Defective phagocyte association during infection of Galleria mellonella with Yersinia pseudotuberculosis is detrimental to both insect host and microbe

Low dose γ-radiation induced effects on wax moth (Galleria mellonella) larvae

Larvae of the greater wax moth Galleria mellonella are common pests of beehives and commercial apiaries, and in more applied settings, these insects act as alternative in vivo bioassays to rodents for studying microbial virulence, antibiotic development, and toxicology. In the current study, our aim was to assess the putative adverse effects of background gamma radiation levels on G. mellonella. To achieve this, we exposed larvae to low (0.014 mGy/h), medium (0.056 mGy/h), and high (1.33 mGy/h) doses of caesium-137 and measured larval pupation events, weight, faecal discharge, susceptibility to bacterial and fungal challenges, immune cell counts, activity, and viability (i.e., haemocyte encapsulation) and melanisation levels. The effects of low and medium levels of radiation were distinguishable from the highest dose rates used - the latter insects weighed the least and pupated earlier. In general, radiation exposure modulated cellular and humoral immunity over time, with larvae showing heightened encapsulation/melanisation levels at the higher dose rates but were more susceptible to bacterial (Photorhabdus luminescens) infection. There were few signs of radiation impacts after 7 days exposure, whereas marked changes were recorded between 14 and 28 days. Our data suggest that G. mellonella demonstrates plasticity at the whole organism and cellular levels when irradiated and offers insight into how such animals may cope in radiologically contaminated environments (e.g. Chornobyl Exclusion Zone).

Implant-associated biofilm infection established in an experimental Galleria mellonella model

The study reports in vivo biofilm infection implemented in an insect model. We mimicked implant-associated biofilm infections in Galleria mellonella larvae using toothbrush bristles and methicillin-resistant Staphylococcus aureus (MRSA). In vivo biofilm formation on bristle was achieved by sequentially injecting a bristle and MRSA into the larval hemocoel. It was found that biofilm formation was in progress without any external sign of infection in most of the bristle-bearing larvae for 12 h after MRSA inoculation. Whereas the activation of the prophenoloxidase system did not affect the preformed in vitro MRSA biofilms, an antimicrobial peptide interfered with in vivo biofilm formation when injected into bristle-bearing larvae infected with MRSA. Finally, our confocal laser scanning microscopic analysis revealed that the biomass of the in vivo biofilm is greater compared to that of the in vitro biofilm and harbors a distribution of dead cells, which might be bacteria and/or host cells.

Effect of acute ultraviolet radiation on Galleria mellonella health and immunity

For humans, acute and chronic overexposure to ultraviolet (UV) radiation can cause tissue damage in the form of sunburn and promote cancer(s). The immune-modulating properties of UV radiation and health-related consequences are not well known. Herein, we used the larvae of the wax moth Galleria mellonella, to determine UV-driven changes in cellular components of innate immunity. From immune cell (haemocyte) reactivity and the production of antimicrobial factors, these insects share many functional similarities with mammalian cellular innate immunity. After exposing insects to UVA or UVB for up to two hours, we monitored larval viability, susceptibility to infection, haemolymph (blood) physiology and faecal discharge. Prolonged exposure of larvae to UVB coincided with decreased survival, enhanced susceptibility to bacterial challenge, melanin synthesis in the haemolymph, compromised haemocyte functionality and changes in faecal (bacterial) content. We contend G. mellonella is a reliable in vivo model for assessing the impact of UV exposure at the whole organism and cellular levels.

Virulent and necrotrophic strategies of Bacillus thuringiensis in susceptible and resistant insects, Galleria mellonella

Bacillus thuringiensis (Bt) is one of the most common entomopathogenic bacteria used as a biopesticide, and source of endotoxin genes for generating insect-resistant transgenic plants. The mechanisms underpinning an insect's susceptibility or resistance to B. thuringiensis are diverse. The bacterial lifecycle does not end with the death of a host, they continue to exploit the cadaver to reproduce and sporulate. Herein, we studied the progression of B. thuringiensis subsp. galleriae infection in two populations of wax moth larvae (Galleria mellonella) to gain further insight into the "arms race" between B. thuringiensis virulence and insect defences. Two doses of B. thuringiensis subsp. galleriae (spore and crystalline toxin mixtures) were administered orally to compare the responses of susceptible (S) and resistant (R) populations at ∼30% mortality each. To investigate B. thuringiensis-insect antibiosis, we used a combination of in vivo infection trials, bacterial microbiome analysis, and RNAi targeting the antibacterial peptide gloverin. Within 48 h post-inoculation, B. thuringiensis-resistant insects purged the midgut of bacteria, i.e., colony forming unit numbers fell below detectable levels. Second, B. thuringiensis rapidly modulated gene expression to initiate sporulation (linked to quorum sensing) when exposed to resistant insects in contrast to susceptible G. mellonella. We reinforce earlier findings that elevated levels of antimicrobial peptides, specifically gloverin, are found in the midgut of resistant insects, which is an evolutionary strategy to combat B. thuringiensis infection via its main portal of entry. A sub-population of highly virulent B. thuringiensis can survive the enhanced immune defences of resistant G. mellonella by disrupting the midgut microbiome and switching rapidly to a necrotrophic strategy, prior to sporulation in the cadaver.

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.

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.

Establishing an invertebrate Galleria mellonella greater wax moth larval model of Neisseria gonorrhoeae infection

Neisseria gonorrhoeae (gonococcus) causes the human sexually transmitted disease gonorrhea. Studying gonococcal pathogenesis and developing new vaccines and therapies to combat the increasing prevalence of multi-antibiotic resistant bacteria has made use of many ex vivo models based on human cells and tissues, and in vivo vertebrate models, for example, rodent, pig and human. The focus of the current study was to examine the utility of the invertebrate greater wax moth Galleria mellonella as an in vivo model of gonococcal infection. We observed that a threshold of ~106 - 107 gonococci/larva was required to kill >50% of larvae (P < 0.05), and increased toxicity correlated with reduced health index scores and pronounced histopathological changes such as increases in the total lesion grade, melanized nodules, hemocyte reaction, and multifocal adipose body degeneration. Larval death was independent of the expression of pilus or Opa protein or LOS sialylation within a single gonococcal species studied, but the model could demonstrate relative toxicity of different isolates. N. meningitidis, N. lacatamica and gonococci all killed larvae equally, but were significantly less toxic (P > 0.05) than Pseudomonas aeruginosa. Larvae primed with nontoxic doses of gonococci were more susceptible to subsequent challenge with homologous and heterologous bacteria, and larval survival was significantly reduced (P < 0.05) in infected larvae after depletion of their hemocytes with clodronate-liposomes. The model was used to test the anti-gonococcal properties of antibiotics and novel antimicrobials. Ceftriaxone (P < 0.05) protected larvae from infection with different gonococcal isolates, but not azithromycin or monocaprin or ligand-coated silver nanoclusters (P > 0.05).

RNAi-mediated suppression of insect metalloprotease inhibitor (IMPI) enhances Galleria mellonella susceptibility to fungal infection

The co-evolutionary arms race between disease-causing agents and their insect victims is ancient and complex - leading to the development of specialised attack and defence strategies. Among such strategies is the capacity of fungal and oomycete pathogens to deploy degradative enzymes, notably proteases, to facilitate infection directly across the integument. To counter these proteases, insects such as the greater wax moth Galleria mellonella release metalloprotease inhibitors and other immune factors to thwart the invading fungus. To date, molecular-based confirmation of insect metalloprotease inhibitor's incontrovertible role in antifungal defence has been lacking. We targeted the IMPI gene for suppression using RNAi and exposed those insects to the entomopathogenic fungus Metarhizium brunneum ARSEF4556. Levels of IMPI were reduced significantly in the integument (10-fold) and fat body (5-fold) of RNAi-treated insects when compared to control larvae, and displayed a significantly higher mortality rate. We also surveyed candidate immune/detoxification gene expression levels (e.g., DOPA decarboxylase, galiomycin) in three tissues (integument, midgut, fat body) in order to gauge any potential non-target effects of RNAi. The loss of IMPI via RNAi compromises antifungal defences and leaves G. mellonella vulnerable to infection.

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.

Infected insect gut reveals differentially expressed proteins for cellular redox, metal resistance and secretion system in Yersinia enterocolitica-Helicoverpa armigera pathogenic model

OBJECTIVE

Mouse infection models are frequently used to study the host-pathogen interaction studies. However, due to several constraints, there is an urgent need for a simple, rapid, easy to handle, inexpensive, and ethically acceptable in vivo model system for studying the virulence of enteropathogens. Thus, the present study was performed to develop the larvae of Helicoverpa armigera as a rapid-inexpensive in vivo model system to evaluate the effect of Yersinia enterocolitica strain 8081 on its midgut via a label-free proteomic approach.

RESULTS

Helicoverpa armigera larvae fed with Yersinia enterocolitica strain 8081 manifested significant reduction in body weight and damage in midgut. On performing label-free proteomic study, secretory systems, putative hemolysin, and two-component system emerged as the main pathogenic proteins. Further, proteome comparison between control and Yersinia added diet-fed (YADF) insects revealed altered cytoskeletal proteins in response to increased melanization (via a prophenoloxidase cascade) and free radical generation. In concurrence, FTIR-spectroscopy, and histopathological and biochemical analysis confirmed gut damage in YADF insects. Finally, the proteome data suggests that the mechanism of infection and the host response in Y. enterocolitica-H. armigera system mimics Yersinia-mammalian gut interactions.

CONCLUSIONS

All data from current study collectively suggest that H. armigera larva can be considered as a potential in vivo model system for studying the enteropathogenic infection by Y. enterocolitica strain 8081.

Nutraceutical intervention protects against bacterial and chemical-induced gastrotoxicity in a non-mammalian model, Galleria mellonella

Preparations of the fungus Cordyceps sinensis and bovine colostrum are considered nutraceuticals due to their anti-inflammatory, repair and gut alimentation properties in mammalian models. To reduce the reliance on rodents in routine experimentation, we gauged the capacity of nutraceuticals to alleviate gastric damage in an insect surrogate, Galleria mellonella. Larvae were reared on standard or supplemented diets - 10% (w/w) colostrum, 10% (w/w) C. sinensis, or 5% + 5% each - prior to receiving an oral dose of the NSAID indomethacin (30 mg/kg) or challenged with the bacterial pathogen Campylobacter jejuni (1-3 x10⁶) via two inoculation routes. Insects reared on a cordyceps-supplemented diet proved most resistant to indomethacin-induced gut leakiness, and displayed stable health indices after C. jejuni challenge (~77% survival). Insects reared on a colostrum-supplemented diet also showed recalcitrance in the gut, but were more sensitive to C. jejuni when injected directly into the body cavity (50% survival). The nutraceutical blend yielded improved health outcomes when compared to the standard diet, but was not as effective as either nutraceutical alone. Our findings represent clear evidence that insects were more resistant to known chemical and microbial agitators when reared on nutraceutical-supplemented diets - toxicological endpoints that are shared with vertebrate studies.