Mycobacterium marinum infection in Drosophila melanogaster for antimycobacterial activity assessment

Drosophila versus Mycobacteria: A model for mycobacterial host-pathogen interactions

Animal models have played an essential role in understanding the host-pathogen interactions of pathogenic mycobacteria, including the Mycobacterium tuberculosis and emerging nontuberculous mycobacteria (NTM) species such as M. avium and M. abscessus. Drosophila melanogaster has become a well-established model for the study of innate immunity and is increasingly being used as a tool to study host-pathogen interactions, in part due to its genetic tractability. The use of D. melanogaster has led to greater understanding of the role of the innate immune system in response to mycobacterial infection, including in vitro RNAi screens and in vivo studies. These studies have identified processes and host factors involved in mycobacterial infection, such as those required for cellular entry, those required to control or resist non-pathogenic mycobacteria, or factors that become dysregulated as a result of mycobacterial infection. Developments in genetic tools for manipulating mycobacterial genomes will allow for more detailed studies into how specific host and pathogen factors interact with one another by using D. melanogaster; however, the full potential of this model has not yet been reached. Here we provide an overview of how D. melanogaster has been used to study mycobacterial infection and discuss the current gaps in our understanding.

Heterologous Expression of ethA and katG in Mycobacterium marinum Enables the Rapid Identification of New Prodrugs Active against Mycobacterium tuberculosis

Screening strategies for antituberculosis compounds using Mycobacterium tuberculosis are time consuming and require biosafety level 3 (BSL3) facilities, which makes the development of high-throughput assays difficult and expensive. Mycobacterium marinum, a close genetic relative of M. tuberculosis, possesses several advantages as a suitable model for tuberculosis drug screening. However, despite the high genetic similarity, there are some obvious differences in susceptibility to some tuberculosis drugs between these two species, especially for the prodrugs ethionamide and isoniazid. In this study, we aimed to improve M. marinum as a model for antituberculosis drug identification by heterologous expression of two common drug activators, EthA and KatG. These two activators were overexpressed in M. marinum, and the strains were tested against ethionamide, isoniazid, and a library of established antimycobacterial compounds from TB Alliance to compare drug susceptibility. Both in vitro and in vivo using zebrafish larvae, these genetically modified M. marinum strains showed significantly higher susceptibility against ethionamide and isoniazid, which require activation by EthA and KatG. More importantly, a strain overexpressing both ethA and katG was potentially more susceptible to approximately 20% of the antituberculosis hit compounds from the TB Alliance library. Most of these compounds were activated by EthA in M. marinum Four of these compounds were selected for further analysis, and three of them showed obvious EthA-dependent activity against M. tuberculosis Overall, our developed M. marinum strains are valuable tools for high-throughput discovery of potential novel antituberculosis prodrugs.

Preclinical Models of Nontuberculous Mycobacteria Infection for Early Drug Discovery and Vaccine Research

Nontuberculous mycobacteria (NTM) represent an increasingly prevalent etiology of soft tissue infections in animals and humans. NTM are widely distributed in the environment and while, for the most part, they behave as saprophytic organisms, in certain situations, they can be pathogenic, so much so that the incidence of NTM infections has surpassed that of Mycobacterium tuberculosis in developed countries. As a result, a growing body of the literature has focused attention on the critical role that drug susceptibility tests and infection models play in the design of appropriate therapeutic strategies against NTM diseases. This paper is an overview of the in vitro and in vivo models of NTM infection employed in the preclinical phase for early drug discovery and vaccine development. It summarizes alternative methods, not fully explored, for the characterization of anti-mycobacterial compounds.

Animal Models of Tuberculosis Vaccine Research: An Important Component in the Fight against Tuberculosis

Tuberculosis (TB), an infectious disease caused by Mycobacterium tuberculosis, is one of the top ten infectious diseases worldwide, and is the leading cause of morbidity from a single infectious agent. M. tuberculosis can cause infection in several species of animals in addition to humans as the natural hosts. Although animal models of TB disease cannot completely simulate the occurrence and development of human TB, they play an important role in studying the pathogenesis, immune responses, and pathological changes as well as for vaccine research. This review summarizes the commonly employed animal models, including mouse, guinea pig, rabbit, rat, goat, cattle, and nonhuman primates, and their characteristics as used in TB vaccine research, and provides a basis for selecting appropriate animal models according to specific research needs. Furthermore, some of the newest animal models used for TB vaccine research (such as humanized animal models, zebrafish, Drosophila, and amoeba) are introduced, and their characteristics and research progress are discussed.

Galleria mellonella: An Infection Model for Screening Compounds Against the Mycobacterium tuberculosis Complex

Drug screening models have a vital role in the development of novel antimycobacterial agents which are urgently needed to tackle drug-resistant tuberculosis (TB). We recently established the larvae of the insect Galleria mellonella (greater wax moth) as a novel infection model for the Mycobacterium tuberculosis complex. Here we demonstrate its use as a rapid and reproducible screen to evaluate antimycobacterial drug efficacy using larvae infected with bioluminescent Mycobacterium bovis BCG lux. Treatment improved larval survival outcome and, with the exception of pyrazinamide, was associated with a significant reduction in in vivo mycobacterial bioluminescence over a 96 h period compared to the untreated controls. Isoniazid and rifampicin displayed the greatest in vivo efficacy and survival outcome. Thus G. mellonella, infected with bioluminescent mycobacteria, can rapidly determine in vivo drug efficacy, and has the potential to significantly reduce and/or replace the number of animals used in TB research.

Combination of Repurposed Drug Diosmin with Amoxicillin-Clavulanic acid Causes Synergistic Inhibition of Mycobacterial Growth

Effective therapeutic regimens for the treatment of tuberculosis (TB) are limited. They are comprised of multiple drugs that inhibit the essential cellular pathways in Mycobacterium tuberculosis (Mtb). The present study investigates an approach which enables a combination of Amoxicillin-Clavulanic acid (AMC) and a repurposed drug for its synergistic effect towards TB treatment. We identified Diosmin (DIO), by targeting the active site residues of L,D-transpeptidase (Ldt) enzymes involved in Mtb cell wall biosynthesis by using a structure-based drug design method. DIO is rapidly converted into aglycone form Diosmetin (DMT) after oral administration. Binding of DIO or DMT towards Ldt enzymes was studied using molecular docking and bioassay techniques. Combination of DIO (or DMT) and AMC exhibited higher mycobactericidal activity against Mycobacterium marinum as compared to individual drugs. Scanning electron microscopy study of M. marinum treated with AMC-DIO and AMC-DMT showed marked cellular leakage. M. marinum infected Drosophila melanogaster fly model showed an increased fly survival of ~60% upon treatment with a combination of AMC and DIO (or DMT). Finally, the enhanced in vitro antimicrobial activity of AMC-DIO was validated against Mtb H37Ra and a MDR clinical isolate. Our results demonstrate the potential for AMC and DIO (or DMT) as a synergistic combination for the treatment of TB.

In Vivo Efficacy of Ellagic Acid against Candida albicans in a Drosophila melanogaster Infection Model

The aim of this study was to evaluate the antifungal activity and the toxicity of ellagic acid (EA) using a Drosophila melanogaster model. Candida albicans bacteria were inoculated into Toll heterozygous flies. Survival curves were obtained for the evaluation of the antimicrobial effect and toxicity of EA. A protective effect of EA against fungal infection in Drosophila melanogaster was observed at nontoxic concentrations. This study showed that EA is a promising tool for the treatment of candidiasis.

Galleria mellonella - a novel infection model for the Mycobacterium tuberculosis complex

Animal models have long been used in tuberculosis research to understand disease pathogenesis and to evaluate novel vaccine candidates and anti-mycobacterial drugs. However, all have limitations and there is no single animal model which mimics all the aspects of mycobacterial pathogenesis seen in humans. Importantly mice, the most commonly used model, do not normally form granulomas, the hallmark of tuberculosis infection. Thus there is an urgent need for the development of new alternative in vivo models. The insect larvae, Galleria mellonella has been increasingly used as a successful, simple, widely available and cost-effective model to study microbial infections. Here we report for the first time that G. mellonella can be used as an infection model for members of the Mycobacterium tuberculosis complex. We demonstrate a dose-response for G. mellonella survival infected with different inocula of bioluminescent Mycobacterium bovis BCG lux, and demonstrate suppression of mycobacterial luminesence over 14 days. Histopathology staining and transmission electron microscopy of infected G. mellonella phagocytic haemocytes show internalization and aggregation of M. bovis BCG lux in granuloma-like structures, and increasing accumulation of lipid bodies within M. bovis BCG lux over time, characteristic of latent tuberculosis infection. Our results demonstrate that G. mellonella can act as a surrogate host to study the pathogenesis of mycobacterial infection and shed light on host-mycobacteria interactions, including latent tuberculosis infection.

Antimycobacterial drug discovery using Mycobacteria-infected amoebae identifies anti-infectives and new molecular targets

Tuberculosis remains a serious threat to human health world-wide, and improved efficiency of medical treatment requires a better understanding of the pathogenesis and the discovery of new drugs. In the present study, we performed a whole-cell based screen in order to complete the characterization of 168 compounds from the GlaxoSmithKline TB-set. We have established and utilized novel previously unexplored host-model systems to characterize the GSK compounds, i.e. the amoeboid organisms D. discoideum and A. castellanii, as well as a microglial phagocytic cell line, BV2. We infected these host cells with Mycobacterium marinum to monitor and characterize the anti-infective activity of the compounds with quantitative fluorescence measurements and high-content microscopy. In summary, 88.1% of the compounds were confirmed as antibiotics against M. marinum, 11.3% and 4.8% displayed strong anti-infective activity in, respectively, the mammalian and protozoan infection models. Additionally, in the two systems, 13–14% of the compounds displayed pro-infective activity. Our studies underline the relevance of using evolutionarily distant pathogen and host models in order to reveal conserved mechanisms of virulence and defence, respectively, which are potential “universal” targets for intervention. Subsequent mechanism of action studies based on generation of over-expresser M. bovis BCG strains, generation of spontaneous resistant mutants and whole genome sequencing revealed four new molecular targets, including FbpA, MurC, MmpL3 and GlpK.

Functional characterisation of the Drosophila cg6568 gene in host defence against Mycobacterium marinum

Mycobacterium marinum is a pathogenic mycobacterial species closely related to Mycobacterium tuberculosis. In this study, we established a mycobacterial infection model of Drosophila melanogaster to characterize the role played by cg6568, a homolog of the human cathelicidin gene, in the innate defense against infection. Drosophila cg6568 was expressed at various levels during all developmental stages, and the expression levels were modulated by M. marinum in a time-dependent manner. 20-hydroxyecdysone induced Drosophila cg6568 transcription both in vitro and in vivo. Using flies expressing cg6568 RNAi, we found that cg6568 was essential both for D. melanogaster survival and the exertion of antimicrobial effects during M. marinum infection. Thus, we named the gene product a cathelicidin-like antimicrobial protein of D. melanogaster (dCAMP). Our results indicate that dCAMP is crucial in terms of the innate D. melanogaster defense during M. marinum infection.

Animals devoid of pulmonary system as infection models in the study of lung bacterial pathogens

Biological disease models can be difficult and costly to develop and use on a routine basis. Particularly, in vivo lung infection models performed to study lung pathologies use to be laborious, demand a great time and commonly are associated with ethical issues. When infections in experimental animals are used, they need to be refined, defined, and validated for their intended purpose. Therefore, alternative and easy to handle models of experimental infections are still needed to test the virulence of bacterial lung pathogens. Because non-mammalian models have less ethical and cost constraints as a subjects for experimentation, in some cases would be appropriated to include these models as valuable tools to explore host-pathogen interactions. Numerous scientific data have been argued to the more extensive use of several kinds of alternative models, such as, the vertebrate zebrafish (Danio rerio), and non-vertebrate insects and nematodes (e.g., Caenorhabditis elegans) in the study of diverse infectious agents that affect humans. Here, we review the use of these vertebrate and non-vertebrate models in the study of bacterial agents, which are considered the principal causes of lung injury. Curiously none of these animals have a respiratory system as in air-breathing vertebrates, where respiration takes place in lungs. Despite this fact, with the present review we sought to provide elements in favor of the use of these alternative animal models of infection to reveal the molecular signatures of host-pathogen interactions.

Novel drug combination for Mycobacterium abscessus disease therapy identified in a Drosophila infection model

OBJECTIVES

Mycobacterium abscessus is known to be the most drug-resistant Mycobacterium and accounts for ∼80% of pulmonary infections caused by rapidly growing mycobacteria. This study reports a new Drosophila melanogaster-M. abscessus infection model that can be used as an in vivo efficacy model for anti-M. abscessus drug potency assessment.

METHODS

D. melanogaster were challenged with M. abscessus, and infected flies were fed with a fly medium containing tigecycline, clarithromycin, linezolid, clofazimine, moxifloxacin, amikacin, cefoxitin, dinitrobenzamide or metronidazole at different concentrations (0, 100 and 500 mg/L). The survival rates of infected flies were plotted and bacterial colonization/dissemination in fly bodies was monitored by cfu determination and green fluorescent protein epifluorescence.

RESULTS

The D. melanogaster-M. abscessus model enabled an assessment of the effectiveness of antibiotic treatment. Tigecycline was the best drug for extending the lifespan of M. abscessus-infected Drosophila, followed by clarithromycin and linezolid. Several different combinations of tigecycline, linezolid and clarithromycin were tested to determine the best combination. Tigecycline (25 mg/L) plus linezolid (500 mg/L) was the best drug combination and its efficacy was superior to conventional regimens, not only in prolonging infected fly survival but also against M. abscessus colonization and dissemination.

CONCLUSIONS

This D. melanogaster-M. abscessus infection/curing methodology may be useful for the rapid evaluation of potential drug candidates. In addition, new combinations using tigecycline and linezolid should be considered as possible next-generation combination therapies to be assessed in higher organisms.

Establishment and validation of whole-cell based fluorescence assays to identify anti-mycobacterial compounds using the Acanthamoeba castellanii-Mycobacterium marinum host-pathogen system

Tuberculosis is considered to be one of the world’s deadliest disease with 2 million deaths each year. The need for new antitubercular drugs is further exacerbated by the emergence of drug-resistance strains. Despite multiple recent efforts, the majority of the hits discovered by traditional target-based screening showed low efficiency in vivo. Therefore, there is heightened demand for whole-cell based approaches directly using host-pathogen systems. The phenotypic host-pathogen assay described here is based on the monitoring of GFP-expressing Mycobacterium marinum during infection of the amoeba Acanthamoeba castellanii. The assay showed straight-forward medium-throughput scalability, robustness and ease of manipulation, demonstrating its qualities as an efficient compound screening system. Validation with a series of known antitubercular compounds highlighted the advantages of the assay in comparison to previously published macrophage-Mycobacterium tuberculosis-based screening systems. Combination with secondary growth assays based on either GFP-expressing D. discoideum or M. marinum allowed us to further fine-tune compound characterization by distinguishing and quantifying growth inhibition, cytotoxic properties and antibiotic activities of the compounds. The simple and relatively low cost system described here is most suitable to detect anti-infective compounds, whether they present antibiotic activities or not, in which case they might exert anti-virulence or host defense boosting activities, both of which are largely overlooked by classical screening approaches.

Drosophila melanogaster: a first step and a stepping-stone to anti-infectives

Following an expansion in the antibiotic drug discovery in the previous century, we now face a bottleneck in the production of new anti-infective drugs. Traditionally, chemical libraries are screened either using in vitro culture systems or in silico to identify and chemically modify small molecules with antimicrobial properties. Nevertheless, almost all compounds passing through in vitro screening fail to pass preclinical trials. Drug screening in Drosophila offers to fill the gap between in vitro and mammalian model host testing by eliminating compounds that are toxic or have reduced bioavailability and by identifying others that may boost innate host defence or selectively reduce microbial virulence in a whole-organism setting. Such alternative screening methods in Drosophila, while low-throughput, may reduce the cost and increase the success rate of preclinical trials.

Drosophila melanogaster model for Mycobacterium abscessus infection

Mycobacterium abscessus is a human pathogen that is responsible for a broad spectrum of tissue infections and disseminated infections in immunodeficient patients. This pathogen is one of the most resistant organisms to chemotherapeutic agents. Therefore, we tested the hypothesis that the fruit fly, Drosophila melanogaster, is a genetically tractable model host for M. abscessus. In this context, we infected D. melanogaster with M. abscessus. This M. abscessus infection results in dissemination in the fly body, followed by death, which is accompanied by severe indirect flight muscle and brain damage. Our data show that M. abscessus can grow and replicate in D. melanogaster w(1118) and that it elicited a humoral immune response, especially of the Toll antimicrobial peptide pathway. To the best of our knowledge, this is the first report that mycobacteria induce the production of antimicrobial peptides in D. melanogaster.