Galleria mellonella: A Novel Invertebrate Model to Distinguish Intestinal Symbionts From Pathobionts

Ascarosides and Symbiotic Bacteria of Entomopathogenic Nematodes Regulate Host Immune Response in Galleria mellonella Larvae

Insects protect themselves through their immune systems. Entomopathogenic nematodes and their bacterial symbionts are widely used for the biocontrol of economically important pests. Ascarosides are pheromones that regulate nematode behaviors, such as aggregation, avoidance, mating, dispersal, and dauer recovery and formation. However, whether ascarosides influence the immune response of insects remains unexplored. In this study, we co-injected ascarosides and symbiotic Photorhabdus luminescens subsp. kayaii H06 bacteria derived from Heterorhabditis bacteriophora H06 into the last instar larvae of Galleria mellonella. We recorded larval mortality and analyzed the expressions of AMPs, ROS/RNS, and LPSs. Our results revealed a process in which ascarosides, acting as enhancers of the symbiotic bacteria, co-induced G. mellonella immunity by significantly increasing oxidative stress responses and secreting AMPs (gallerimycin, gloverin, and cecropin). This led to a reduction in color intensity and the symbiotic bacteria load, ultimately resulting in delayed host mortality compared to either ascarosides or symbiotic bacteria. These findings demonstrate the cross-kingdom regulation of insects and symbiotic bacteria by nematode pheromones. Furthermore, our results suggest that G. mellonella larvae may employ nematode pheromones secreted by IJs to modulate insect immunity during early infection, particularly in the presence of symbiotic bacteria, for enhancing resistance to invasive bacteria in the hemolymph.

Humoral immune response of Galleria mellonella after mono- and co-injection with Hypericum perforatum extract and Candida albicans

Galleria mellonella is used as a model organism to study the innate immune response of insects. In this study, the humoral immune response was assessed by examining phenoloxidase activity, fungal burden, and the expression of phenoloxidase and antimicrobial peptide genes at different time point following separate and combined injections of Hypericum perforatum extract and a nonlethal dose of Candida albicans. The administration of a plant extract at low doses increased phenoloxidase activity, while higher doses had no effect. Similarly, co-injection of a low dose of the extract with the pathogen allowed half of the yeast cells to survive after 24 h. Co-injection of plant extract with the pathogen decreased the phenoloxidase activity at the end of 4 h compared to C. albicans mono-injection. The phenoloxidase gene expressions was reduced in all experimental conditions with respect to the control. When plant extracts and the pathogen were administered together, gallerimycin and hemolin gene expressions were considerably higher compared to mono-injections of plant extracts and the pathogen. The results of this study reveal that gene activation and regulatory mechanisms may change for each immune gene, and that recognition and signaling pathways may differ depending on the involved immunoregulator.

Immunophenotyping of hemocytes from infected Galleria mellonella larvae as an innovative tool for immune profiling, infection studies and drug screening

In recent years, there has been a considerable increasing interest in the use of the greater wax moth Galleria mellonella as an animal model. In vivo pharmacological tests, concerning the efficacy and the toxicity of novel compounds are typically performed in mammalian models. However, the use of the latter is costly, laborious and requires ethical approval. In this context, G. mellonella larvae can be considered a valid option due to their greater ease of use and the absence of ethical rules. Furthermore, it has been demonstrated that the immune system of these invertebrates has similarity with the one of mammals, thus guaranteeing the reliability of this in vivo model, mainly in the microbiological field. To better develop the full potential of this model, we present a novel approach to characterize the hemocyte population from G. mellonella larvae and to highlight the immuno modulation upon infection and treatments. Our approach is based on the detection in isolated hemocytes from G. mellonella hemolymph of cell membrane markers typically expressed by human immune cells upon inflammation and infection, for instance CD14, CD44, CD80, CD163 and CD200. This method highlights the analogies between G. mellonella larvae and humans. Furthermore, we provide an innovative tool to perform pre-clinical evaluations of the efficacy of antimicrobial compounds in vivo to further proceed with clinical trials and support drug discovery campaigns.

Establishing Galleria mellonella as an invertebrate model for the emerging multi-host pathogen Helcococcus ovis

Helcococcus ovis (H. ovis) can cause disease in a broad range of animal hosts, including humans, and has been described as an emerging bacterial pathogen in bovine metritis, mastitis, and endocarditis. In this study, we developed an infection model that showed H. ovis can proliferate in the hemolymph and induce dose-dependent mortality in the invertebrate model organism Galleria mellonella (G. mellonella). We applied the model and identified H. ovis isolates with attenuated virulence originating from the uterus of a healthy post-partum dairy cow (KG38) and hypervirulent isolates (KG37, KG106) originating from the uterus of cows with metritis. Medium virulence isolates were also isolated (KG36, KG104) from the uterus of cows with metritis. A major advantage of this model is that a clear differentiation in induced mortality between H. ovis isolates was detected in just 48 h, resulting in an effective infection model able to identify virulence differences between H. ovis isolates with a short turnaround time. Histopathology showed G. mellonella employs hemocyte-mediated immune responses to H. ovis infection, which are analogous to the innate immune response in cows. In summary, G. mellonella can be used as an invertebrate infection model for the emerging multi-host pathogen Helcococcus ovis.

A quantitative micro-tomographic gut atlas of the lepidopteran model insect Manduca sexta

The tobacco hornworm is used extensively as a model system for ecotoxicology, immunology and gut physiology. Here, we established a micro-computed tomography approach based on the oral application of the clinical contrast agent iodixanol, allowing for a high-resolution quantitative analysis of the Manduca sexta gut. This technique permitted the identification of previously unknown and understudied structures, such as the crop or gastric ceca, and revealed the underlying complexity of the hindgut folding pattern, which is involved in fecal pellet formation. The acquired data enabled the volume rendering of all gut parts, the reliable calculation of their volumes, and the virtual endoscopy of the entire alimentary tract. It can provide information for accurate orientation in histology uses, enable quantitative anatomical phenotyping in three dimensions, and allow the calculation of locally effective midgut concentrations of applied chemicals. This atlas will provide critical insights into the evolution of the alimentary tract in lepidopterans.

Galleria mellonella-A Model for the Study of aPDT-Prospects and Drawbacks

Galleria mellonella is a promising in vivo model insect used for microbiological, medical, and pharmacological research. It provides a platform for testing the biocompatibility of various compounds and the kinetics of survival after an infection followed by subsequent treatment, and for the evaluation of various parameters during treatment, including the host-pathogen interaction. There are some similarities in the development of pathologies with mammals. However, a limitation is the lack of adaptive immune response. Antimicrobial photodynamic therapy (aPDT) is an alternative approach for combating microbial infections, including biofilm-associated ones. aPDT is effective against Gram-positive and Gram-negative bacteria, viruses, fungi, and parasites, regardless of whether they are resistant to conventional treatment. The main idea of this comprehensive review was to collect information on the use of G. mellonella in aPDT. It provides a collection of references published in the last 10 years from this area of research, complemented by some practical experiences of the authors of this review. Additionally, the review summarizes in brief information on the G. mellonella model, its advantages and methods used in the processing of material from these larvae, as well as basic knowledge of the principles of aPDT.

Studies of Streptococcus anginosus Virulence in Dictyostelium discoideum and Galleria mellonella Models

For many years, Streptococcus anginosus has been considered a commensal colonizing the oral cavity, as well as the gastrointestinal and genitourinary tracts. However, recent epidemiological and clinical data designate this bacterium as an emerging opportunistic pathogen. Despite the reported pathogenicity of S. anginosus, the molecular mechanism underpinning its virulence is poorly described. Therefore, our goal was to develop and optimize efficient and simple infection models that can be applied to examine the virulence of S. anginosus and to study host-pathogen interactions. Using 23 S. anginosus isolates collected from different infections, including severe and superficial infections, as well as an attenuated strain devoid of CppA, we demonstrate for the first time that Dictyostelium discoideum is a suitable model for initial, fast, and large-scale screening of virulence. Furthermore, we found that another nonvertebrate animal model, Galleria mellonella, can be used to study the pathogenesis of S. anginosus infection, with an emphasis on the interactions between the pathogen and host innate immunity. Examining the profile of immune defense genes, including antimicrobial peptides, opsonins, regulators of nodulation, and inhibitors of proteases, by quantitative PCR (qPCR) we identified different immune response profiles depending on the S. anginosus strain. Using these models, we show that S. anginosus is resistant to the bactericidal activity of phagocytes, a phenomenon confirmed using human neutrophils. Notably, since we found that the data from these models corresponded to the clinical severity of infection, we propose their further application to studies of the virulence of S. anginosus.

Tenuazonic acid alters immune and physiological reactions and susceptibility to pathogens in Galleria mellonella larvae

Tenuazonic acid (TeA) is synthesized by phytopathogenic and opportunistic fungi and is detected in a broad range of foods. This natural compound is of interest in terms of toxicity to animals, but its mechanisms of action on insects are poorly understood. We administered TeA orally at different concentrations (0.2-5.0 mg/[gram of a growth medium]) to the model insect Galleria mellonella, with subsequent estimation of physiological, histological, and immunological parameters in different tissues (midgut, fat body, and hemolymph). Susceptibility of the TeA-treated larvae to pathogenic microorganisms Beauveria bassiana and Bacillus thuringiensis was also analyzed. The feeding of TeA to the larvae led to a substation delay of larval growth, apoptosis-like changes in midgut cells, and an increase in midgut bacterial load. A decrease in activities of detoxification enzymes and downregulation of genes Nox, lysozyme, and cecropin in the midgut and/or hemocoel tissues were detected. By contrast, genes gloverin, gallerimycin, and galiomycin and phenoloxidase activity proved to be upregulated in the studied tissues. Hemocyte density did not change under the influence of TeA. TeA administration increased susceptibility of the larvae to B. bassiana but diminished their susceptibility to B. thuringiensis. The results indicate that TeA disturbs wax moth gut physiology and immunity and also exerts a systemic action on this insect. Mechanisms underlying the observed changes in wax moth susceptibility to the pathogens are discussed.

Spiramycin Disarms Pseudomonas aeruginosa without Inhibiting Growth

Spiramycin is a 16-membered macrolide antibiotic currently used in therapy to treat infections caused by Gram-positive bacteria responsible for respiratory tract infections, and it is also effective against some Gram-negative bacteria and against Toxoplasma spp. In contrast, Pseudomonas aeruginosa, which is one of the pathogens of most concern globally, is intrinsically resistant to spiramycin. In this study we show that spiramycin inhibits the expression of virulence determinants in P. aeruginosa in the absence of any significant effect on bacterial multiplication. In vitro experiments demonstrated that production of pyoverdine and pyocyanin by an environmental strain of P. aeruginosa was markedly reduced in the presence of spiramycin, as were biofilm formation, swarming motility, and rhamnolipid production. Moreover, treatment of P. aeruginosa with spiramycin sensitized the bacterium to H2O2 exposure. The ability of spiramycin to dampen the virulence of the P. aeruginosa strain was confirmed in a Galleria mellonella animal model. The results demonstrated that when G. mellonella larvae were infected with P. aeruginosa, the mortality after 24 h was >90%. In contrast, when the spiramycin was injected together with the bacterium, the mortality dropped to about 50%. Furthermore, marked reduction in transcript levels of the antimicrobial peptides gallerimycin, gloverin and moricin, and lysozyme was found in G. mellonella larvae infected with P. aeruginosa and treated with spiramycin, compared to the larvae infected without spiramycin treatment suggesting an immunomodulatory activity of spiramycin. These results lay the foundation for clinical studies to investigate the possibility of using the spiramycin as an anti-virulence and anti-inflammatory drug for a more effective treatment of P. aeruginosa infections, in combination with other antibiotics.

Genomic and phenotypic comparison of Prevotella intermedia strains possessing different virulence in vivo

Prevotella intermedia readily colonizes healthy dental biofilm and is associated with periodontal diseases. The viscous exopolysaccharide (EPS)-producing capability is known as a major virulence factor of P. intermedia 17 (Pi17). However, the inter-strain difference in P. intermedia regarding virulence-associated phenotype is not well studied. We compared in vivo virulence and whole genome sequences using five wild-type strains: ATCC 49046 (Pi49046), ATCC 15032 (Pi15032), ATCC 15033 (Pi15033), ATCC 25611 (Pi25611), and Pi17. Non-EPS producing Pi25611 was the least virulent in insect and mammalian models. Unexpectedly, Pi49046 did not produce viscous EPS but was the most virulent, followed by Pi17. Genomes of the five strains were quite similar but revealed subtle differences such as copy number variations and single nucleotide polymorphisms. Variations between strains were found in genes encoding glycosyltransferases and genes involved in the acquisition of carbohydrates and iron/haem. Based on these genetic variations, further analyses were performed. Phylogenetic and structural analyses discovered phosphoglycosyltransferases of Pi49046 and Pi17 have evolved to contain additional loops that may confer substrate specificity. Pi17, Pi15032, and Pi15033 displayed increased growth by various carbohydrates. Meanwhile, Pi49046 exhibited the highest activities for haemolysis and haem accumulation, as well as co-aggregation with Porphyromonas gingivalis harbouring fimA type II, which is more tied to periodontitis than other fimA types. Collectively, subtle genetic differences related to glycosylation and acquisition of carbohydrates and iron/haem may contribute to the diversity of virulence and phenotypic traits among P. intermedia strains. These variations may also reflect versatile strategies for within-host adaptation of P. intermedia.

High-throughput screening of caterpillars as a platform to study host-microbe interactions and enteric immunity

Mammalian models of human disease are expensive and subject to ethical restrictions. Here, we present an independent platform for high-throughput screening, using larvae of the tobacco hornworm Manduca sexta, combining diagnostic imaging modalities for a comprehensive characterization of aberrant phenotypes. For validation, we use bacterial/chemical-induced gut inflammation to generate a colitis-like phenotype and identify significant alterations in morphology, tissue properties, and intermediary metabolism, which aggravate with disease progression and can be rescued by antimicrobial treatment. In independent experiments, activation of the highly conserved NADPH oxidase DUOX, a key mediator of gut inflammation, leads to similar, dose-dependent alterations, which can be attenuated by pharmacological interventions. Furthermore, the developed platform could differentiate pathogens from mutualistic gastrointestinal bacteria broadening the scope of applications also to microbiomics and host-pathogen interactions. Overall, larvae-based screening can complement mammals in preclinical studies to explore innate immunity and host-pathogen interactions, thus representing a substantial contribution to improve mammalian welfare.

Lepidopteran insects: emerging model organisms to study infection by enteropathogens

The in vivo analysis of a pathogen is a critical step in gaining greater knowledge of pathogen biology and host-pathogen interactions. In the last two decades, there has been a notable rise in the number of studies on developing insects as a model for studying pathogens, which provides various benefits, such as ethical acceptability, relatively short life cycle, and cost-effective care and maintenance relative to routinely used rodent infection models. Furthermore, lepidopteran insects provide many advantages, such as easy handling and tissue extraction due to their large size relative to other invertebrate models, like Caenorhabditis elegans. Additionally, insects have an innate immune system that is highly analogous to vertebrates. In the present review, we discuss the components of the insect's larval immune system, which strengthens its usage as an alternative host, and present an updated overview of the research findings involving lepidopteran insects (Galleria mellonella, Manduca sexta, Bombyx mori, and Helicoverpa armigera) as infection models to study the virulence by enteropathogens due to the homology between insect and vertebrate gut.

Dissecting the invasion of Galleria mellonella by Yersinia enterocolitica reveals metabolic adaptations and a role of a phage lysis cassette in insect killing

The human pathogen Yersinia enterocolitica strain W22703 is characterized by its toxicity towards invertebrates that requires the insecticidal toxin complex (Tc) proteins encoded by the pathogenicity island Tc-PAIYe. Molecular and pathophysiological details of insect larvae infection and killing by this pathogen, however, have not been dissected. Here, we applied oral infection of Galleria mellonella (Greater wax moth) larvae to study the colonisation, proliferation, tissue invasion, and killing activity of W22703. We demonstrated that this strain is strongly toxic towards the larvae, in which they proliferate by more than three orders of magnitude within six days post infection. Deletion mutants of the genes tcaA and tccC were atoxic for the insect. W22703 ΔtccC, in contrast to W22703 ΔtcaA, initially proliferated before being eliminated from the host, thus confirming TcaA as membrane-binding Tc subunit and TccC as cell toxin. Time course experiments revealed a Tc-dependent infection process starting with midgut colonisation that is followed by invasion of the hemolymph where the pathogen elicits morphological changes of hemocytes and strongly proliferates. The in vivo transcriptome of strain W22703 shows that the pathogen undergoes a drastic reprogramming of central cell functions and gains access to numerous carbohydrate and amino acid resources within the insect. Strikingly, a mutant lacking a phage-related holin/endolysin (HE) cassette, which is located within Tc-PAIYe, resembled the phenotypes of W22703 ΔtcaA, suggesting that this dual lysis cassette may be an example of a phage-related function that has been adapted for the release of a bacterial toxin.

Citrobacter freundii, a natural associate of the Colorado potato beetle, increases larval susceptibility to Bacillus thuringiensis

BACKGROUND

We assume that certain representatives of gut microflora mediate immune changes during dysbiosis, accelerating septicemia caused by Bacillus thuringiensis.

RESULTS

Co-introduction of Citrobacter freundii with Bacillus thuringiensis var. tenebrionis (morrisoni) (Bt) led to an increase in Colorado potato beetle (CPB) larval mortality to 69.0% (1.3-5×) and a synergistic effect was observed from day 1 to day 6. Ultrathin sections of the CPB midgut showed autophagosome formation and partial destruction of gut microvilli under the influence of Bt, which was accompanied by pronounced hypersecretion of the endoplasmic reticulum with apocrine vesicle formation and oncotic changes in cells under the action of C. freundii. The destruction of gut tissues was accompanied by suppression of detoxification processes under the action of the bacteria and a decrease (2.8-3.5×) in the concentration of lipid oxidation products during Bt infection. In the first hours post combined treatment, we registered a slight increase in the total hemocyte count (THC) especially a predomination (1.4×) of immune-competent plasmatocytes. Oral administration of symbiotic and entomopathogenic bacteria to the CPB larvae significantly decreased the THC (1.4×) after 24 h and increased (1.1-1.5×) the detoxifying enzymes level in the lymph. These changes are likely to be associated with the destruction of hemocytes and the need to remove the toxic products of reactive oxygen species.

CONCLUSION

The obtained results indicate that feeding of C. freundii and B. thuringiensis to the CPB larvae is accompanied by tissue changes that significantly affect the cellular and humoral immunity of the insect, increasing its susceptibility to Bt. © 2022 Society of Chemical Industry.

Symbiont-mediated immune priming in animals through an evolutionary lens

Protective symbionts can defend hosts from parasites through several mechanisms, from direct interference to modulating host immunity, with subsequent effects on host and parasite fitness. While research on symbiont-mediated immune priming (SMIP) has focused on ecological impacts and agriculturally important organisms, the evolutionary implications of SMIP are less clear. Here, we review recent advances made in elucidating the ecological and molecular mechanisms by which SMIP occurs. We draw on current works to discuss the potential for this phenomenon to drive host, parasite, and symbiont evolution. We also suggest approaches that can be used to address questions regarding the impact of immune priming on host-microbe dynamics and population structures. Finally, due to the transient nature of some symbionts involved in SMIP, we discuss what it means to be a protective symbiont from ecological and evolutionary perspectives and how such interactions can affect long-term persistence of the symbiosis.

A first glimpse into the transcriptomic changes induced by the PaV1 infection in the gut of Caribbean spiny lobsters, Panulirus argus (Latreille, 1804) (Decapoda: Achelata: Palinuridae)

The Caribbean spiny lobster, Panulirus argus (Latreille, 1804) supports important fisheries in the Caribbean region. This species is affected by a deadly virus, Panulirus argus Virus 1 (PaV1), the only known pathogenic virus for this species. As infection progresses, the effects of PaV1 on its host become systemic, with far reaching impacts on the host's physiology, including structural injuries to its gastrointestinal organs, such as the hepatopancreas and the gut. This last one becomes highly compromised in the last stages of infection. Since the gut is a key organ for the physiological stability of lobsters, we compared the transcriptomic changes in the gut of juvenile individuals of Panulirus argus naturally infected with PaV1. In the RNA-Seq analysis, we obtained a total of 485 × 10⁶ raw reads. After cleaning, reads were de novo assembled into 68,842 transcripts and 50,257 unigenes. The length of unigenes ranged from 201 bp to 28,717 bp, with a N50 length of 2079, and a GC content of 40.61%. In the differential gene expression analysis, we identified a total of 3,405 non redundant differential transcripts, of which 1,920 were up-regulated and 1,485 were down-regulated. We found alterations in transcripts encoding for proteins involved in transcriptional regulation, splicing, postraductional regulation, protein signaling, transmembrane transport, cytoskeletal regulation, and proteolysis, among others. This is the first insight into the transcriptomic regulation of PaV1-P. argus interaction. The information generated can help to unravel the molecular mechanisms that may intervene in the gut during PaV1 infection.

Enterococcus innesii sp. nov., isolated from the wax moth Galleria mellonella

Four bacterial strains were isolated from two different colony sources of the wax moth Galleria mellonella. They were characterized by a polyphasic approach including 16S rRNA gene sequence analysis, core-genome analysis, average nucleotide identity (ANI) analysis, digital DNA-DNA hybridization (dDDH), determination of G+C content, screening of antibiotic resistance genes, and various phenotypic analyses. Initial analysis of 16S rRNA gene sequence identities indicated that strain GAL7^T was potentially very closely related to Enterococcus casseliflavus and Enterococcus gallinarum, having 99.5-99.9 % sequence similarity. However, further analysis of whole genome sequences revealed a genome size of 3.69 Mb, DNA G+C content of 42.35 mol%, and low dDDH and ANI values between the genomes of strain GAL7^T and closest phylogenetic relative E. casseliflavus NBRC 100478^T of 59.0 and 94.5 %, respectively, indicating identification of a putative new Enterococcus species. In addition, all novel strains encoded the atypical vancomycin-resistance gene vanC-4. Results of phylogenomic, physiological and phenotypic characterization confirmed that strain GAL7^T represented a novel species within the genus Enterococcus, for which the name Enterococcus innesii sp. nov. is proposed. The type strain is GAL7^T (=DSM 112306^T=NCTC 14608^T).

The diarrhetic shellfish-poisoning toxin, okadaic acid, provokes gastropathy, dysbiosis and susceptibility to bacterial infection in a non-rodent bioassay, Galleria mellonella

Diarrhetic shellfish-poisoning (DSP) toxins such as okadaic acid and dinophysistoxins harm the human gastrointestinal tract, and therefore, their levels are regulated to an upper limit of 160 μg per kg tissue to protect consumers. Rodents are used routinely for risk assessment and studies concerning mechanisms of toxicity, but there is a general move toward reducing and replacing vertebrates for these bioassays. We have adopted insect larvae of the wax moth Galleria mellonella as a surrogate toxicology model. We treated larvae with environmentally relevant doses of okadaic acid (80–400 μg/kg) via intrahaemocoelic injection or gavage to determine marine toxin-related health decline: (1) whether pre-exposure to a sub-lethal dose of toxin (80 μg/kg) enhances susceptibility to bacterial infection, or (2) alters tissue pathology and bacterial community (microbiome) composition of the midgut. A sub-lethal dose of okadaic acid (80 μg/kg) followed 24 h later by bacterial inoculation (2 × 10⁵Escherichia coli) reduced larval survival levels to 47%, when compared to toxin (90%) or microbial challenge (73%) alone. Histological analysis of the midgut depicted varying levels of tissue disruption, including nuclear aberrations associated with cell death (karyorrhexis, pyknosis), loss of organ architecture, and gross epithelial displacement into the lumen. Moreover, okadaic acid presence in the midgut coincided with a shift in the resident bacterial population over time in that substantial reductions in diversity (Shannon) and richness (Chao-1) indices were observed at 240 μg toxin per kg. Okadaic acid-induced deterioration of the insect alimentary canal resembles those changes reported for rodent bioassays.

Innate Immune Responses of Galleria mellonella to Mycobacterium bovis BCG Challenge Identified Using Proteomic and Molecular Approaches

The larvae of the insect Galleria mellonella, have recently been established as a non-mammalian infection model for the Mycobacterium tuberculosis complex (MTBC). To gain further insight into the potential of this model, we applied proteomic (label-free quantification) and transcriptomic (gene expression) approaches to characterise the innate immune response of G. mellonella to infection with Mycobacterium bovis BCG lux over a 168 h time course. Proteomic analysis of the haemolymph from infected larvae revealed distinct changes in the proteome at all time points (4, 48, 168 h). Reverse transcriptase quantitative PCR confirmed induction of five genes (gloverin, cecropin, IMPI, hemolin, and Hdd11), which encoded proteins found to be differentially abundant from the proteomic analysis. However, the trend between gene expression and protein abundance were largely inconsistent (20%). Overall, the data are in agreement with previous phenotypic observations such as haemocyte internalization of mycobacterial bacilli (hemolin/β-actin), formation of granuloma-like structures (Hdd11), and melanization (phenoloxidase activating enzyme 3 and serpins). Furthermore, similarities in immune expression in G. mellonella, mouse, zebrafish and in vitro cell-line models of tuberculosis infection were also identified for the mechanism of phagocytosis (β-actin). Cecropins (antimicrobial peptides), which share the same α-helical motif as a highly potent peptide expressed in humans (h-CAP-18), were induced in G. mellonella in response to infection, giving insight into a potential starting point for novel antimycobacterial agents. We believe that these novel insights into the innate immune response further contribute to the validation of this cost-effective and ethically acceptable insect model to study members of the MTBC.

Gut Commensal-Induced IκBζ Expression in Dendritic Cells Influences the Th17 Response

Intestinal commensal bacteria can have a large impact on the state of health and disease of the host. Regulation of Th17 cell development by gut commensals is known to contribute to their dichotomous role in promoting gut homeostasis and host defense, or development of autoimmune diseases. Yet, the underlying mechanisms remain to be fully elucidated. One candidate factor contributing to Th17 differentiation, and the expression of which could be influenced by commensals is the atypical nuclear IκB protein IκBζ. IκBζ acts as a transcriptional regulator of the expression of Th17-related secondary response genes in many cell types including dendritic cells (DCs). Insights into the regulation of IκBζ in DCs could shed light on how these immune sentinel cells at the interface between commensals, innate and adaptive immune system drive an immune-tolerogenic or inflammatory Th17 cell response. In this study, the influence of two gut commensals of low (Bacteroides vulgatus) or high (Escherichia coli) immunogenicity on IκBζ expression in DCs and its downstream effects was analyzed. We observed that the amount of IκBζ expression and secretion of Th17-inducing cytokines correlated with the immunogenicity of these commensals. However, under immune-balanced conditions, E. coli also strongly induced an IκBζ-dependent secretion of anti-inflammatory IL-10, facilitating a counter-regulative Treg response as assessed in in vitro CD4⁺ T cell polarization assays. Yet, in an in vivo mouse model of T cell-induced colitis, prone to inflammatory and autoimmune conditions, administration of E. coli promoted an expansion of rather pro-inflammatory T helper cell subsets whereas administration of B. vulgatus resulted in the induction of protective T helper cell subsets. These findings might contribute to the development of new therapeutic strategies for the treatment of autoimmune diseases using commensals or commensal-derived components.

Overview of rearing and testing conditions and a guide for optimizing Galleria mellonella breeding and use in the laboratory for scientific purposes

The greater wax moth Galleria mellonella is an increasingly popular and consolidated alternative infection model to assess microbial virulence and the effectiveness of antimicrobial compounds. The lack of G. mellonella suppliers aiming at scientific purposes and a lack of well-established protocols for raising and testing these animals may impact results and reproducibility between different laboratories. In this review, we discuss the state of the art of rearing the larvae in situ, providing an overview of breeding and testing conditions commonly used and their influence on larval health and experiments results, from setting up the environment, providing the ideal diet, understanding the effects of pretreatments, choosing the best testing conditions, to exploring the most from the results obtained. Meanwhile, we guide the reader through the most practical ways of dealing with G. mellonella to achieve successful experiments.

UtilisingGalleria mellonella larvae for studying in vivo activity of conventional and novel antimicrobial agents

The immune response of insects displays many structural and functional similarities to the innate immune response of mammals. As a result of these conserved features, insects may be used for evaluating microbial virulence or for testing the in vivo efficacy and toxicity of antimicrobial compounds and results show strong similarities to those from mammals. Galleria mellonella larvae are widely used in this capacity and have the advantage of being easy to use, inexpensive to purchase and house, and being free from the ethical and legal restrictions that relate to the use of mammals in these tests. Galleria mellonella larvae may be used to assess the in vivo toxicity and efficacy of novel antimicrobial compounds. A wide range of antibacterial and antifungal therapies have been evaluated in G. mellonella larvae and results have informed subsequent experiments in mammals. While insect larvae are a convenient and reproducible model to use, care must be taken in their use to ensure accuracy of results. The objective of this review is to provide a comprehensive account of the use of G. mellonella larvae for assessing the in vivo toxicity and efficacy of a wide range of antibacterial and antifungal agents.

Interplay between Fungal Infection and Bacterial Associates in the Wax Moth Galleria mellonella under Different Temperature Conditions

Various insect bacterial associates are involved in pathogeneses caused by entomopathogenic fungi. The outcome of infection (fungal growth or decomposition) may depend on environmental factors such as temperature. The aim of this study was to analyze the bacterial communities and immune response of Galleria mellonella larvae injected with Cordyceps militaris and incubated at 15 °C and 25 °C. We examined changes in the bacterial CFUs, bacterial communities (Illumina MiSeq 16S rRNA gene sequencing) and expression of immune, apoptosis, ROS and stress-related genes (qPCR) in larval tissues in response to fungal infection at the mentioned temperatures. Increased survival of larvae after C. militaris injection was observed at 25 °C, although more frequent episodes of spontaneous bacteriosis were observed at this temperature compared to 15 °C. We revealed an increase in the abundance of enterococci and enterobacteria in the midgut and hemolymph in response to infection at 25 °C, which was not observed at 15 °C. Antifungal peptide genes showed the highest expression at 25 °C, while antibacterial peptides and inhibitor of apoptosis genes were strongly expressed at 15 °C. Cultivable bacteria significantly suppressed the growth of C. militaris. We suggest that fungi such as C. militaris may need low temperatures to avoid competition with host bacterial associates.

Refining the Galleria mellonella Model by Using Stress Marker Genes to Assess Clostridioides difficile Infection and Recuperation during Phage Therapy

The Galleria mellonella is an effective model for probing Clostridioides difficile interactions with phages. Despite valuable insights from this model, the larvae are not easily amenable to assessing detailed clinical responses to either bacteria or phages. Here, larval survival, colonisation and toxin levels were compared to expression profiles of 17 G. mellonella stress genes to monitor Clostridiodes difficile infection (CDI), and recuperation during phage therapy. The larvae were infected with a ribotype 014/020 isolate and treated with an optimised phage cocktail. Larvae treated prophylactically with phages and the phage-control larval group were protected, showing the highest survival, and low C. difficile colonisation and toxin rates, compared to co-infection, remedial and bacterial-control larval groups. Expression of growth (9) and reproduction (2) genes were enhanced within prophylaxis and phage-control larval groups compared to the co-infection, remedial and bacterial control groups. In contrast, expression of infection (2), humoral (1) and cellular (3) immunity genes declined in the prophylactic and phage-control groups but increased in the co-infection, remedial and bacterial control larvae. The molecular markers augment the survival, colonisation and toxin data and allow detailed monitoring of CDI and recovery. This data support the use of stress marker genes as tools to analyse clinical symptoms in this model.

Inoculation With a Microbe Isolated From the Negev Desert Enhances Corn Growth

Corn (Zea mays L.) is not only an important food source, but also has numerous uses, including for biofuels, fillers for cosmetics, glues, and so on. The amount of corn grown in the U.S. has significantly increased since the 1960's and with it, the demand for synthetic fertilizers and pesticides/fungicides to enhance its production. However, the downside of the continuous use of these products, especially N and P fertilizers, has been an increase in N₂O emissions and other greenhouse gases into the atmosphere as well as run-off into waterways that fuel pollution and algal blooms. These approaches to agriculture, especially if exacerbated by climate change, will result in decreased soil health as well as human health. We searched for microbes from arid, native environments that are not being used for agriculture because we reasoned that indigenous microbes from such soils could promote plant growth and help restore degraded soils. Employing cultivation-dependent methods to isolate bacteria from the Negev Desert in Israel, we tested the effects of several microbial isolates on corn in both greenhouse and small field studies. One strain, Dietzia cinnamea 55, originally identified as Planomicrobium chinense, significantly enhanced corn growth over the uninoculated control in both greenhouse and outside garden experiments. We sequenced and analyzed the genome of this bacterial species to elucidate some of the mechanisms whereby D. cinnamea 55 promoted plant growth. In addition, to ensure the biosafety of this previously unknown plant growth promoting bacterial (PGPB) strain as a potential bioinoculant, we tested the survival and growth of Caenorhabditis elegans and Galleria mellonella (two animal virulence tests) as well as plants in response to D. cinnamea 55 inoculation. We also looked for genes for potential virulence determinants as well as for growth promotion.

Genomic characterization of multidrug-resistant ESBL-producing Escherichia coli ST58 causing fatal colibacillosis in critically endangered Brazilian merganser (Mergus octosetaceus)

Even though antimicrobial‐resistant bacteria have begun to be detected in wildlife, raising important issues related to their transmission and persistence of clinically important pathogens in the environment, little is known about the role of these bacteria on wildlife health, especially on endangered species. The Brazilian merganser (Mergus octosetaceus) is one of the most threatened waterfowl in the world, classified as Critically Endangered by the International Union for Conservation of Nature. In 2019, a fatal case of sepsis was diagnosed in an 8‐day‐old Brazilian merganser inhabiting a zoological park. At necropsy, major gross lesions were pulmonary and hepatic congestion. Using microbiologic and genomic methods, we identified a multidrug‐resistant (MDR) extended‐spectrum β‐lactamase (ESBL) CTX‐M‐8‐producing Escherichia coli (designed as PMPU strain) belonging to the international clone ST58, in coelomic cavity, oesophagus, lungs, small intestine and cloaca samples. PMPU strain harboured a broad resistome against antibiotics (cephalosporins, tetracyclines, aminoglycosides, sulphonamides, trimethoprim and quinolones), domestic/hospital disinfectants and heavy metals (arsenic, mercury, lead, copper and silver). Additionally, the virulence of E. coli PMPU strain was confirmed using a wax moth (Galleria mellonella) infection model, and it was supported by the presence of virulence genes encoding toxins, adherence factors, invasins and iron acquisition systems. Broad resistome and virulome of PMPU contributed to therapeutic failure and death of the animal. In brief, we report for the first time a fatal colibacillosis by MDR ESBL‐producing E. coli in critically endangered Brazilian merganser, highlighting that besides colonization, critical priority pathogens are threatening wildlife. E. coli ST58 clone has been previously reported in humans, food‐producing animals, wildlife and environment, supporting broad adaptation and persistence at human–animal–environment interface.

Galleria mellonella as a consolidated in vivo model hosts: New developments in antibacterial strategies and novel drug testing

A greater ethical conscience, new global rules and a modified perception of ethical consciousness entail a more rigorous control on utilizations of vertebrates for in vivo studies. To cope with this new scenario, numerous alternatives to rodents have been proposed. Among these, the greater wax moth Galleria mellonella had a preponderant role, especially in the microbiological field, as demonstrated by the growing number of recent scientific publications. The reasons for its success must be sought in its peculiar characteristics such as the innate immune response mechanisms and the ability to grow at a temperature of 37°C. This review aims to describe the most relevant features of G. mellonella in microbiology, highlighting the most recent and relevant research on antibacterial strategies, novel drug tests and toxicological studies. Although solutions for some limitations are required, G. mellonella has all the necessary host features to be a consolidated in vivo model host.

Candidate chemosensory genes identified from the greater wax moth, Galleria mellonella, through a transcriptomic analysis

The greater wax moth, Galleria mellonella Linnaeus (Lepidoptera: Galleriinae), is a ubiquitous pest of the honeybee, and poses a serious threat to the global honeybee industry. G. mellonella pheromone system is unusual compared to other lepidopterans and provides a unique olfactory model for pheromone perception. To better understand the olfactory mechanisms in G. mellonella, we conducted a transcriptomic analysis on the antennae of both male and female adults of G. mellonella using high-throughput sequencing and annotated gene families potentially involved in chemoreception. We annotated 46 unigenes coding for odorant receptors, 25 for ionotropic receptors, two for sensory neuron membrane proteins, 22 for odorant binding proteins and 20 for chemosensory proteins. Expressed primarily in antennae were all the 46 odorant receptor unigenes, nine of the 14 ionotropic receptor unigenes, and two of the 22 unigenes coding for odorant binding proteins, suggesting their putative roles in olfaction. The expression of some of the identified unigenes were sex-specific, suggesting that they may have important functions in the reproductive behavior of the insect. Identification of the candidate unigenes and initial analyses on their expression profiles should facilitate functional studies in the future on chemoreception mechanisms in this species and related lepidopteran moths.

Indomethacin-induced gut damage in a surrogate insect model, Galleria mellonella

Indomethacin is a non-steroidal anti-inflammatory drug that causes gastric ulceration and increased 'leakiness' in rat models, and is used routinely as a toxicology assay to screen novel compounds for repair and restitution properties. We set out to establish conditions for indomethacin-induced gut damage in wax-moth (Galleria mellonella) larvae with a view to reducing the need for rodents in such experimentation. We administered indomethacin (0.5-7.5 µg/larva; 2-30 mg/kg) to G. mellonella via intrahaemocoelic injection and gavage (force-feeding) and monitored survival and development, blood cell (haemocyte) numbers, and changes in gut permeability. Increased levels of gut leakiness were observed within the first 4- to 24 h by tracking fluorescent microspheres in the faeces and haemolymph (blood equivalent). Additionally, we recorded varying levels of tissue damage in histological sections of the insect midgut, including epithelial sloughing and cell necrosis. Degeneration of the midgut was accompanied by significant increases in detoxification-associated activities (superoxide dismutase and glutathione-S-transferase). Herein, we present the first evidence that G. mellonella larvae force-fed indomethacin display broad symptoms of gastric damage similar to their rodent counterparts.