Haemocoel injection of PirA1B1 to Galleria mellonella larvae leads to disruption of the haemocyte immune functions

Effect of Benzyl Alcohol on Main Defense System Components of Galleria mellonella (Lepidoptera)

Benzyl alcohol (E1519) is an aromatic alcohol used in the pharmaceutical and food industry. It is used to protect food products against microorganisms during storage, as a flavoring in the production of chocolate and confectionery products, as an important ingredient in fragrance, and as a preservative in medical products. However, little is known of its effect on insects. The main aim of this study was to determine the influence of benzyl alcohol on the defense systems of the wax moth Galleria mellonella, i.e., its cuticular lipid composition and critical elements of its immune system. A gas chromatography/mass spectrometry (GC/MS) analysis found benzyl alcohol treatment to elicit significant quantitative and qualitative differences in cuticular free fatty acid (FFA) profiles. Our findings indicate that benzyl alcohol treatment increased the levels of HSP70 and HSP90 and decreased those of HSF1, histamine, and cysteinyl leukotriene. Benzyl alcohol application also increased dismutase level in the hemolymph and lowered those of catalase and 8-OHdG. The treatment also had negative effects on G. mellonella hemocytes and a Sf9 cell line in vitro: 48-h treatment resulted in morphological changes, with the remaining cells being clearly spindle-shaped with numerous granules. The high insecticidal activity of compound and its lack of toxicity towards vertebrates suggest it could be an effective insecticide.

Transcriptome and Metabolome Analyses of Thitarodes xiaojinensis in Response to Ophiocordyceps sinensis Infection

Ophiocordyceps sinensis exhibits more than 5 months of vegetative growth in Thitarodes xiaojinensis hemocoel. The peculiar development process of O. sinensis has been elucidated through morphological observation and omics technology; however, little information has been reported regarding the changes that occur in the host T. xiaojinensis. The RNA sequencing data showed that when O. sinensis blastospores were in the proliferative stage, the greatest change in the infected larval fat body was the selectively upregulated immune recognition and antimicrobial peptide genes. When O. sinensis blastospores were in the stationary stage, the immune pathways of T. xiaojinensis reverted to normal levels, which coincides with the successful settlement of O. sinensis. Pathway enrichment analysis showed a higher expression of genes involved in energy metabolism pathway in this stage. Metabolomic analyses revealed a reduction of amino acids and lipids in hemolymph, but an upregulation of lipids in the fat body of the host larvae after O. sinensis infection. We present the first transcriptome integrated with the metabolome study of T. xiaojinensis infected by O. sinensis. It will improve our understanding of the interaction mechanisms between the host and entomopathogenic fungi, and facilitate future functional studies of genes and pathways involved in these interactions.

Enterobacter Strain IPPBiotE33 Displays a Synergistic Effect with Bacillus thuringiensis Bt185

The discovery and isolation of new non-Bt insecticidal bacteria and genes are significant for the development of new biopesticides against coleopteran pests. In this study, we evaluated the insecticidal activity of non-Bt insecticidal bacteria, PPBiotE33, IPPBiotC41, IPPBiotA42 and IPPBiotC43, isolated from the peanut rhizosphere. All these strains showed insecticidal activity against first- and third-instar larvae of Holotrichia parallela, Holotrichia oblita, Anomala corpulenta and Potosia brevitarsis. IPPBiotE33 showed the highest toxicity among the four strains and exhibited virulence against Colaphellus bowringi. The genome of IPPBiotE33 was sequenced, and a new protein, 03673, with growth inhibition effects on C. bowringi was obtained. In addition, IPPBiotE33 had a synergistic effect with Bacillus thuringiensis Bt185 against H. parallela in bioassays and back-inoculation experiments with peanut seedlings. IPPBiotE33 induced a decrease in hemocytes and an increase in phenol oxidase activity in H. parallela hemolymph, known as the immunosuppressive effect, which mediated synergistic activity with Bt185. This study increased our knowledge of the new insecticidal strain IPPBiotE33 and shed new light on the research on new insecticidal coaction mechanisms and new blended pesticides.

In Vitro and In Vivo Biocompatibility of Natural and Synthetic Pseudomonas aeruginosa Pyomelanin for Potential Biomedical Applications

Bacteria are the source of many bioactive compounds, including polymers with various physiological functions and the potential for medical applications. Pyomelanin from Pseudomonas aeruginosa, a nonfermenting Gram-negative bacterium, is a black-brown negatively charged extracellular polymer of homogentisic acid produced during L-tyrosine catabolism. Due to its chemical properties and the presence of active functional groups, pyomelanin is a candidate for the development of new antioxidant, antimicrobial and immunomodulatory formulations. This work aimed to obtain bacterial water-soluble (Pyo_sol), water-insoluble (Pyo_insol) and synthetic (sPyo) pyomelanin variants and characterize their chemical structure, thermosensitivity and biosafety in vitro and in vivo (Galleria mallonella). FTIR analysis showed that aromatic ring connections in the polymer chains were dominant in Pyo_sol and sPyo, whereas Pyo_insol had fewer C_ar-C_ar links between rings. The differences in chemical structure influence the solubility of various forms of pyomelanins, their thermal stability and biological activity. Pyo_sol and Pyo_insol showed higher biological safety than sPyo. The obtained results qualify Pyo_sol and Pyo_insol for evaluation of their antimicrobial, immunomodulatory and proregenerative activities.

Galleria mellonella as an infection model for the virulent Mycobacterium tuberculosis H37Rv

Tuberculosis (TB), caused by Mycobacterium tuberculosis (MTB), is a leading cause of infectious disease mortality. Animal infection models have contributed substantially to our understanding of TB, yet their biological and non-biological limitations are a research bottleneck. There is a need for more ethically acceptable, economical, and reproducible TB infection models capable of mimicking key aspects of disease. Here, we demonstrate and present a basic description of how Galleria mellonella (the greater wax moth, Gm) larvae can be used as a low cost, rapid, and ethically more acceptable model for TB research. This is the first study to infect Gm with the fully virulent MTB H37Rv, the most widely used strain in research. Infection of Gm with MTB resulted in a symptomatic lethal infection, the virulence of which differed from both attenuated Mycobacterium bovis BCG and auxotrophic MTB strains. The Gm-MTB model can also be used for anti-TB drug screening, although CFU enumeration from Gm is necessary for confirmation of mycobacterial load reducing activity of the tested compound. Furthermore, comparative virulence of MTB isogenic mutants can be determined in Gm. However, comparison of mutant phenotypes in Gm against conventional models must consider the limitations of innate immunity. Our findings indicate that Gm will be a practical, valuable, and advantageous additional model to be used alongside existing models to advance tuberculosis research.

Galleria mellonella as a Novel In Vivo Model to Screen Natural Product-Derived Modulators of Innate Immunity

Immunomodulators are drugs that either stimulate or suppress the immune system in response to an immunopathological disease or cancer. The majority of clinically approved immunomodulators are either chemically synthesised (e.g., dexamethasone) or protein-based (e.g., monoclonal antibodies), whose uses are limited due to toxicity issues, poor bioavailability, or prohibitive cost. Nature is an excellent source of novel compounds, as it is estimated that almost half of all licenced medicines are derived from nature or inspired by natural product (NP) structures. The clinical success of the fungal-derived immunosuppressant cyclosporin A demonstrates the potential of natural products as immunomodulators. Conventionally, the screening of NP molecules for immunomodulation is performed in small animal models; however, there is a growing impetus to replace animal models with more ethical alternatives. One novel approach is the use of Galleria melonella larvae as an in vivo model of immunity. Despite lacking adaptive antigen-specific immunity, this insect possesses an innate immune system comparable to mammals. In this review, we will describe studies that have used this alternative in vivo model to assess the immunomodulating activity of synthetic and NP-derived compounds, outline the array of bioassays employed, and suggest strategies to enhance the use of this model in future research.

Prior infection of Galleria mellonella with sublethal dose of Bt elicits immune priming responses but incurs metabolic changes

Invertebrate immune priming has attracted wide attention of biologists in recent years because it challenges core notions about the disparate nature of acquired and innate immunity. However, the metabolic switch and energetic cost during eliciting immune priming are poorly investigated issues, which could widen and deepen our understanding of the physiological mechanism of immune priming. In this study, using sublethal dose of Bacillus thuringiensis (Bt) as an elicitor, we detected typical immune priming responses in Galleria mellonella. We found that the intensity of immune priming is positively correlated with the levels of antimicrobial peptides and phagocytosis ability of hemocytes. Subsequently, we employed LC-MS/MS-based untargeted metabolomics techniques to analyze the metabolic changes in the fat body of G. mellonella larvae during immune priming. The results showed that there were 74 and 56 significantly altered metabolites in positive and negative ion mode, respectively, after Bt priming. Most of the differential metabolites were enriched in the following metabolic pathways: amino acid biosynthesis, carbon metabolism, aminoacyl-tRNA biosynthesis and ABC transporters. The energetic cost of immune priming was depicted mainly in the slow growth of body mass and decreased levels of sucrose, lactose, D-ribulose 1,5-bisphosphate, Glycerate-3P and isocitric acid, which are enriched in carbon metabolism and involved in energy production. Meanwhile, correlation and interaction network analysis showed negative correlations between carbohydrates and metabolites involved in amino acid biosynthesis, suggesting that amino acids acted as the main energy source and helped the organisms synthesize immune effectors to participate in the immune priming response. Our results pave the way for uncovering the physiological mechanism of insect immune priming and discovering novel targets for Bt insecticide.

Dodecanol, metabolite of entomopathogenic fungus Conidiobolus coronatus, affects fatty acid composition and cellular immunity of Galleria mellonella and Calliphora vicina

One group of promising pest control agents are the entomopathogenic fungi; one such example is Conidiobolus coronatus, which produces a range of metabolites. Our present findings reveal for the first time that C. coronatus also produces dodecanol, a compound widely used to make surfactants and pharmaceuticals, and enhance flavors in food. The main aim of the study was to determine the influence of dodecanol on insect defense systems, i.e. cuticular lipid composition and the condition of insect immunocompetent cells; hence, its effect was examined in detail on two species differing in susceptibility to fungal infection: Galleria mellonella and Calliphora vicina. Dodecanol treatment elicited significant quantitative and qualitative differences in cuticular free fatty acid (FFA) profiles between the species, based on gas chromatography analysis with mass spectrometry (GC/MS), and had a negative effect on G. mellonella and C. vicina hemocytes and a Sf9 cell line in vitro: after 48 h, almost all the cells were completely disintegrated. The metabolite had a negative effect on the insect defense system, suggesting that it could play an important role during C. coronatus infection. Its high insecticidal activity and lack of toxicity towards vertebrates suggest it could be an effective insecticide.

In vivo transcriptome analysis provides insights into host-dependent expression of virulence factors by Yersinia entomophaga MH96, during infection of Galleria mellonella

The function of microbes can be inferred from knowledge of genes specifically expressed in natural environments. Here, we report the in vivo transcriptome of the entomopathogenic bacterium Yersinia entomophaga MH96, captured during initial, septicemic, and pre-cadaveric stages of intrahemocoelic infection in Galleria mellonella. A total of 1285 genes were significantly upregulated by MH96 during infection; 829 genes responded to in vivo conditions during at least one stage of infection, 289 responded during two stages of infection, and 167 transcripts responded throughout all three stages of infection compared to in vitro conditions at equivalent cell densities. Genes upregulated during the earliest infection stage included components of the insecticidal toxin complex Yen-TC (chi1, chi2, and yenC1), genes for rearrangement hotspot element containing protein yenC3, cytolethal distending toxin cdtAB, and vegetative insecticidal toxin vip2. Genes more highly expressed throughout the infection cycle included the putative heat-stable enterotoxin yenT and three adhesins (usher-chaperone fimbria, filamentous hemagglutinin, and an AidA-like secreted adhesin). Clustering and functional enrichment of gene expression data also revealed expression of genes encoding type III and VI secretion system-associated effectors. Together these data provide insight into the pathobiology of MH96 and serve as an important resource supporting efforts to identify novel insecticidal agents.

Txp40, a protein from Photorhabdus akhurstii, conferred potent insecticidal activity against the larvae of Helicoverpa armigera, Spodoptera litura and S. exigua

BACKGROUND

Txp40, a 37 kDa protein, previously characterized from the Gram-negative bacterium Photorhabdus akhurstii (symbiotically associates with insect-parasitic nematode, Heterorhabditis indica), conferred insecticidal activity against Galleria mellonella. Here, the biological activity of Txp40 was evaluated against economically important insects, including Helicoverpa armigera, Spodoptera litura and S. exigua.

RESULTS

When both intra-hemocoel injected and orally fed to test insects, comparatively greater oral LD₅₀ (187.7-522 ng g^-1 ) than injection LD₅₀ (32.33-150.6 ng g^-1 ) was obtained with Txp40 derived from P. akhurstii strain IARI-SGMG3. Injection of purified Txp40 caused a dose-dependent reduction in the total circulatory hemocytes and hemocyte viability of fourth-instar larvae of the test insects at 12 h post incubation; unlike healthy cells toxin-treated ones displayed aggregated distribution. Injection of Txp40 significantly elevated the phenoloxidase activity of insect hemolymph, which potentially led to unrestrained melanization reaction and ultimately larval death. Histological analyses showed the primary site of action of Txp40 in the insect midgut. Extensive damage to midgut epithelium 24 h after injection of the Txp40 explains the access of the toxin from hemocoel to midgut via leaky septate junctions. In silico analyses suggested that Txp40 can potentially interact with H. armigera midgut receptor proteins cadherin, ATP-binding cassettes, aminopeptidase N1 and alkaline phosphatase to exert toxicity.

CONCLUSION

We propose Txp40 as an attractive alternative to Cry toxins of Bacillus thuringiensis, the transgenic expression of which is reported to cause resistance development in insects. © 2019 Society of Chemical Industry.

Identification of Arylphorin interacting with the insecticidal protein PirAB from Xenorhabdus nematophila by yeast two-hybrid system

PirAB toxin was initially found in the Photorhabdus luminescens TT01 strain and is a demonstrated binary toxin with high insecticidal activity. In this paper, we co-expressed the pirAB gene of Xenorhabdus nematophila HB310 in a prokaryotic expression system, and we found that the PirAB protein showed high hemocoel insecticidal activity against Galleria mellonella, Helicoverpa armigera and Spodoptera exigua. LD₅₀ values were 1.562, 2.003 and 2.17 μg/larvae for G. mellonella, H. armigera, and S. exigua, respectively (p > 0.05). Additionally, PirAB-interaction proteins were identified from G. mellonella by 6 × His Protein Pulldown combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS). Of which, arylphorin of G. mellonella showed the highest matching rate. A protein domain conservative structure analysis indicated that arylphorin has three domains including Hemocyanin-N, Hemocyanin-M, and Hemocyanin-C. Among these protein domains, Hemocyanin-C has immune and recognition functions. Further, Hemocyanin-C domain of arylphorin was identified to interact with PirA but not PirB by Yeast two-hybrid system. These findings reveal, for the first time, new host protein interacting with PirAB. The identification of interaction protein may serve as the foundation for further study on the function and insecticidal mechanism of this binary toxin from Xenorhabdus.

Conidiobolus coronatus induces oxidative stress and autophagy response in Galleria mellonella larvae

Cell homeostasis requires the correct levels of reactive oxygen species (ROS) to be maintained as these regulate the proliferation and differentiation of cells, and control the immune response and inflammation. High levels of ROS can cause oxidative stress, leading to protein, lipid and DNA damage, or even cell death. Under physiological conditions, the rate of autophagy remains stable; however, it can be accelerated by a number of exogenous stimuli such as oxidative stress, starvation or hypoxia, leading to cell death. The present paper examines the effect of Conidiobolus coronatus infection on the immune response, oxidative stress processes and autophagy in the greater wax moth, Galleria mellonella. Fungal infection was found to result in the disorganization of the cytoskeleton of the larval immune cells and the enhancement of oxidative defense processes. Lipid peroxidation and autophagy were also induced in the hemocytes. Our findings show that G. mellonella is an ideal model for exploring immune mechanisms.