Galleria mellonella as a model host for human pathogens: recent studies and new perspectives

Effects of antimicrobial photodynamic therapy with photodithazine® on methicillin-resistant Staphylococcus aureus (MRSA): Studies in biofilms and experimental model with Galleria mellonella

Staphylococcus aureus infections are a severe health problem due to the high mortality rate. Conventional treatment of these infections is via the administration of antibiotics. However, its indiscriminate use can select resistant microorganisms. Thus, it is necessary to develop alternatives for antibiotic therapy. Antimicrobial Photodynamic Therapy (aPDT), a therapeutic method that associates a photosensitizer (PS), a light source with adequate wavelength to the PS, interacts with molecular oxygen generating reactive oxygen species responsible for cell inactivation, is a viable alternative. This work aimed to analyze, in vitro and in vivo, the action of aPDT with PS Photodithazine® (PDZ) on the methicillin-resistant S. aureus (MRSA) strain. In the in vitro method, the S. aureus biofilm was incubated with PDZ at 50 and 75 μg.mL-1 for 15 min, adopting the light dose of 25, 50, and 100 J/cm2. In addition, PS interaction, formation of reactive oxygen species (ROS), bacterial metabolism, adhesion, bacterial viability, and biofilm structure were evaluated by scanning electron microscopy. Subsequently, the strain was inoculated into models of Galleria mellonella, and the survival curve, health scale, blood cell analysis, and CFU/mL of S. aureus in the hemolymph were analyzed after aPDT. In the in vitro results, bacterial reduction was observed in the different PDZ concentrations, highlighting the parameters of 75 μg.mL-1 of PDZ and 100 J/cm2. As for in vivo results, aPDT increased survival and stimulated the immune system of G. mellonella infected by S. aureus. aPDT proved effective in both models, demonstrating its potential as an alternative therapy in treating MRSA bacterial infections.

Amphotericin B-loaded natural latex dressing for treating Candida albicans wound infections using Galleria mellonella model

Amphotericin B (AmB) is the gold standard for antifungal drugs. However, AmB systemic administration is restricted because of its side effects. Here, we report AmB loaded in natural rubber latex (NRL), a sustained delivery system with low toxicity, which stimulates angiogenesis, cell adhesion and accelerates wound healing. Physicochemical characterizations showed that AmB did not bind chemically to the polymeric matrix. Electronic and topographical images showed small crystalline aggregates from AmB crystals on the polymer surface. About 56.6% of AmB was released by the NRL in 120 h. However, 33.6% of this antifungal was delivered in the first 24 h due to the presence of AmB on the polymer surface. The biomaterial's excellent hemo- and cytocompatibility with erythrocytes and human dermal fibroblasts (HDF) confirmed its safety for dermal wound application. Antifungal assay against Candida albicans showed that AmB-NRL presented a dose-dependent behavior with an inhibition halo of 30.0 ± 1.0 mm. Galleria mellonella was employed as an in vivo model for C. albicans infection. Survival rates of 60% were observed following the injection of AmB (0.5 mg.mL-1) in G. mellonella larvae infected by C. albicans. Likewise, AmB-NRL (0.5 mg.mL-1) presented survival rates of 40%, inferring antifungal activity against fungus. Thus, NRL adequately acts as an AmB-sustained release matrix, which is an exciting approach, since this antifungal is toxic at high concentrations. Our findings suggest that AmB-NRL is an efficient, safe, and reasonably priced ($0.15) dressing for the treatment of cutaneous fungal infections.

Changes in the apolipophorin III in Galleria mellonella larvae treated with Pseudomonas aeruginosa exotoxin A

In the present study, we have demonstrated a correlation in time between changes in the amount of apolipophorin III (apoLp-III) in the fat body and hemocytes of Galleria mellonella larvae challenged with Pseudomonas aeruginosa exotoxin A (exoA). An increase in the amount of apoLp-III was detected 1-8 h after the challenge; then, a temporary decrease was observed after 15 h followed by an increase in the level of apoLp-III, however to a different extent. The profile of apoLp-III forms in the hemolymph, hemocytes, and fat body of the exoA-challenged larvae was analyzed using two-dimensional electrophoresis (IEF/SDS-PAGE) and immunoblotting with anti-apoLp-III antibodies. Two apoLp-III forms differing in isoelectric point values estimated at ∼ 6.5 and ∼ 6.1 in the hemolymph and ∼ 6.5 and ∼ 5.9 in the hemocytes as well as one isoform with pI ∼ 6.5 in the fat body with an additional apoLp-III-derived polypeptide with estimated pI ∼ 6.9 were detected in the control insects. The injection of exoA caused a significant decrease in the abundance of both apoLp-III isoforms in the insect hemolymph. In the hemocytes, a decrease in the amount of the pI ∼ 5.9 isoform was detected, while the major apoLp-III isoform (pI ∼ 6.5) remained unchanged. In addition, appearance of an additional apoLp-III-derived polypeptide with an estimated pI ∼ 5.2 was observed. Interestingly, there were no statistically significant differences in the amount of the main isoform in the fat body between the control and exoA-challenged insects, but the polypeptide with pI ∼ 6.9 disappeared completely. It should be noted that the decrease in the amount of apoLp-III and other proteins was especially noticeable at the time points when exoA was detected in the studied tissues.

Insight into the Antibacterial Action of Iodinated Imine, an Analogue of Rafoxanide: a Comprehensive Study of Its Antistaphylococcal Activity

In this study, we have focused on a multiparametric microbiological analysis of the antistaphylococcal action of the iodinated imine BH77, designed as an analogue of rafoxanide. Its antibacterial activity against five reference strains and eight clinical isolates of Gram-positive cocci of the genera Staphylococcus and Enterococcus was evaluated. The most clinically significant multidrug-resistant strains, such as methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant S. aureus (VRSA), and vancomycin-resistant Enterococcus faecium, were also included. The bactericidal and bacteriostatic actions, the dynamics leading to a loss of bacterial viability, antibiofilm activity, BH77 activity in combination with selected conventional antibiotics, the mechanism of action, in vitro cytotoxicity, and in vivo toxicity in an alternative animal model, Galleria mellonella, were analyzed. The antistaphylococcal activity (MIC) ranged from 15.625 to 62.5 μM, and the antienterococcal activity ranged from 62.5 to 125 μM. Its bactericidal action; promising antibiofilm activity; interference with nucleic acid, protein, and peptidoglycan synthesis pathways; and nontoxicity/low toxicity in vitro and in vivo in the Galleria mellonella model were found to be activity attributes of this newly synthesized compound. In conclusion, BH77 could be rightfully minimally considered at least as the structural pattern for future adjuvants for selected antibiotic drugs. IMPORTANCE Antibiotic resistance is among the largest threats to global health, with a potentially serious socioeconomic impact. One of the strategies to deal with the predicted catastrophic future scenarios associated with the rapid emergence of resistant infectious agents lies in the discovery and research of new anti-infectives. In our study, we have introduced a rafoxanide analogue, a newly synthesized and described polyhalogenated 3,5-diiodosalicylaldehyde-based imine, that effectively acts against Gram-positive cocci of the genera Staphylococcus and Enterococcus. The inclusion of an extensive and comprehensive analysis for providing a detailed description of candidate compound-microbe interactions allows the valorization of the beneficial attributes linked to anti-infective action conclusively. In addition, this study can help with making rational decisions about the possible involvement of this molecule in advanced studies or may merit the support of studies focused on related or derived chemical structures to discover more effective new anti-infective drug candidates.

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.

Antifungal activity and potential mechanism of action of caspofungin in combination with ribavirin against Candida albicans

The number of invasive fungal infections has increased dramatically, resulting in high morbidity and mortality among immunocompromised patients. With increasing use of caspofungin (CAS), resistant strains have emerged frequently and led to limitations in the treatment of patients with severe invasive Candida albicans infections. Combination therapy is an important method to deal with this issue. As such, this study investigated the activity of CAS in combination with ribavirin (RBV) against C. albicans. The results of this in-vitro study showed that the minimum inhibitory concentrations (MICs) of CAS and RBV when they were used as monotherapy were 0.5-1 μg/mL and 2-8 μg/mL, respectively, while the MIC of CAS decreased from 0.5-1 μg/mL to 0.0625-0.25 μg/mL when used in combination with RBV, with a fractional inhibitory concentration index (FICI) ≤0.5. In addition, the RBV + CAS combination group displayed synergistic effects against C. albicans biofilm over 4 h; the sessile MIC (sMIC) of CAS decreased from 0.5-1 µg/mL to 0.0625-0.25µg/mL and the sMIC of RBV decreased from 4-16 µg/mL to 1-2 µg/mL, with FICI <0.5. The survival of C. albicans-infected Galleria mellonella was prolonged, the fungal burden was decreased, and the area of tissue damage was reduced after combination therapy. Further study showed that the mechanisms of action of the synergistic effect were related to the inhibition of biofilm formation, the inhibition of hyphal growth, and the activation of metacaspases, but were not related to the accumulation of reactive oxygen species. It is hoped that these findings will contribute to the understanding of drug resistance in C. albicans, and provide new insights for the application of RBV.

Secretions from Serratia marcescens Inhibit the Growth and Biofilm Formation of Candida spp. and Cryptococcus neoformans

Candida spp. and Cryptococcus are conditional pathogenic fungi that commonly infect immunocompromised patients. Over the past few decades, the increase in antifungal resistance has prompted the development of new antifungal agents. In this study, we explored the potential antifungal effects of secretions from Serratia marcescens on Candida spp. and Cryptococcus neoformans. We confirmed that the supernatant of S. marcescens inhibited fungal growth, suppressed hyphal and biofilm formation, and downregulated the expression of hyphae-specific genes and virulence-related genes in Candida spp. and C. neoformans. Furthermore, the S. marcescens supernatant retained biological stability after heat, pH, and protease K treatment. The chemical profile of the S. marcescens supernatant was characterized by ultra-high-performance liquid chromatography-linear ion trap/orbitrap high resolution mass spectrometry analysis and a total of 61 compounds with an mzCloud best match of greater than 70 were identified. In vivo, treatment with the S. marcescens supernatant reduced the mortality of fungi-infected Galleria mellonella. Taken together, our results revealed that the stable antifungal substances in the supernatant of S. marcescens have promising potential applications in the development of new antifungal agents.

Comprehensive insight into anti-staphylococcal and anti-enterococcal action of brominated and chlorinated pyrazine-based chalcones

The greatest threat and medicinal impact within gram-positive pathogens are posed by two bacterial genera, Staphylococcus and Enterococcus. Chalcones have a wide range of biological activities and are recognized as effective templates in medicinal chemistry. This study provides comprehensive insight into the anti-staphylococcal and anti-enterococcal activities of two recently published brominated and chlorinated pyrazine-based chalcones, CH-0y and CH-0w. Their effects against 4 reference and 12 staphylococcal and enterococcal clinical isolates were evaluated. Bactericidal action, the activity in combination with selected conventional antibiotics, the study of post-antimicrobial effect (PAE, PAE/SME), and in vitro and in vivo toxicity, were included. In CH-0y, anti-staphylococcal activity ranging from MIC = 15.625 to 62.5 μM, and activity against E. faecium from 31.25 to 62.5 μM was determined. In CH-0w, anti-staphylococcal activity ranging from 31.25 to 125 μM, and activity against E. faecium and E. faecalis (62.5 μM) was revealed. Both CH-0y and CH-0w showed bactericidal action, beneficial impact on bacterial growth delay within PAE and PAE/SME studies, and non/low toxicity in vivo. Compared to CH-0w, CH-0y seems to have higher anti-staphylococcal and less toxic potential. In conclusion, chalcones CH-0y and CH-0w could be considered as structural pattern for future adjuvants to selected antibiotic drugs.

Screening the immunotoxicity of different food preservative agents on the model organism Galleria mellonella L. (Lepidoptera: Pyralidae) larvae

Immunotoxic effects of sodium benzoate (SB, E211), sodium nitrate (SNa, E251), and sodium nitrite (SNi, E250), a few of the most common food preservatives, on the model organism Galleria mellonella L. (Lepidoptera: Pyralidae) larvae were investigated in this study. The last instar larvae were used for all experimental analyses. For this purpose, median lethal doses of SB, SNa, and SNi were applied to the larvae by the force-feeding method. We found that force-feeding G. mellonella larvae with SB, SNa, and SNi significantly reduced the larval total hemocyte counts, prohemocyte, and granulocyte ratios but increased plasmatocyte, spherulocyte, and oenocyte ratios, as well as the hemocyte mitotic indices and micronucleus frequency. The spreading ability of hemocytes and hemocyte-mediated immune responses were lower in the SB, SNa-, and SNi-treated larval groups compared to controls. Apoptotic indices were higher in all larval groups treated with food preservatives, but increments in necrotic indices were only significantly higher in SNi-treated larvae compared to controls. Our research shows that SB, SNa, and SNi have immunotoxic and cytotoxic potential on G. mellonella larvae. Thus, we suggest that G. mellonella larvae can be used as preliminary in vivo models to screen the immunotoxic effects of food preservative agents.

Antimicrobial Photodynamic Therapy Mediated by Fotenticine and Methylene Blue on Planktonic Growth, Biofilms, and Burn Infections of Acinetobacter baumannii

Antimicrobial photodynamic therapy (aPDT) is considered a promising alternative strategy to control Acinetobacter baumannii infections. In this study, we evaluated the action of aPDT mediated by a new photosensitizer derivative from chlorin e-6 (Fotoenticine—FTC) on A. baumannii, comparing its effects with methylene blue (MB). For this, aPDT was applied on A. baumannii in planktonic growth, biofilms, and burn infections in Galleria mellonella. The absorption of FTC and MB by bacterial cells was also evaluated using microscopic and spectrophotometric analysis. The results of planktonic cultures showed that aPDT reduced the number of viable cells compared to the non-treated group for the reference and multidrug-resistant A. baumannii strains. These reductions varied from 1.4 to 2 log10 CFU for FTC and from 2 log10 CFU to total inhibition for MB. In biofilms, aPDT with MB reduced 3.9 log10 CFU of A. baumannii, whereas FTC had no effect on the cell counts. In G. mellonella, only MB-mediated aPDT had antimicrobial activity on burn injuries, increasing the larvae survival by 35%. Both photosensitizers were internalized by bacterial cells, but MB showed a higher absorption compared to FTC. In conclusion, MB had greater efficacy than FTC as a photosensitizer in aPDT against A. baumannii.

Cryptococcus spp. and Cryptococcosis: focusing on the infection in Brazil

Cryptococcosis is a global fungal infection caused by the Cryptococcus neoformans/Cryptococcus gattii yeast complex. This infection is acquired by inhalation of propagules such as basidiospores or dry yeast, initially causing lung infections with the possibility of progressing to the meninges. This infection mainly affects immunocompromised HIV and transplant patients; however, immunocompetent patients can also be affected. This review proposes to evaluate cryptococcosis focusing on studies of this mycosis in Brazilian territory; moreover, recent advances in the understanding of its virulence mechanism, animal models in research are also assessed. For this, literature review as realized in PubMed, Scielo, and Brazilian legislation. In Brazil, cryptococcosis has been identified as one of the most lethal fungal infections among HIV patients and C. neoformans VNI and C. gattii VGII are the most prevalent genotypes. Moreover, different clinical settings published in Brazil were described. As in other countries, cryptococcosis is difficult to treat due to a limited therapeutic arsenal, which is highly toxic and costly. The presence of a polysaccharide capsule, thermo-tolerance, production of melanin, biofilm formation, mechanisms for iron use, and morphological alterations is an important virulence mechanism of these yeasts. The introduction of cryptococcosis as a compulsory notification disease could improve data regarding incidence and help in the management of these infections.

Anti-EFG1 2′-OMethylRNA oligomer inhibits Candida albicans filamentation and attenuates the candidiasis in Galleria mellonella

EFG1 is a central transcriptional regulator of filamentation that is an important virulence factor of Candida albicans. This study serves to assess in vivo the applicability of the anti-EFG1 2′-OMethylRNA oligomer for inhibiting C.albicans filamentation and to attenuate candidiasis, using the Galleria mellonella model. For that, larvae infected with a lethal concentration of C. albicans cells were treated with a single dose and with a double dose of the anti-EFG1 2′OMe oligomer (at 40 and 100 nM). The anti-EFG1 2′OMe oligomer toxicity and effect on larvae survival was evaluated. No evidence of anti-EFG1 2′OMe oligomer toxicity was observed and the treatment with double dose of 2′OMe oligomer empowered larvae survival over 24 h (by 90%–100%) and prolonged its efficacy until 72 h of infection (by 30%). Undoubtedly, this work validates the in vivo therapeutic potential of anti-EFG1 2′OMe oligomer for controlling C. albicans infections.

Zinc Oxide Nanoparticles Prime a Protective Immune Response in Galleria mellonella to Defend Against Candida albicans

Purpose: Zinc oxide nanoparticles (ZnO-NPs) have exerted antimicrobial properties. However, there is insufficient evaluation regarding the in vivo antifungal activity of ZnO-NPs. This study aimed to investigate the efficacy and mechanism of ZnO-NPs in controlling Candida albicans in the invertebrate Galleria mellonella. Methods: Galleria mellonella larvae were injected with different doses of ZnO-NPs to determine their in vivo toxicity. Non-toxic doses of ZnO-NPs were chosen for prophylactic injection in G. mellonella followed by C. albicans infection. Then the direct in vitro antifungal effect of ZnO-NPs against C. albicans was evaluated. In addition, the mode of action of ZnO-NPs was assessed in larvae through different assays: quantification of hemocyte density, morphology observation of hemocytes, characterization of hemocyte aggregation and phagocytosis, and measurement of hemolymph phenoloxidase (PO) activity. Results: Zinc oxide nanoparticles were non-toxic to the larvae at relatively low concentrations (≤20 mg/kg). ZnO-NP pretreatment significantly prolonged the survival of C. albicans-infected larvae and decreased the fungal dissemination and burden in the C. albicans-infected larvae. This observation was more related to the activation of host defense rather than their fungicidal capacities. Specifically, ZnO-NP treatment increased hemocyte density, promoted hemocyte aggregation, enhanced hemocyte phagocytosis, and activated PO activity in larvae. Conclusion: Prophylactic treatment with lower concentrations of ZnO-NPs protects G. mellonella from C. albicans infection. The innate immune response primed by ZnO-NPs may be part of the reason for the protective effects. This study provides new evidence of the capacity of ZnO-NPs in enhancing host immunity and predicts that ZnO-NPs will be attractive for further anti-infection applications.

Antisense locked nucleic acid gapmers to control Candida albicans filamentation

Whereas locked nucleic acid (LNA) has been extensively used to control gene expression, it has never been exploited to control Candida virulence genes. Thus, the main goal of this work was to compare the efficacy of five different LNA-based antisense oligonucleotides (ASO) with respect to the ability to control EFG1 gene expression, to modulate filamentation and to reduce C. albicans virulence. In vitro, all LNA-ASOs were able to significantly reduce C. albicans filamentation and to control EFG1 gene expression. Using the in vivo Galleria mellonella model, important differences among the five LNA-ASOs were revealed in terms of C. albicans virulence reduction. The inclusion of PS-linkage and palmitoyl-2'-amino-LNA chemical modification in these five LNA gapmers proved to be the most promising combination, increasing the survival of G. mellonella by 40%. Our work confirms that LNA-ASOs are useful tools for research and therapeutic development in the candidiasis field.

The DEAD-box RNA helicase RhlE2 is a global regulator of Pseudomonas aeruginosa lifestyle and pathogenesis

RNA helicases perform essential housekeeping and regulatory functions in all domains of life by binding and unwinding RNA molecules. The bacterial RhlE-like DEAD-box RNA helicases are among the least well studied of these enzymes. They are widespread especially among Proteobacteria, whose genomes often encode multiple homologs. The significance of the expansion and diversification of RhlE-like proteins for bacterial fitness has not yet been established. Here, we study the two RhlE homologs present in the opportunistic pathogen Pseudomonas aeruginosa. We show that, in the course of evolution, RhlE1 and RhlE2 have diverged in their biological functions, molecular partners and RNA-dependent enzymatic activities. Whereas RhlE1 is mainly needed for growth in the cold, RhlE2 also acts as global post-transcriptional regulator, affecting the level of hundreds of cellular transcripts indispensable for both environmental adaptation and virulence. The global impact of RhlE2 is mediated by its unique C-terminal extension, which supports the RNA unwinding activity of the N-terminal domain as well as an RNA-dependent interaction with the RNase E endonuclease and the cellular RNA degradation machinery. Overall, our work reveals how the functional and molecular divergence between two homologous RNA helicases can contribute to bacterial fitness and pathogenesis.

A comparison of the production of antimicrobial peptides and proteins by Galleria mellonella larvae in response to infection with two Pseudomonas aeruginosa strains differing in the profile of secreted proteases

The work presents identification of antimicrobial peptides and proteins (AMPs) in the hemolymph of Galleria mellonella larvae infected with two Pseudomonas aeruginosa strains (ATCC 27,853 and PA18), differing in the profile of secreted proteases. The insects were immunized with bacteria cultivated in rich (LB) and minimal (M9) media, which resulted in appearance of a similar broad set of AMPs in the hemolymph. Among them, 13 peptides and proteins were identified, i.e. proline-rich peptides 1 and 2, lebocin-like anionic peptide 1 and anionic peptide 2, defensin/galiomicin, cecropin, cecropin D-like peptide, apolipophoricin, gallerimycin, moricin-like peptide B, lysozyme, apolipophorin III, and superoxide dismutase. Bacterial strain- and/or medium-dependent changes in the level of proline-rich peptide 1, anionic peptide 1 and 2, moricin-like peptide B, cecropin D-like and gallerimycin were observed. The analysis of the expression of genes encoding cecropin, gallerimycin, and galiomicin indicated that they were differently affected by the bacterial strain but mainly by the medium used for bacterial culture. The highest expression was found for the LB medium. In addition to the antibacterial and antifungal activity, proteolytic activity was detected in the hemolymph of the P. aeruginosa-infected insects. Based on these results and those presented in our previous reports, it can be postulated that the appearance of AMPs in G. mellonella hemolymph can be triggered not only by P. aeruginosa pathogen associated molecular patterns (PAMPs) but also by bacterial extracellular proteases secreted during infection. However, although there were no qualitative differences in the set of AMPs depending on the P. aeruginosa strain and medium, differences in the level of particular AMPs synthesized in response to the bacteria used were observed.

Evaluation of metal-based antimicrobial compounds for the treatment of bacterial pathogens

Antimicrobial resistance (AMR) is one of the greatest global health challenges of modern times and its prevalence is rising worldwide. AMR within bacteria reduces the efficacy of antibiotics and increases both the morbidity and the mortality associated with bacterial infections. Despite this growing risk, few antibiotics with a novel mode of action are being produced, leading to a lack of antibiotics that can effectively treat bacterial infections with AMR. Metals have a history of antibacterial use but upon the discovery of antibiotics, often became overlooked as antibacterial agents. Meanwhile, metal-based complexes have been used as treatments for other diseases, such as the gold-containing drug auranofin, used to treat rheumatoid arthritis. Metal-based antibacterial compounds have novel modes of action that provide an advantage for the treatment of bacterial infections with resistance to conventional antibiotics. In this review, the antibacterial activity, mode of action, and potential for systemic use of a number of metal-based antibacterial complexes are discussed. The current limitations of these compounds are highlighted to determine if metal-based agents are a potential solution for the treatment of bacterial infections, especially those resistant to conventional antibiotics.

Antifungal activity and potential mechanism of Asiatic acid alone and in combination with fluconazole against Candida albicans

Candida albicans (C. albicans) infection remains a challenge to clinicians due to the limited available antifungals. With the widespread use of antifungals in the clinic, the drug resistance has been emerging continuously, especially fluconazole. Therefore, searching for new antifungals, active constituents of natural or traditional medicines, and approaches to overcome antifungals resistance is needed. This study investigated the activity of Asiatic acid (AA) alone and in combination with fluconazole (FLC) against C. albicans in vitro and in vivo. The in vitro studies indicated that the drug combination had a synergistic effect on FLC-resistant C. albicans, with fractional inhibitory concentration index (FICI) of 0.25. And when AA at the dose of 32 µg/mL, the drug combination group could decrease the sessile minimum inhibitory concentration (sMIC) of FLC from > 1024 µg/mL to 0.125-0.25 µg/mL within 8 h against C. albicans biofilms, even with the FICI > 0.5. In vivo, the antifungal efficacy of AA used alone and in combination with FLC was evaluated by Galleria mellonella (G. mellonella) larvae. The drug combination group prolonged the survival rate and reduced tissue invasion of larvae infected with resistant C. albicans. Furthermore, mechanism studies indicated that the antifungal effects of AA in combination with FLC might be associated with the inhibition of drug efflux pump, the accumulation of reactive oxygen species (ROS) and the inhibition of hyphal growth. These findings might provide novel insights for overcoming drug resistance of C. albicans and bring new reference data for the development and application of AA in future.

Galleria mellonella as a screening tool to study virulence factors of Aspergillus fumigatus

The invertebrate Galleria mellonella has increasingly and widely been used in the last few years to study complex host–microbe interactions. Aspergillus fumigatus is one of the most pathogenic fungi causing life-threatening diseases in humans and animals. Galleria mellonella larvae has been proven as a reliable model for the analysis of pathogenesis and virulence factors, enable to screen a large number of A. fumigatus strains. This review describes the different uses of G. mellonella to study A. fumigatus and provides a comparison of the different protocols to trace fungal pathogenicity. The review also includes a summary of the diverse mutants tested in G. mellonella, and their respective contribution to A. fumigatus virulence. Previous investigations indicated that G. mellonella should be considered as an interesting tool even though a mammalian model may be required to complete and verify initial data.

Utility of Galleria mellonella larvae for evaluating nanoparticle toxicology

The use of nanoparticles in consumer products is currently on the rise, so it is important to have reliable methods to predict any associated toxicity effects. Traditional in vitro assays fail to mimic true physiological responses of living organisms against nanoparticles whereas murine in vivo models are costly and ethically controversial. For these reasons, this study aimed to evaluate the efficacy of Galleria mellonella as an alternative, non-rodent in vivo model for examining nanoparticle toxicity. Silver, selenium, and functionalized gold nanoparticles were synthesized, and their toxicity was assessed in G. mellonella larvae. The degree of acute toxicity effects caused by each type of NP was efficiently detected by an array of indicators within the larvae: LD₅₀ calculation, hemocyte proliferation, NP distribution, behavioral changes, and histological alterations. G. mellonella larvae are proposed as a nanotoxicological model that can be used as a bridge between in vitro and in vivo murine assays in order to obtain better predictions of NP toxicity.

Galleria mellonella larvae exhibit a weight-dependent lethal median dose when infected with methicillin-resistant Staphylococcus aureus

Galleria mellonella is a recognised model to study antimicrobial efficacy; however, standardisation across the scientific field and investigations of methodological components are needed. Here, we investigate the impact of weight on mortality following infection with Methicillin-resistant Staphylococcus aureus (MRSA). Larvae were separated into six weight groups (180-300 mg at 20 mg intervals) and infected with a range of doses of MRSA to determine the 50% lethal dose (LD50), and the 'lipid weight' of larvae post-infection was quantified. A model of LD50 values correlated with weight was developed. The LD50 values, as estimated by our model, were further tested in vivo to prove our model. We establish a weight-dependent LD50 in larvae against MRSA and demonstrate that G. mellonella is a stable model within 180-260 mg. We present multiple linear models correlating weight with: LD50, lipid weight, and larval length. We demonstrate that the lipid weight is reduced as a result of MRSA infection, identifying a potentially new measure in which to understand the immune response. Finally, we demonstrate that larval length can be a reasonable proxy for weight. Refining the methodologies in which to handle and design experiments involving G. mellonella, we can improve the reliability of this powerful model.

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.

In vitro and in vivo activity of chelerythrine against Candida albicans and underlying mechanisms

Aim: To evaluate whether chelerythrine (CHT) exhibited antifungal activity against Candida albicans in vitro and in vivo and to explore the underlying mechanisms. Materials & methods: Broth microdilution assay and Galleria mellonella model were used to evaluate the antifungal effect in vitro and in vivo, respectively. Mechanism studies were investigated by morphogenesis observation, Fluo-3/AM, DCFH-DA and rhodamine6G assay, respectively. Results: CHT exhibited antifungal activity against C. albicans and preformed biofilms with minimum inhibitory concentrations ranged from 2 to 16 μg/ml. Besides, CHT protected G. mellonella larvae infected by C. albicans. Mechanisms studies revealed that CHT inhibited hyphal growth, increased intracellular calcium concentration, induced accumulation of reactive oxygen species and inhibited drug transporter activity. Conclusion: CHT exhibited antifungal activity against C. albicans.

Antifungal activity of ribavirin used alone or in combination with fluconazole against Candida albicans is mediated by reduced virulence

The incidence of fungal infections has increased continuously in recent years, and drug resistance, especially resistance to fluconazole (FLC), has emerged. To overcome this challenge, research on the antifungal activities of non-antifungal agents has gained more attention. In this study, we determined the anti-Candida activity of ribavirin (RBV), an antiviral drug commonly used in the clinic, and found that RBV displayed potent antifungal activity when used alone or in combination with FLC in vitro and in vivo. In vitro, the MIC₈₀ values of RBV were 2-4 µg/mL for FLC-susceptible Candida albicans and 8 µg/mL for FLC-resistant C. albicans. When RBV at a dose of 1 µg/mL was combined with FLC, significant synergistic effects were exhibited against FLC-resistant C. albicans, and the MICs of FLC decreased from >512 µg/mL to 0.25-1 µg/mL. Synergism was also exhibited against C. albicans biofilms. In vivo, RBV plus FLC significantly improved the survival of infected Galleria mellonella larvae compared with the FLC-treated group over a 4-day period and attenuated the damage of FLC-resistant C. albicans to G. mellonella larvae tissue. Furthermore, mechanistic studies indicated that the antifungal effects of RBV used alone or in combination with FLC might be associated with inhibition of biofilm formation, reduced extracellular phospholipase activity and inhibition of hyphal growth, but is not related to promotion of FLC uptake and inhibition of FLC efflux. These results provide a promising direction for overcoming drug resistance and for expanding the clinical application of existing drugs.

Voriconazole inhibits cross-kingdom interactions between Candida albicans and Actinomyces viscosus through the ergosterol pathway

Candida albicans and Actinomyces viscosus are prominent microbes associated with dental root caries. The aim of this study was to investigate the effect of C. albicans on A. viscosus biofilms and to identify the mechanisms associated with this interaction. A. viscosus and C. albicans strains (wide-type and mutants) were used to form biofilms in vitro and in vivo, which were subsequently analysed by crystal violet assay and scanning electron microscopy (SEM) to investigate the effect of C. albicans on A. viscosus growth. A viable plate count and survival curve for C. albicans mutants and A. viscosus combinations were used to identify which C. albicans pathway was crucial for cross-kingdom interactions. Voriconazole was used to block their interactions both in vitro and in vivo. SEM, fluorescence in situ hybridisation (FISH), quantitative PCR and survival curve analyses were performed to evaluate the activity of voriconazole on C. albicans and A. viscosus interactions. The biomass and virulence of mixed-species biofilms were significantly enhanced compared with the A. viscosus biofilm alone. However, this was not observed in the mixed-species biofilms with the C. albicans mutant erg11Δ/Δ in vitro and in vivo, indicating that azoles may work on the mixed-species biofilms. As expected, voriconazole can effectively reduce the biomass of mixed-species biofilms. A high concentration of voriconazole (1 µg/mL) reduced the abundance of C. albicans, whilst a low voriconazole concentration (0.25 µg/mL) blocked their interactions similar to the effect of the erg11Δ/Δ mutant. Voriconazole may be a candidate strategy to combat root caries pathogens.

Diet influences the bacterial and free fatty acid profiles of the cuticle of Galleria mellonella larvae

The evolutionary success of insects is arguably due to their ability to build up a complex, highly-adaptable and very effective defense system against numerous pathogens, including entomopathogenic fungi. This system relies on the humoral immune system and cellular defense reactions. The first line of defense against biological pathogens is a cuticle formed of several layers. The cuticular lipids may contain hydrocarbons, free fatty acids (FFA), alcohols, waxes, glycerides, aldehydes and sterols. Cuticular fatty acids may also play a role in defending against fungal invasion. Our present findings show that the diet of insects can have a significant effect on their sensitivity and defense response to pathogens; for example, while G. mellonella larvae fed on beeswax had a similar appearance to those reared on a semi-artificial diet, they possessed a different cuticular free fatty acid (FFA) profile to those fed on a semi-artificial diet, and were less sensitive to Conidiobolus coronatus infection. It is possible that the presence of heneicosenoic acid (C21:1) and other long-chain free fatty acids (C22:0, C24:0, C26:0), as well as Brevibacillus laterosporus bacteria, on the cuticle of larvae fed on beeswax, plays a protective role against fungal invasion. Insect pests represent a global problem. An understanding of the basic mechanisms underlying the fungal infection of insects might provide a clearer insight into their defenses, thus allowing the design of more effective, and environmentally-friendly, means of controlling them. The greater wax moth is an excellent model for the study of immunology resistance. Knowledge of the influence of diet on pathogen resistance in insects can be also useful for creating a model of human diseases caused by pathogens, such as Candia albicans.

Current Status and Trends in Alternative Models to Study Fungal Pathogens

Fungal infections affect over a billion people, with mortality rates estimated at 1⁻2 million per year [...].

A new class of prophylactic metallo-antibiotic possessing potent anti-cancer and anti-microbial properties

A family of metallo-antibiotics of general formula [Cu(N,N)(CipA)Cl] where N,N is a phenanthrene ligand and CipA is a derivative of the clinically used fluoroquinolone antibiotic ciprofloxacin – targeting immunocompromised cancer patients undergoing chemotherapy.

Non-mammalian Hosts and Photobiomodulation: Do All Life-forms Respond to Light?

Photobiomodulation (PBM), also known as low-level laser (light) therapy, was discovered over 50 years ago, but only recently has it been making progress toward wide acceptance. PBM originally used red and near-infrared (NIR) lasers, but now other wavelengths and non-coherent light-emitting diodes (LEDs) are being explored. The almost complete lack of side effects makes the conduction of controlled clinical trials relatively easy. Laboratory research has mainly concentrated on mammalian cells (normal or cancer) in culture, and small rodents (mice and rats) as models of different diseases. A sizeable body of work was carried out in the 1970s and 1980s in Russia looking at various bacterial and fungal cells. The present review covers some of these studies and a recent number of papers that have applied PBM to so-called "model organisms." These models include flies (Drosophila), worms (Caenorhabditis elegans), fish (zebrafish) and caterpillars (Galleria). Much knowledge about the genomics and proteomics, and many reagents for these organisms already exist. They are inexpensive to work with and have lower regulatory barriers compared to vertebrate animals. Other researchers have studied different models (snails, sea urchins, Paramecium, toads, frogs and chickens). Plants may respond to NIR light differently from visible light (photosynthesis and photomorphogenesis) but PBM in plants has not been much studied. Veterinarians routinely use PBM to treat non-mammalian patients. The conclusion is that red or NIR light does indeed have significant biologic effects conserved over many different kingdoms, and perhaps it is true that "all life-forms respond to light."

Immune Response of Galleria mellonella against Human Fungal Pathogens

In many aspects, the immune response against pathogens in insects is similar to the innate immunity in mammals. This has caused a strong interest in the scientific community for the use of this model in research of host⁻pathogen interactions. In recent years, the use of Galleria mellonella larvae, an insect belonging to the Lepidoptera order, has emerged as an excellent model to study the virulence of human pathogens. It is a model that offers many advantages; for example, it is easy to handle and establish in every laboratory, the larvae have a low cost, and they tolerate a wide range of temperatures, including human temperature 37 °C. The immune response of G. mellonella is innate and is divided into a cellular component (hemocytes) and humoral component (antimicrobial peptides, lytic enzymes, and peptides and melanin) that work together against different intruders. It has been shown that the immune response of this insect has a great specificity and has the ability to distinguish between different classes of microorganisms. In this review, we delve into the different components of the innate immune response of Galleria mellonella, and how these components manifest in the infection of fungal pathogens including Candida albicans, Aspergillus fumigatus, Cryptococcus neoformans, and Histoplasma capsulatum.

Recent Advances in the Use of Galleria mellonella Model to Study Immune Responses against Human Pathogens

The use of invertebrates for in vivo studies in microbiology is well established in the scientific community. Larvae of Galleria mellonella are a widely used model for studying pathogenesis, the efficacy of new antimicrobial compounds, and immune responses. The immune system of G. mellonella larvae is structurally and functionally similar to the innate immune response of mammals, which makes this model suitable for such studies. In this review, cellular responses (hemocytes activity: phagocytosis, nodulation, and encapsulation) and humoral responses (reactions or soluble molecules released in the hemolymph as antimicrobial peptides, melanization, clotting, free radical production, and primary immunization) are discussed, highlighting the use of G. mellonella as a model of immune response to different human pathogenic microorganisms.

Antifungal activity of immunosuppressants used alone or in combination with fluconazole

Fungal infections remain a challenge to clinicians due to the limited available antifungals. With the increasing use of antifungals in clinical practice, drug resistance has been emerging continuously, especially to fluconazole (FLC). Thus, a search for new antifungals and approaches to overcome antifungal resistance is needed. However, the development of new antifungals is usually costly and time consuming; discovering the antifungal activity of non-antifungal agents is one way to address these problems. Interestingly, some researchers have demonstrated that several classes of immunosuppressants (calcineurin inhibitors, glucocorticoids, etc) also displayed potent antifungal activity when used alone or in combination with antifungals, especially with FLC. Some of them could increase FLC's susceptibility against resistant Candida albicans significantly reversing fungal resistance to FLC. This article reviews the antifungal activities of immunosuppressants used alone or in combination with antifungals and their potential antifungal mechanisms that have been discovered so far. Although immunosuppressive agents have been identified as risk factors for fungal infection, we believe these findings are very important for overcoming drug resistance and developing new antifungals.

From moths to caterpillars: Ideal conditions for Galleria mellonella rearing for in vivo microbiological studies

Galleria mellonella is a well-accepted insect model for the study of pathogen-host interactions and antimicrobial compounds. The main advantages of this model include the low cost of maintenance, the fast life cycle, the possibility of using a large number of caterpillars and the innate immune system, which is evolutionarily conserved relative to mammals. Because of these advantages, different research groups have been working to implement the rearing of G. mellonella in laboratory conditions. This protocol describes our experience in the rearing of G. mellonella caterpillars for experimental infection models and the influence of different artificial diets on developmental and physiological parameters. Here, we suggest a diet composition that benefits the life cycle of G. mellonella by accelerating the larval phase length and increasing the caterpillar weight. This diet also stimulated the immune system of G. mellonella by increasing the hemolymph volume and hemocyte concentration. In addition, our rearing protocol generated caterpillars that are more resistant to infection by Staphylococcus aureus, Escherichia coli and Candida albicans. A standard G. mellonella rearing protocol is fundamental to minimize external influences on the results, and this simple and easy protocol can support researchers starting to rear G. mellonella.

Pathogenesis of the Candida parapsilosis Complex in the Model Host Caenorhabditis elegans

Caenorhabditiselegans is a valuable tool as an infection model toward the study of Candida species. In this work, we endeavored to develop a C. elegans-Candidaparapsilosis infection model by using the fungi as a food source. Three species of the C. parapsilosis complex (C. parapsilosis (sensu stricto), Candida orthopsilosis and Candida metapsilosis) caused infection resulting in C. elegans killing. All three strains that comprised the complex significantly diminished the nematode lifespan, indicating the virulence of the pathogens against the host. The infection process included invasion of the intestine and vulva which resulted in organ protrusion and hyphae formation. Importantly, hyphae formation at the vulva opening was not previously reported in C. elegans-Candida infections. Fungal infected worms in the liquid assay were susceptible to fluconazole and caspofungin and could be found to mount an immune response mediated through increased expression of cnc-4, cnc-7, and fipr-22/23. Overall, the C. elegans-C. parapsilosis infection model can be used to model C. parapsilosis host-pathogen interactions.

Genome sequence analysis of a hypermucoviscous/hypervirulent and MDR CTX-M-15/K19/ST29 Klebsiella pneumoniae isolated from human infection

The emergence of hypervirulent Klebsiella pneumoniae (hvKP) with multidrug resistance (MDR) profile is a worrisome public health issue. We report the first draft genome sequence of a hypermucoviscous (positive string test) and MDR K. pneumoniae serotype K19, belonging to ST29, isolated from human infection. This strain harboured multiple antimicrobial resistance genes, including blaCTX-M-15, besides yersiniabactin and type 3 fimbriae virulence genes. In vivo experiments carried out with the Galleria mellonella infection model revealed that K. pneumoniae K19/ST29 killed 100% of the larvae at 24 h post-infection, in a similar way to the known hypermucoviscous hvKP K1/ST23 lineage.

Differences in Virulence Between Legionella pneumophila Isolates From Human and Non-human Sources Determined in Galleria mellonella Infection Model

Legionella pneumophila is a ubiquitous bacterium in freshwater environments and in many man-made water systems capable of inducing pneumonia in humans. Despite its ubiquitous character most studies on L. pneumophila virulence focused on clinical strains and isolates from man-made environments, so little is known about the nature and extent of virulence variation in strains isolated from natural environments. It has been established that clinical isolates are less diverse than man-made and natural environmental strains, suggesting that only a subset of environmental isolates is specially adapted to infect humans. In this work we intended to determine if unrelated L. pneumophila strains, isolated from different environments and with distinct virulence-related genetic backgrounds, displayed differences in virulence, using the Wax Moth Galleria mellonella infection model. We found that all tested strains were pathogenic in G. mellonella, regardless of their origin. Indeed, a panoply of virulence-related phenotypes was observed sustaining the existence of significant differences on the ability of L. pneumophila strains to induce disease. Taken together our results suggest that the occurrence of human infection is not related with the increased capability of some strains to induce disease since we also found a concentration threshold above which L. pneumophila strains are equally able to cause disease. In addition, no link could be established between the sequence-type (ST) and L. pneumophila pathogenicity. We envision that in man-made water distribution systems environmental filtering selection and biotic competition acts structuring L. pneumophila populations by selecting more resilient and adapted strains that can rise to high concentration if no control measures are implemented. Therefore, public health strategies based on the sequence based typing (STB) scheme analysis should take into account that the major disease-associated clones of L. pneumophila were not related with higher virulence in G. mellonella infection model, and that potential variability of virulence-related phenotypes was found within the same ST.

Evaluating Different Virulence Traits of Klebsiella pneumoniae Using Dictyostelium discoideum and Zebrafish Larvae as Host Models

Multiresistant and invasive hypervirulent Klebsiella pneumoniae strains have become one of the most urgent bacterial pathogen threats. Recent analyses revealed a high genomic plasticity of this species, harboring a variety of mobile genetic elements associated with virulent strains, encoding proteins of unknown function whose possible role in pathogenesis have not been addressed. K. pneumoniae virulence has been studied mainly in animal models such as mice and pigs, however, practical, financial, ethical and methodological issues limit the use of mammal hosts. Consequently, the development of simple and cost-effective experimental approaches with alternative host models is needed. In this work we described the use of both, the social amoeba and professional phagocyte Dictyostelium discoideum and the fish Danio rerio (zebrafish) as surrogate host models to study K. pneumoniae virulence. We compared three K. pneumoniae clinical isolates evaluating their resistance to phagocytosis, intracellular survival, lethality, intestinal colonization, and innate immune cells recruitment. Optical transparency of both host models permitted studying the infective process in vivo, following the Klebsiella-host interactions through live-cell imaging. We demonstrated that K. pneumoniae RYC492, but not the multiresistant strains 700603 and BAA-1705, is virulent to both host models and elicits a strong immune response. Moreover, this strain showed a high resistance to phagocytosis by D. discoideum, an increased ability to form biofilms and a more prominent and irregular capsule. Besides, the strain 700603 showed the unique ability to replicate inside amoeba cells. Genomic comparison of the K. pneumoniae strains showed that the RYC492 strain has a higher overall content of virulence factors although no specific genes could be linked to its phagocytosis resistance, nor to the intracellular survival observed for the 700603 strain. Our results indicate that both zebrafish and D. discoideum are advantageous host models to study different traits of K. pneumoniae that are associated with virulence.

Clinical strains of Lactobacillus reduce the filamentation of Candida albicans and protect Galleria mellonella against experimental candidiasis

Candida albicans is the most common human fungal pathogen and can grow as yeast or filaments, depending on the environmental conditions. The filamentous form is of particular interest because it can play a direct role in adherence and pathogenicity. Therefore, the purpose of this study was to evaluate the effects of three clinical strains of Lactobacillus on C. albicans filamentation as well as their probiotic potential in pathogen-host interactions via an experimental candidiasis model study in Galleria mellonella. We used the reference strain Candida albicans ATCC 18804 and three clinical strains of Lactobacillus: L. rhamnosus strain 5.2, L. paracasei strain 20.3, and L. fermentum strain 20.4. First, the capacity of C. albicans to form hyphae was tested in vitro through association with the Lactobacillus strains. After that, we verified the ability of these strains to attenuate experimental candidiasis in a Galleria mellonella model through a survival curve assay. Regarding the filamentation assay, a significant reduction in hyphae formation of up to 57% was observed when C. albicans was incubated in the presence of the Lactobacillus strains, compared to a control group composed of only C. albicans. In addition, when the larvae were pretreated with Lactobacillus spp. prior to C. albicans infection, the survival rate of G. mellonela increased in all experimental groups. We concluded that Lactobacillus influences the growth and expression C. albicans virulence factors, which may interfere with the pathogenicity of these microorganisms.

Immunomodulatory effect of photodynamic therapy in Galleria mellonella infected with Porphyromonas gingivalis

Porphyromonas gingivalis is an important pathogen in the development of periodontal disease. Our study investigated if the treatment with antimicrobial photodynamic therapy (aPDT) that employs a nontoxic dye, followed by irradiation with harmless visible light can attenuate the experimental infection of P. gingivalis in Galleria mellonella. Firstly, different concentrations of P. gingivalis ranging from 10² to 10⁶ cells/larva were injected into the animal to obtain a lethal concentration. Next, the following groups of G. mellonella infected with P. gingivalis were evaluated: inoculation of the photosensitizer and application of laser (P + L+), inoculation of physiologic solution and application of laser (P-L+), inoculation the photosensitizer without laser (P + L-) and inoculation of physiologic solution without Laser (P-L-). The effects of aPDT on infection by P. gingivalis were evaluated by survival curve analysis and hemocytes count. A lethal concentration of 10⁶ cells/larva was adopted for evaluating the effects of aPDT on experimental infection with P. gingivalis. We found that after 120 s of PDT application, the death of G. mellonella was significantly lower compared to the control groups (p = 0.0010). Moreover, the hemocyte density in the P+L+ group was increased by 9.6 × 10⁶ cells/mL (2.62-fold increase) compared to the infected larvae with no treatment (L-P- group) (p = 0.0175). Finally, we verified that the aPDT led to a significant reduction of the number of P. gingivalis cells in G. mellonella hemolymph. In conclusion, PDT application was effective against P. gingivalis infection by increasing the survival of G. mellonella and was able to increase the circulating hemocytes indicating that PDT activates the G. mellonella immune system.

Immunity of the greater wax moth Galleria mellonella

Investigation of insect immune mechanisms provides important information concerning innate immunity, which in many aspects is conserved in animals. This is one of the reasons why insects serve as model organisms to study virulence mechanisms of human pathogens. From the evolutionary point of view, we also learn a lot about host-pathogen interaction and adaptation of organisms to conditions of life. Additionally, insect-derived antibacterial and antifungal peptides and proteins are considered for their potential to be applied as alternatives to antibiotics. While Drosophila melanogaster is used to study the genetic aspect of insect immunity, Galleria mellonella serves as a good model for biochemical research. Given the size of the insect, it is possible to obtain easily hemolymph and other tissues as a source of many immune-relevant polypeptides. This review article summarizes our knowledge concerning G. mellonella immunity. The best-characterized immune-related proteins and peptides are recalled and their short characteristic is given. Some other proteins identified at the mRNA level are also mentioned. The infectious routes used by Galleria natural pathogens such as Bacillus thuringiensis and Beauveria bassiana are also described in the context of host-pathogen interaction. Finally, the plasticity of G. mellonella immune response influenced by abiotic and biotic factors is described.

Lactobacillus paracasei modulates the immune system of Galleria mellonella and protects against Candida albicans infection

Probiotics have been described as a potential strategy to control opportunistic infections due to their ability to stimulate the immune system. Using the non-vertebrate model host Galleria mellonella, we evaluated whether clinical isolates of Lactobacillus spp. are able to provide protection against Candida albicans infection. Among different strains of Lactobacillus paracasei, Lactobacillus rhamnosus and Lactobacillus fermentum, we verified that L. paracasei 28.4 strain had the greatest ability to prolong the survival of larvae infected with a lethal dose of C. albicans. We found that the injection of 107 cells/larvae of L. paracasei into G. mellonella larvae infected by C. albicans increased the survival of these insects compared to the control group (P = 0.0001). After that, we investigated the immune mechanisms involved in the protection against C. albicans infection, evaluating the number of hemocytes and the gene expression of antifungal peptides. We found that L. paracasei increased the hemocyte quantity (2.38 x 106 cells/mL) in relation to the control group (1.29 x 106 cells/mL), indicating that this strain is capable of raising the number of circulating hemocytes into the G. mellonella hemolymph. Further, we found that L. paracasei 28.4 upregulated genes that encode the antifungal peptides galiomicin and gallerymicin. In relation to the control group, L. paracasei 28.4 increased gene expression of galiomicin by 6.67-fold and 17.29-fold for gallerymicin. Finally, we verified that the prophylactic provision of probiotic led to a significant reduction of the number of fungal cells in G. mellonella hemolymph. In conclusion, L. paracasei 28.4 can modulate the immune system of G. mellonella and protect against candidiasis.

Lactobacillus rhamnosus inhibits Candida albicans virulence factors in vitro and modulates immune system in Galleria mellonella

AIM

The aim of this study was to evaluate the potential anti-Candida effects of Lactobacillus rhamnosus ATCC 9595 on Candida albicans ATCC 18804 using in vitro and in vivo models.

METHODS AND RESULTS

The in vitro analysis evaluated the effects of L. rhamnosus on C. albicans's biofilm formation by CFU count and metabolic activity, filamentation capacity, and adhesion (ALS3 and HWP1) and transcriptional regulatory gene (BCR1 and CPH1) expression. The in vitro results showed that both the L. rhamnosus cells and supernatant reduced C. albicans biofilm formation, filamentation and gene expression. In the in vivo study, the treatment with L. rhamnosus supernatant increased 80% the survival of Galleria mellonella larvae infected with C. albicans. Furthermore, the supernatant of L. rhamnosus recruited haemocytes into the haemolymph (2·1-fold increase).

CONCLUSIONS

Lactobacillus rhamnosus reduced the biofilm formation and filamentation of C. albicans in vitro by negatively regulating all studied C. albicans genes. Lactobacillus rhamnosus protected G. mellonella against experimental candidiasis in vivo.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study is the first study to report the anti-Candida properties of L. rhamnosus ATCC 9595. The supernatant of this strain has immunomodulatory effects on the G. mellonella model and protects the larvae against pathogens.