01-07. Likewise, 20 MPMs isolates were delineated into seven additional M. ulcerans genomospecies, 01-07. A taxonomic card explaining the ecology, hosts of isolation and the plasmid harboured is provided for each taxon.

The immunology of other mycobacteria: M. ulcerans, M. leprae

Mycobacterial pathogens can be categorized into three broad groups: Mycobacterium tuberculosis complex causing tuberculosis, M. leprae and M. lepromatosis causing leprosy, and atypical mycobacteria, or non-tuberculous mycobacteria (NTM), responsible for a wide range of diseases. Among the NTMs, M. ulcerans is responsible for the neglected tropical skin disease Buruli ulcer (BU). Most pathogenic mycobacteria, including M. leprae, evade effector mechanisms of the humoral immune system by hiding and replicating inside host cells and are furthermore excellent modulators of host immune responses. In contrast, M. ulcerans replicates predominantly extracellularly, sheltered from host immune responses through the cytotoxic and immunosuppressive effects of mycolactone, a macrolide produced by the bacteria. In the year 2018, 208,613 new cases of leprosy and 2713 new cases of BU were reported to WHO, figures which are notoriously skewed by vast underreporting of these diseases.

Buruli ulcer: The Efficacy of Innate Immune Defense May Be a Key Determinant for the Outcome of Infection With Mycobacterium ulcerans

Buruli ulcer (BU) is a neglected, tropical infectious disease of the skin and the subcutaneous tissue caused by Mycobacterium ulcerans. This pathogen has emerged as a new species from a common ancestor with Mycobacterium marinum by acquisition of the virulence plasmid pMUM. The plasmid encodes enzymes required for the synthesis of the macrolide toxin mycolactone, which has cytotoxic and immunosuppressive activities. In advanced BU lesions, extracellular clusters of M. ulcerans reside in necrotic subcutaneous tissue and are protected from infiltrating leukocytes by the cytotoxic activity of secreted mycolactone. Several lines of evidence indicate that elements of the innate immune system eliminate in many cases the initial inoculum before bacterial clusters can form and that therefore exposure to M. ulcerans leads only in a minority of individuals to the characteristic chronic necrotizing BU lesions. It is assumed that phagocytes play a key role in early host defense against M. ulcerans. Antibodies against bacterial surface structures seem to have less potential to enhance innate immunity than T_H1 cell responses. Precise innate and adaptive immune effector mechanisms leading to protective immunity are however unclear, complicating the development of effective vaccines, the most desired solution to control BU. The tuberculosis vaccine Mycobacterium bovis Bacillus Calmette–Guérin (BCG) has limited short-term protective activity against BU. Whether this effect is due to the broad antigenic cross-reactivity between M. bovis and M. ulcerans or is at least partly mediated by a non-specific enhanced responsiveness of innate immune cells to secondary stimulation, recently described as “trained immunity” or “innate immune memory” is unknown but has major implications for vaccine design. Current vaccine research and development activities are focusing on recombinant BCG, subunit vaccines with selected M. ulcerans proteins, and the neutralization of mycolactone.

Mycobacterium ulcerans mycolactones-fungi crosstalking

The opportunistic pathogen Mycobacterium ulcerans, which is responsible for Buruli ulcer, synthesizes a series of plasmid-encoded macrolide exotoxins termed mycolactones. These toxins destabilize cell membranes and induce apoptosis-associated pleiotropic effects including tissue destruction, analgesic and anti-inflammatory effects. Despite its medical interest, M. ulcerans is primarily an environmental mycobacterium and the primary functions of mycolactones in the natural ecosystems are unknown. High throughput biochemical profiling findings suggested that M. ulcerans may interact with fungi. Here, we report that semi-purified and purified mycolactones significantly enhance spore germination of Scedosporium apiospermum, Fusarium equiseti and Mucor circinelloides; and that M. ulcerans mycolactones significantly attract colonies of M. circinelloides whereas no significant effect was observed on S. apiospermum and F. equiseti. These experimental results suggest that mycolactones exhibit a chemoattractant activity independent of their cytotoxicity. In natural ecosystems, M. ulcerans mycolactones may act as spore germination inducers and chemoattractants for some fungi, suggesting a novel role for this unique class of mycobacterial toxins in natural ecosystems.

Membrane perturbing properties of toxin mycolactone from Mycobacterium ulcerans

Mycolactone is the exotoxin produced by Mycobacterium ulcerans and is the virulence factor behind the neglected tropical disease Buruli ulcer. The toxin has a broad spectrum of biological effects within the host organism, stemming from its interaction with at least two molecular targets and the inhibition of protein uptake into the endoplasmic reticulum. Although it has been shown that the toxin can passively permeate into host cells, it is clearly lipophilic. Association with lipid carriers would have substantial implications for the toxin’s distribution within a host organism, delivery to cellular targets, diagnostic susceptibility, and mechanisms of pathogenicity. Yet the toxin’s interactions with, and distribution in, lipids are unknown. Herein we have used coarse-grained molecular dynamics simulations, guided by all-atom simulations, to study the interaction of mycolactone with pure and mixed lipid membranes. Using established techniques, we calculated the toxin’s preferential localization, membrane translocation, and impact on membrane physical and dynamical properties. The computed water-octanol partition coefficient indicates that mycolactone prefers to be in an organic phase rather than in an aqueous environment. Our results show that in a solvated membrane environment the exotoxin mainly localizes in the water-membrane interface, with a preference for the glycerol moiety of lipids, consistent with the reported studies that found it in lipid extracts of the cell. The calculated association constant to the model membrane is similar to the reported association constant for Wiskott-Aldrich syndrome protein. Mycolactone is shown to modify the physical properties of membranes, lowering the transition temperature, compressibility modulus, and critical line tension at which pores can be stabilized. It also shows a tendency to behave as a linactant, a molecule that localizes at the boundary between different fluid lipid domains in membranes and promotes inter-mixing of domains. This property has implications for the toxin’s cellular access, T-cell immunosuppression, and therapeutic potential.

Mycolactone is a macrolide exotoxin secreted by Mycobacterium ulcerans, which causes a skin disease called Buruli ulcer, a neglected emerging disease. It is the third most common mycobacterial disease after tuberculosis and leprosy. Studies have shown how mycolactone plays a pivotal role in Buruli ulcer pathogenesis, and identified it as an attractive therapeutic target. This multifunctional cytotoxin exerts multiple local and global responses, including ulcerative, analgesic, and anti-inflammatory effects. Prompted by its lipid-like structure, we used extensive multi-resolution simulations to probe mycolactone’s interactions with model membranes. Our results suggest that mycolactone is sequestered in membranes where it alters several dynamical, physical, and mechanical properties. It also behaves as a linactant, localizing at the interface between lipid domains and decreasing the inter-domain line tension. Our results shed light on how mycolactone permeates host cell membranes and is distributed between lipid and aqueous environments. These findings have significant implications for the toxin’s distribution in the host environment and mechanisms of pathogenicity. Understanding the toxin’s distribution and mechanism of trafficking will have ramifications for targeted diagnostics, therapeutic approaches, and our understanding of Buruli ulcer pathogenesis.

Buruli Ulcer, a Prototype for Ecosystem-Related Infection, Caused by Mycobacterium ulcerans

Buruli ulcer is a noncontagious disabling cutaneous and subcutaneous mycobacteriosis reported by 33 countries in Africa, Asia, Oceania, and South America. The causative agent, Mycobacterium ulcerans, derives from Mycobacterium marinum by genomic reduction and acquisition of a plasmid-borne, nonribosomal cytotoxin mycolactone, the major virulence factor. M. ulcerans-specific sequences have been readily detected in aquatic environments in food chains involving small mammals. Skin contamination combined with any type of puncture, including insect bites, is the most plausible route of transmission, and skin temperature of <30°C significantly correlates with the topography of lesions. After 30 years of emergence and increasing prevalence between 1970 and 2010, mainly in Africa, factors related to ongoing decreasing prevalence in the same countries remain unexplained. Rapid diagnosis, including laboratory confirmation at the point of care, is mandatory in order to reduce delays in effective treatment. Parenteral and potentially toxic streptomycin-rifampin is to be replaced by oral clarithromycin or fluoroquinolone combined with rifampin. In the absence of proven effective primary prevention, avoiding skin contamination by means of clothing can be implemented in areas of endemicity. Buruli ulcer is a prototype of ecosystem pathology, illustrating the impact of human activities on the environment as a source for emerging tropical infectious diseases.

The chemistry and biology of mycolactones

Mycolactones are a group of macrolides excreted by the human pathogen Mycobacterium ulcerans, which exhibit cytotoxic, immunosuppressive and analgesic properties. As the virulence factor of M. ulcerans, mycolactones are central to the pathogenesis of the neglected disease Buruli ulcer, a chronic and debilitating medical condition characterized by necrotic skin ulcers. Due to their complex structure and fascinating biology, mycolactones have inspired various total synthesis endeavors and structure-activity relationship studies. Although this review intends to cover all synthesis efforts in the field, special emphasis is given to the comparison of conceptually different approaches and to the discussion of more recent contributions. Furthermore, a detailed discussion of molecular targets and structure-activity relationships is provided.

Mycobacterium marinum

Mycobacterium marinum is a well-known pathogenic mycobacterium for skin and soft tissue infections and is associated with fishes and water. Among nontuberculous mycobacteria (NTM), it is the leading cause of extrarespiratory human infections worldwide. In addition, there is a specific scientific interest in M. marinum because of its genetic relatedness to Mycobacterium tuberculosis and because experimental infection of M. marinum in fishes mimics tuberculosis pathogenesis. Microbiological characteristics include the fact that it grows in 7 to 14 days with photochromogenic colonies and is difficult to differentiate from Mycobacterium ulcerans and other mycolactone-producing NTM on a molecular basis. The diagnosis is highly suspected by the mode of infection, which is related to the hobby of fishkeeping, professional handling of marine shells, or swimming in nonchlorinated pools. Clinics distinguished skin and soft tissue lesions (typically sporotrichoid or subacute hand nodules) and lesions disseminated to joint and bone, often related with the local use of corticosteroids. In clinical microbiology, microscopy and culture are often negative because growth requires low temperature (30°C) and several weeks to succeed in primary cultivation. The treatment is not standardized, and no randomized control trials have been done. Therapy is a combination of surgery and antimicrobial agents such as cyclines and rifampin, with successful outcome in most of the skin diseases but less frequently in deep tissue infections. Prevention can be useful with hand protection recommendations for professionals and all persons manipulating fishes or fish tank water and use of alcohol disinfection after contact.

Mycolactone-producing Mycobacterium marinum infection in captive Hong Kong warty newts and pathological evidence of impaired host immune function

A mass mortality event of captive Hong Kong warty newts Paramesotriton hongkongensis with non-granulomatous necrotic lesions occurred in Taipei Zoo, Taiwan, in 2014. Clinically, the sick newts were lethargic and often covered with water mold Saprolegnia sp. on the skin of the body trunk or extremities. Predominant pathological findings were multifocal non-granulomatous necrotic lesions in the liver, spleen, and kidneys and severe skin infection with Saprolegnia sp., with deep invasion and involvement of underlying muscles. The possibility of ranavirus infection was ruled out by negative PCR results. Unexpectedly, abundant intralesional acid-fast positive bacilli were found in the necrotic lesions of the liver, spleen, and kidney in all 14 sick newts. PCR targeting the hsp65, ITS region, and partial 16S rRNA genes was performed, and the sequence identity from amplified amplicons of hsp65 and partial 16S rRNA genes was 100% identical to that of the corresponding gene fragment of Mycobacterium marinum. Further molecular investigations demonstrated that the current M. marinum was a mycolactone-producing mycobacterium with the presence of esxA/esxB genes. Mycolactone is a plasmid-encoded, immunosuppressive, and cytotoxic toxin. The possible immunosuppression phenomenon characterized by systemic non-granulomatous necrotic lesions caused by M. marinum and the unusual deep invasive infection caused by water mold might be associated with the immunosuppressive effect of mycolactone. Therefore, it should be noted that non-granulomatous necrotic lesions in amphibians can be caused not only by ranavirus infection but also by mycobacteriosis.

Molecular Characterization of Mycolactone Producing Mycobacteria from Aquatic Environments in Buruli Ulcer Non-Endemic Areas in Côte d'Ivoire

Non-tuberculous mycobacteria (NTM), particularly mycolactone producing mycobacteria (MPM), are bacteria found in aquatic environments causing skin diseases in humans like Buruli ulcer (BU). Although the causative agent for BU, Mycobacterium ulcerans has been identified and associated with slow-moving water bodies, the real transmission route is still unknown. This study aimed to characterize MPMs from environmental aquatic samples collected in a BU non-endemic community, Adiopodoumé, in Côte d’Ivoire. Sixty samples were collected in four types of matrices (plant biofilms, water filtrate residues, plant detritus and soils) from three water bodies frequently used by the population. Using conventional polymerase chain reaction (PCR), MPMs were screened for the 16S ribosomal RNA (rRNA) mycobacterial gene, the IS2404 insertion sequence, and MPM enoyl reductase (ER) gene. Variable Number Tandem Repeat (VNTR) typing with loci 6, 19, mycobacterial interspersed repetitive unit 1 (MIRU1) and sequence type 1(ST1) was performed to discriminate between different MPMs. Our findings showed 66.7%, 57.5% and 43.5% of positivity respectively for 16S rRNA, IS2404 and ER. MPM discrimination using VNTR typing did not show any positivity and therefore did not allow precise MPM distinction. Nevertheless, the observed contamination of some water bodies in a BU non-endemic community by MPMs suggests the possibility of pathogen dissemination and transmission to humans. These aquatic environments could also serve as reservoirs that should be considered during control and prevention strategies.

Recent advances: role of mycolactone in the pathogenesis and monitoring of Mycobacterium ulcerans infection/Buruli ulcer disease

Infection of subcutaneous tissue with Mycobacterium ulcerans can lead to chronic skin ulceration known as Buruli ulcer. The pathogenesis of this neglected tropical disease is dependent on a lipid‐like toxin, mycolactone, which diffuses through tissue away from the infecting organisms. Since its identification in 1999, this molecule has been intensely studied to elucidate its cytotoxic and immunosuppressive properties. Two recent major advances identifying the underlying molecular targets for mycolactone have been described. First, it can target scaffolding proteins (such as Wiskott Aldrich Syndrome Protein), which control actin dynamics in adherent cells and therefore lead to detachment and cell death by anoikis. Second, it prevents the co‐translational translocation (and therefore production) of many proteins that pass through the endoplasmic reticulum for secretion or placement in cell membranes. These pleiotropic effects underpin the range of cell‐specific functional defects in immune and other cells that contact mycolactone during infection. The dose and duration of mycolactone exposure for these different cells explains tissue necrosis and the paucity of immune cells in the ulcers. This review discusses recent advances in the field, revisits older findings in this context and highlights current developments in structure‐function studies as well as methodology that make mycolactone a promising diagnostic biomarker.

Detection of Mycobacterium ulcerans DNA in the Environment, Ivory Coast

Background

Ivory Coast is a West African country with the highest reported cases of Buruli ulcer, a disabling subcutaneous infection due to Mycobacterium ulcerans. However, the prevalence of environmental M. ulcerans is poorly known in this country.

Methods

We collected 496 environmental specimens consisting of soil (n = 100), stagnant water (n = 200), plants (n = 100) and animal feces (n = 96) in Ivory Coast over five months in the dry and wet seasons in regions which are free of Buruli ulcer (control group A; 250 specimens) and in regions where the Buruli ulcer is endemic (group B; 246 specimens). After appropriate total DNA extraction incorporating an internal control, the M. ulcerans IS2404 and KR-B gene were amplified by real-time PCR in samples. In parallel, a calibration curve was done for M. ulcerans Agy99 IS2404 and KR-B gene.

Results

Of 460 samples free of PCR inhibition, a positive real-time PCR detection of insertion sequence IS2404 and KR-B gene was observed in 1/230 specimens in control group A versus 9/230 specimens in group B (P = 0.02; Fisher exact test). Positive specimens comprised seven stagnant water specimens, two feces specimens confirmed to be of Thryonomys swinderianus (agouti) origin by real-time PCR of the cytb gene; and one soil specimen. Extrapolation from the calibration curves indicated low inoculums ranging from 1 to 10² mycobacteria/mL.

Conclusion

This study confirms the presence of M. ulcerans in the watery environment surrounding patients with Buruli ulcer in Ivory Coast. It suggests that the agouti, which is in close contacts with populations, could play a role in the environmental cycle of M. ulcerans, as previously suggested for the closely related possums in Australia.

Short Convergent Synthesis of the Mycolactone Core Through Lithiation-Borylation Homologations

Using iterative lithiation-borylation homologations, the mycolactone toxin core has been synthesized in 13 steps and 17% overall yield. The rapid build-up of molecular complexity, high convergence and high stereoselectivity are noteworthy features of this synthesis.

Heterogeneity among Mycobacterium ulcerans from French Guiana revealed by multilocus variable number tandem repeat analysis (MLVA)

Buruli ulcer is an emerging and neglected tropical disease caused by Mycobacterium ulcerans. Few cases have been reported so far in the Americas. With 250 cases reported since 1969, French Guiana is the only Buruli ulcer endemic area in the continent. Thus far, no genetic diversity studies of strains of M. ulcerans from French Guiana have been reported. Our goal in the present study was to examine the genetic diversity of M. ulcerans strains in this region by using the Multilocus Variable Number Tandem Repeat Analysis (MLVA) approach. A total of 23 DNA samples were purified from ulcer biopsies or derived from pure cultures. MVLA was used in the study of six previously-described Variable Number of Tandem Repeat (VNTR) markers. A total of three allelic combinations were characterized in our study: genotype I which has been described previously, genotype III which is very similar to genotype I, and genotype II which has distinctly different characteristics in comparison with the other two genotypes. This high degree of genetic diversity appears to be uncommon for M. ulcerans. Further research based on complete genome sequencing of strains belonging to genotypes I and II is in progress and should lead soon to a better understanding of genetic specificities of M. ulcerans strains from French Guiana.

Bacterial zoonoses of fishes: a review and appraisal of evidence for linkages between fish and human infections

Human contact with and consumption of fishes presents hazards from a range of bacterial zoonotic infections. Whereas many bacterial pathogens have been presented as fish-borne zoonoses on the basis of epidemiological and phenotypic evidence, genetic identity between fish and human isolates is not frequently examined or does not provide support for transmission between these hosts. In order to accurately assess the zoonotic risk from exposure to fishes in the context of aquaculture, wild fisheries and ornamental aquaria, it is important to critically examine evidence of linkages between bacteria infecting fishes and humans. This article reviews bacteria typically presented as fish-borne zoonoses, and examines the current strength of evidence for this classification. Of bacteria generally described as fish-borne zoonoses, only Mycobacterium spp., Streptococcus iniae, Clostridium botulinum, and Vibrio vulnificus appear to be well-supported as zoonoses in the strict sense. Erysipelothrix rhusiopathiae, while transmissible from fishes to humans, does not cause disease in fishes and is therefore excluded from the list. Some epidemiological and/or molecular linkages have been made between other bacteria infecting both fishes and humans, but more work is needed to elucidate routes of transmission and the identity of these pathogens in their respective hosts at the genomic level.

Late onset of the serological response against the 18 kDa small heat shock protein of Mycobacterium ulcerans in children

A previous survey for clinical cases of Buruli ulcer (BU) in the Mapé Basin of Cameroon suggested that, compared to older age groups, very young children may be less exposed to Mycobacterium ulcerans. Here we determined serum IgG titres against the 18 kDa small heat shock protein (shsp) of M. ulcerans in 875 individuals living in the BU endemic river basins of the Mapé in Cameroon and the Densu in Ghana. While none of the sera collected from children below the age of four contained significant amounts of 18 kDa shsp specific antibodies, the majority of sera had high IgG titres against the Plasmodium falciparum merozoite surface protein 1 (MSP-1). These data suggest that exposure to M. ulcerans increases at an age which coincides with the children moving further away from their homes and having more intense environmental contact, including exposure to water bodies at the periphery of their villages.

Although M. ulcerans, the causative agent of Buruli ulcer (BU), was identified in 1948, its transmission pathways and environmental reservoirs remain poorly understood. The occurrence of M. ulcerans infections in endemic countries in West and Central Africa is highly focal and associated with stagnant and slow flowing water bodies. BU is often described as a disease mainly affecting children <15 years of age. However, taking the population age distribution into account, our recent longitudinal survey for BU in the Mapé Dam Region of Cameroon revealed that clinical cases of BU among children <5 years are relatively rare. In accordance with these findings, data of the present sero-epidemiological study indicate that children <4 years old are less exposed to M. ulcerans than older children. Sero-conversion is associated with age, which may be due to age-related changes in behavioural factors, such as a wider movement radius of older children, including more frequent contact with water bodies at the periphery of their villages.

Mycobacterium ulcerans fails to infect through skin abrasions in a guinea pig infection model: implications for transmission

Transmission of M. ulcerans, the etiological agent of Buruli ulcer, from the environment to humans remains an enigma despite decades of research. Major transmission hypotheses propose 1) that M. ulcerans is acquired through an insect bite or 2) that bacteria enter an existing wound through exposure to a contaminated environment. In studies reported here, a guinea pig infection model was developed to determine whether Buruli ulcer could be produced through passive inoculation of M. ulcerans onto a superficial abrasion. The choice of an abrasion model was based on the fact that most bacterial pathogens infecting the skin are able to infect an open lesion, and that abrasions are extremely common in children. Our studies show that after a 90d infection period, an ulcer was present at intra-dermal injection sites of all seven animals infected, whereas topical application of M. ulcerans failed to establish an infection. Mycobacterium ulcerans was cultured from all injection sites whereas infected abrasion sites healed and were culture negative. A 14d experiment was conducted to determine how long organisms persisted after inoculation. Mycobacterium ulcerans was isolated from abrasions at one hour and 24 hours post infection, but cultures from later time points were negative. Abrasion sites were qPCR positive up to seven days post infection, but negative at later timepoints. In contrast, M. ulcerans DNA was detected at intra-dermal injection sites throughout the study. M. ulcerans was cultured from injection sites at each time point. These results suggest that injection of M. ulcerans into the skin greatly facilitates infection and lends support for the role of an invertebrate vector or other route of entry such as a puncture wound or deep laceration where bacteria would be contained within the lesion. Infection through passive inoculation into an existing abrasion appears a less likely route of entry.

Genetic diversity of PCR-positive, culture-negative and culture-positive Mycobacterium ulcerans isolated from Buruli ulcer patients in Ghana

Culture of Mycobacterium ulcerans from Buruli ulcer patients has very low sensitivity. Thus confirmation of M. ulcerans infection is primarily based on PCR directed against IS2404. In this study we compare the genotypes obtained by variable number of tandem repeat analysis of DNA from IS2404-PCR positive cultures with that obtained from IS2404 positive, culture-negative tissue. A significantly greater genetic heterogeneity was found among culture-negative samples compared with that found in cultured strains but a single genotype is over-represented in both sample sets. This study provides evidence that both the focal location of bacteria in a lesion as well as differences in the ability to culture a particular genotype may underlie the low sensitivity of culture. Though preliminary, data from this work also suggests that mycobacteria previously associated with fish disease (M. pseudoshottsii) may be pathogenic for humans.

History, biology and chemistry of Mycobacterium ulcerans infections (Buruli ulcer disease)

Mycobacterium ulcerans infections (Buruli ulcer disease) have a long history that can be traced back 150 years. The successive discoveries of the mycobacteria in 1948 and of mycolactone A/B in 1999, the toxin responsible for this dramatic necrotic skin disease, resulted in a paradigm shift concerning the disease itself and in a broader sense, delineated an entirely new role for bioactive polyketides as virulence factors. The fascinating history, biology and chemistry of M. ulcerans infections are discussed in this review.

Genotyping Tools for Mycobacterium ulcerans-Drawbacks and Future Prospects

Mycobacterium ulcerans infection (Buruli ulcer) is a neglected but treatable skin disease endemic in over 30 countries. M. ulcerans is an environmental mycobacteria with an elusive mode of transmission to humans. Ecological and Molecular epidemiological studies to identify reservoirs and transmission vectors are important for source tracking infections especially during outbreaks and elucidating transmission routes. Research efforts have therefore focused on genotyping strains of the mycobacteria from clinical and environmental samples. This review discusses genotyping tools for differentiating M. ulcerans strains from other environmental and Mycolactone Producing Mycobacteria (MPMs). We highlight tools that have been adapted from related fields and propose ways these could be enhanced to resolve intra-species variation for epidemiological, transmission, evolutionary studies, and detection of emerging drug resistant strains. In the wake of increasing cases of Buruli ulcer, cumulative efforts including improvement in diagnostic methods and fine-tuning of genotyping tools are crucial to complement public health efforts in reducing infections.

Clinical considerations on Buruli ulcer employing two molecular tests for the detection of Mycobacterium ulcerans in 100 skin biopsies

OBJECTIVES

Buruli ulcer (BU) is an infected cutaneous lesion, the etiological agent of which is Mycobacterium ulcerans. Diagnosis is confirmed by the identification of acid-fast bacilli and culture. In clinically suspicious forms with negative bacteriological or Ziehl-Neelsen (ZN) findings, molecular tests are used. This study compared the concordance of nested polymerase chain reaction (PCR) (targeting IS2404) and PCR (targeting IS2606) in different clinical situations.

METHODS

A total of 57 samples were sourced from 39 BU patients. Control samples (n = 43) were obtained from non-BU ulcers in 38 patients. Samples were divided into two pieces and submitted to, respectively, histological examination and ZN staining, and PCR. Subsamples submitted to PCR were divided and submitted to nested PCR IS2404 and PCR IS2606, respectively.

RESULTS

Of the 57 BU biopsies, positive results were obtained by nested PCR in 18 (31.6%) and by IS2606 PCR in 37 (64.9%) cases. Sequencing of the positive samples confirmed the specificity of amplicons in all nested PCR samples and in 26 of 37 (70.2%) samples positive to IS2606. Hence, nested PCR was more specific (100% vs. 93%) and less sensitive (32% vs. 46%) than IS2606 PCR. In the BU samples, nested PCR was negative in 15 instances, and IS2606 PCR was negative in 11 instances in which ZN histology had been positive (false negatives). Both PCRs were positive in six ZN-negative smears.

CONCLUSIONS

We considered 57 samples from 39 BU patients in various clinical stages and at different times after the beginning of therapy. These provided positive results in 18 cases with IS2404 nested PCR and in 37 cases with PCR IS2606; only 26 of the latter remained positive subsequent to sequencing. Hence, even if IS2404 PCR is considered more specific, in subjects who appear to fail to respond to therapy, it is advisable to also carry out IS2606 PCR. A possible interpretation of the discordance between the two techniques due to unavoidable technical errors as well as to different sensitivity of the two tests at M. ulcerans DNA low concentration (i.e. in recent infection and in well-treated cases) is discussed.

Buruli ulcer and mycolactone-producing mycobacteria

Buruli ulcer (BU) is an emerging human disease caused by Mycobacterium ulcerans, which mainly affects the extremities. It is most endemic in sub-Saharan Africa; however, it has been reported worldwide, including in some non-tropical areas. "M. ulcerans subsp. shinshuense" is proposed as a subspecies of M. ulcerans, which have been reported from Japan and China. A total of 35 BU cases have been reported as of November 2012. Although M. ulcerans is categorized as nontuberculous mycobacteria, it has some unique characteristics that could only be observed in this bacterium. It possesses a giant virulent plasmid, composed of 174-kbp nucleotides, coding polyketide synthase to produce macrolide toxin called mycolactone. The discovery of such a linkage of plasmid and its pathogenesis has not been reported in other human disease-causing mycobacteria.

Structure-activity relationship studies on the macrolide exotoxin mycolactone of Mycobacterium ulcerans

Background

Mycolactones are a family of polyketide-derived macrolide exotoxins produced by Mycobacterium ulcerans, the causative agent of the chronic necrotizing skin disease Buruli ulcer. The toxin is synthesized by polyketide synthases encoded by the virulence plasmid pMUM. The apoptotic, necrotic and immunosuppressive properties of mycolactones play a central role in the pathogenesis of M. ulcerans.

Methodology/Principal Findings

We have synthesized and tested a series of mycolactone derivatives to conduct structure-activity relationship studies. Flow cytometry, fluorescence microscopy and Alamar Blue-based metabolic assays were used to assess activities of mycolactones on the murine L929 fibroblast cell line. Modifications of the C-linked upper side chain (comprising C12–C20) caused less pronounced changes in cytotoxicity than modifications in the lower C5-O-linked polyunsaturated acyl side chain. A derivative with a truncated lower side chain was unique in having strong inhibitory effects on fibroblast metabolism and cell proliferation at non-cytotoxic concentrations. We also tested whether mycolactones have antimicrobial activity and found no activity against representatives of Gram-positive (Streptococcus pneumoniae) or Gram-negative bacteria (Neisseria meningitis and Escherichia coli), the fungus Saccharomyces cerevisae or the amoeba Dictyostelium discoideum.

Conclusion

Highly defined synthetic compounds allowed to unambiguously compare biological activities of mycolactones expressed by different M. ulcerans lineages and may help identifying target structures and triggering pathways.

Buruli ulcer is a chronic necrotizing skin disease caused by Mycobacterium ulcerans. The characteristic histopathological features of Buruli ulcer, severe destruction of subcutaneous tissue with minimal inflammation in the core of the lesion, are primarily attributed to the cytotoxic activity of mycolactone, the macrolide exotoxin of M. ulcerans. Different geographical lineages of M. ulcerans produce different structural variants of mycolactone. By using highly defined synthetic mycolactones, including both naturally occurring molecular species and additional non-natural variants, we have assessed the influence of the structure of the C-linked upper side chain and the lower C5-O-linked polyunsaturated acyl side chain on biological activity. Changes in the lower side chain affected the cytotoxic activity against mammalian cells more profoundly than changes in the upper side chain. Mycolactone A/B had no antimicrobial activity against Gram-positive and Gram-negative bacteria and was also inactive against Saccharomyces and Dictyostelium.

Fish and amphibians as potential reservoirs of Mycobacterium ulcerans, the causative agent of Buruli ulcer disease

Background

Buruli ulcer is a skin disease often associated with proximity to certain water bodies in Africa. Much remains unknown about the reservoir and transmission of this disease. Previous studies have suggested that fish may concentrate Mycobacterium ulcerans, the etiological agent of the disease, in their gills and intestines and serve as passive reservoirs of the bacterium. The objective of this study was to test the hypothesis that fish and amphibians serve as natural reservoirs of M. ulcerans or other closely related mycolactone-producing mycobacteria.

Methods

Polymerase chain reaction targeting the enoyl reductase (ER) domain present in mlsA, which is required for mycolactone production, was used to screen water, fish, and amphibians from water bodies in Ghana for the presence of mycolactone-producing mycobacteria, and positive specimens were subjected to variable number tandem repeat (VNTR) typing.

Results

The use of VNTR typing revealed the presence of Mycobacterium liflandii in a tadpole and a fish, and M. ulcerans in an adult frog. Similarity percentage analysis (SIMPER) showed that the predatory cichlid Hemichromis bimaculatus was associated with ER-positive water bodies. No amphibian species or fish-feeding guild served as a reliable indicator of the presence of mycolactone-producing mycobacteria in a water body, and there was no significant difference between fish and amphibian positivity rates (P-value=0.106). There was a significant difference between water bodies in the total number of ER-positive specimens (P-value=0.0164).

Conclusions

Although IS2404-positive tadpoles and fish have been reported, this is the first VNTR confirmation of M. ulcerans or M. liflandii in wild amphibian and fish populations in West Africa. Results from this study suggest that amphibians should be carefully examined as potential reservoirs for M. ulcerans in West Africa, and that H. bimaculatus may be useful as an indicator of habitats likely to support mycolactone-producing mycobacteria.

Environmental distribution and seasonal prevalence of Mycobacterium ulcerans in Southern Louisiana

Mycobacterium ulcerans is an emerging environmental pathogen that causes debilitating, ulcerative disease in humans and other vertebrates. The majority of human cases occur in tropical and temperate regions of Africa and Australia, and outbreaks of piscine mycobacteriosis caused by M. ulcerans have been reported in disparate geographic locations spanning the globe. While exposure to a natural body of water is the most common risk factor for human infection, the environmental distribution of M. ulcerans in aquatic habitats has not been extensively studied. Although no human cases have been reported in the United States, a strain of M. ulcerans has been identified as the cause of a piscine mycobacteriosis in Striped bass (Morone saxatilis) within the Chesapeake Bay. Infected fish exhibit bright red ventral and lateral dermal lesions. We observed a possible outbreak causing similar lesions on red drum (Sciaenops ocellatus) in wetlands of southern Louisiana and detected M. ulcerans-specific genetic markers in lesion samples from these fish. Based on these findings, we studied the geographic and seasonal prevalence of these markers across southern Louisiana. M. ulcerans was detected in each of the nine areas sampled across the state. M. ulcerans prevalence was significantly lower in the fall samples, and the low prevalence coincided with decreased nutrient levels and an increase in water temperature. To our knowledge, this is the first study of M. ulcerans biomarkers in the southern United States.

Genotypic characteristics of a Mycobacterium sp. isolated from yellowtail Seriola quinqueradiata and striped jack Pseudocaranx dentex in Japan

In Japan, a Mycobacterium marinum-like mycobacterium was isolated from the yellowtail, Seriola quinqueradiata. The species was identified as M. marinum by a commercial mycobacterial DNA-DNA hybridization kit. Nevertheless, PCR restriction analysis of the DNA of its RNA polymerase β-subunit gene definitively showed that this Mycobacterium sp. was M. ulcerans. PCR analysis revealed the genotypic characteristics of M. ulcerans in the Mycobacterium sp., only the mup053 gene sequence being absent, as has been found previously in other piscine mycobacteria such as M. marinum strains DL240490 and DL045 and M. pseudoshottsii. With one exception, this Mycobacterium sp. and M. pseudoshottsii had identical 16S rRNA gene sequences, which is also probably true of M. marinum strains DL240490 and DL045. Similarly, according to comparisons of the 16S rRNA gene, ITS region, and hsp65 gene sequences, this Mycobacterium sp. is more closely related to M. pseudoshottsii than to M. ulcerans or M. marinum. A PCR product of approximately 2000 bp was amplified from region of difference 9 in the Mycobacterium sp. The nucleotide sequence revealed insertion of IS2404, the sequence of which is 1366 bp long. The novel single nucleotide polymorphisms identified in this region distinguished this Mycobacterium sp. from M. marinum strain DL240490 and M. pseudoshottsii. The present findings raise the possibility that these species have a common ancestor. Further studies are required to improve our understanding of the relationship between their geographical origin and genetic diversity.

Mycobacterium ulcerans causes minimal pathogenesis and colonization in medaka (Oryzias latipes): an experimental fish model of disease transmission

Mycobacterium ulcerans causes Buruli ulcer in humans, a progressive ulcerative epidermal lesion due to the mycolactone toxin produced by the bacterium. Molecular analysis of M. ulcerans reveals it is closely related to Mycobacterium marinum, a pathogen of both fish and man. Molecular evidence from diagnostic PCR assays for the insertion sequence IS2404 suggests an association of M. ulcerans with fish. However, fish infections by M. ulcerans have not been well documented and IS2404 has been found in other mycobacteria. We have thus, employed two experimental approaches to test for M. ulcerans in fish. We show here for the first time that M. ulcerans with or without the toxin does not mount acute or chronic infections in Japanese Medaka "Oryzias latipes" even at high doses. Moreover, M. ulcerans-infected medaka do not exhibit any visible signs of infection nor disease and the bacteria do not appear to replicate over time. In contrast, similar high doses of the wild-type M. marinum or a mycolactone-producing M. marinum "DL" strain are able to mount an acute disease with mortality in medaka. Although these results would suggest that M. ulcerans does not mount infections in fish we have evidence that CLC macrophages from goldfish are susceptible to mycolactones.

Photodegradation of the Mycobacterium ulcerans toxin, mycolactones: considerations for handling and storage

BACKGROUND

Mycolactones are toxins secreted by M. ulcerans, the etiological agent of Buruli ulcer. These toxins, which are the main virulence factors of the bacilli, are responsible for skin lesions. Considering their specificity for M. ulcerans and their presence in skin lesions even at early stages, mycolactones are promising candidates for the development of a diagnostic tool for M. ulcerans infection. Stability of purified mycolactones towards light and heat has not yet been investigated, despite the importance of such parameters in the selection of strategies for a diagnosis tool development. In this context, the effects of UV, light and temperature on mycolactone stability and biological activity were studied.

METHODOLOGY/PRINCIPAL FINDINGS

To investigate the effect of these physical parameters, mycolactones were exposed to different wavelengths in several solvents and temperatures. Structural changes and biological activity were monitored. Whilst high temperature had no effect on mycolactones, UV irradiation (UV-A, UV-B and UV-C) and sunlight exposure caused a considerable degradation, as revealed by LC-MS and NMR analysis, correlated with a loss of biological activity. Moreover, effect of UVs on mycolactone caused a photodegradation rather than a phototransformation due to the identification of degradation product.

CONCLUSION/SIGNIFICANCE

This study demonstrates the high sensitivity of mycolactones to UVs as such it defines instructions for storage and handling.

A quick and cost effective method for the diagnosis of Mycobacterium ulcerans infection

Background

Buruli ulcer (BU), a neglected tropical skin disease caused by Mycobacterium ulcerans, has been reported in over 30 countries worldwide and is highly endemic in rural West and Central Africa. The mode of transmission remains unknown and treatment is the only alternative to disease control. Early and effective treatment to prevent the morbid effects of the disease depends on early diagnosis; however, current diagnosis based on clinical presentation and microscopy has to be confirmed by PCR and other tests in reference laboratories. As such confirmed BU diagnosis is either late, inefficient, time consuming or very expensive, and there is the need for an early diagnosis tool at point of care facilities. In this paper we report on a simple, quick and inexpensive diagnostic test that could be used at point of care facilities, in resource-poor settings.

Methods

The methodology employed is based on the loop mediated isothermal amplification (LAMP) technique. Four sets of Primers, targeting the mycolactone encoding plasmid genome sequence of M. ulcerans were designed. The BU-LAMP assay was developed and tested on five M. ulcerans strains from patients in Ghana and two American Type Culture Control (ATCC) reference isolates; Ghana #970321 (D19F9) and Benin #990826 (D27D14). We also tested the assay on other closely related, mycolactone-producing mycobacterial strains; M. marinum 1218, M. marinum DL240490, M. liflandii and M. pseudoshotsii, as well as experimentally infected laboratory animal and clinical samples.

Results

The results revealed a high specificity of the BU-LAMP assay for selectively detecting M. ulcerans. Compared to the conventional IS-2404 PCR, the new assay is cheaper and simpler and ten times more sensitive. Test results can be obtained within 1 hour.

Conclusions

This study indicates that the BU-LAMP assay could be suitable for early disease diagnosis and application in low-resource health facilities.

Characterization of photochromogenic Mycobacterium spp. from Chesapeake Bay striped bass Morone saxatilis

A large diversity of Mycobacterium spp. has been isolated from striped bass Morone saxatilis in Chesapeake Bay, USA. The new species M. shottsii and M. pseudoshottsii are the dominant isolates, while the classical fish pathogen M. marinum is found much less frequently. M. fortuitum and M. chelonae, other Mycobacterium spp. known to commonly infect fishes, have not yet been aseptically isolated from striped bass within Chesapeake Bay. While M. pseudoshottsii and M. shottsii have been phenotypically and genotypically characterized, other less common mycobacterial isolates have not. In the present study, we describe 17 photochromogenic isolates from Chesapeake Bay striped bass using phenotypic characterization and multilocus sequencing of 16S rRNA, hsp65 and rpoB genes. Genetic characterization reveals that these isolates are related to widely divergent portions of the mycobacterial phylogeny; however, some interesting trends are observed, such as a majority of isolates (10/17) belonging to the M. simiae-related grouping. Five additional isolates were assigned to the slow-growing mycobacteria (including 2 identified as M. marinum), while 2 are clearly shown to belong genetically to the fast-growing mycobacteria.

Highly stereoselective total synthesis of fully hydroxy-protected mycolactones A and B and their stereoisomerization upon deprotection

Unprecedentedly efficient and highly (≥98 %) stereoselective syntheses of mycolactones A and B side chains relied heavily on Pd-catalyzed alkenylation (Negishi version) and were completed in 11 longest linear steps from ethyl (S)-3-hydroxybutyrate in 12% and 11% overall yield, respectively, roughly corresponding to an average of 82% yield per step. The synthesis of mycolactone core was realized by using Pd-catalyzed alkenyl-allyl coupling and an epoxide-opening reaction with a trialkylalkenylaluminate as key steps. Fully hydroxy-protected mycolactones A and B of ≥98% isomeric purity were synthesized successfully for the first time. However, unexpected 4:3-5:4 inseparable mixtures of mycolactones A and B were obtained upon deprotection.