Silkworm as an experimental animal for research on fungal infections

Strategy to Identify Virulence-Related Genes of the Pathogenic Fungus Trichosporon asahii Using an Efficient Gene-Targeting System

Trichosporon asahii is a pathogenic fungus that causes severe deep-seated mycosis in immunocompromised patients with neutropenia. Understanding the molecular mechanisms of T. asahii infection will facilitate the development of new therapeutic and preventive strategies. Two main obstacles have prevented the identification of virulence-related genes in T. asahii using molecular genetic techniques: the lack of experimental animal infection models for easy evaluation of T. asahii virulence and the lack of genetic recombination technology for T. asahii. To address these issues, we developed a silkworm infection model to quantitatively evaluate T. asahii virulence and a genetic recombination method to generate gene-deficient T. asahii mutants, enabling the identification of virulence factors of T. asahii. In this review, we propose a strategy for identifying virulence-related factors in T. asahii using a silkworm infection model and an efficient gene-targeting system.

The composition and function of bacterial communities in Bombyx mori (Lepidoptera: Bombycidae) changed dramatically with infected fungi: A new potential to culture Cordyceps cicadae

Cordyceps cicadae (Hypocreales: Cordycipitaceae) is a renowned entomopathogenic fungus used as herbal medicine in China. However, wild C. cicadae resources have been threatened by heavy harvesting. We hypothesised that Bombyx mori L. (Lepidoptera: Bombycidae) could be a new alternative to cultivate C. cicadae due to the low cost of rearing. Bacterial communities are crucial for the formation of Cordyceps and for promoting the production of metabolites. To better understand the bacterial community structure associated with Cordyceps, three Claviciptaceae fungi were used to explore the pathogenicity of the silkworms. Here, fifth-instar silkworms were infected with C. cicadae, Cordyceps cateniannulata (Hypocreales: Cordycipitaceae) and Beauveria bassiana (Hypocreales: Cordycipitaceae). Subsequently, we applied high-throughput sequencing to explore the composition of bacterial communities in silkworms. Our results showed that all three fungi were highly pathogenic to silkworms, which suggests that silkworms have the potential to cultivate Cordyceps. After fungal infection, the diversity of bacterial communities in silkworms decreased significantly, and the abundance of Staphylococcus increased in mummified larvae, which may play a role in the death process when the host suffers infection by entomopathogenic fungi. Furthermore, there were high similarities in the bacterial community composition and function in the C. cicadae and C. cateniannulata infected samples, and the phylogenetic analysis suggested that these similarities may be related to the fungal phylogenetic relationship. Our findings reveal that infection with different entomopathogenic fungi affects the composition and function of bacterial communities in silkworms and that the bacterial species associated with Cordyceps are primarily host dependent, while fungal infection affects bacterial abundance.

Bombyx mori UFBP1 regulates apoptosis and promotes BmNPV proliferation by affecting the expression of ER chaperone BmBIP

Ubiquitin-fold modifier 1 (UFM1) is attached to protein substrates through the sequential activity of an E1 (UBA5) - E2 (UFC1) - E3 (UFL1) cascade. UFBP1 is a conserved UFL1-interacting protein in mammals. However, to date, no study has been conducted on UFBP1 in silkworm. In this study, we identified a UFBP1 ortholog in the B. mori genome. Spatiotemporal expression profiles showed that BmUFBP1 expression was high in the midgut and fatbody, and at the moth stage. BmUFBP1 knockdown inhibited ER chaperone BmBIP expression and BmNPV proliferation, while BmUFBP1 overexpression increased BmNPV proliferation, and BmBIP rescued BmUFBP1-regulated BmNPV proliferation. Mechanistically, Apoptosis and ATF6 signaling are involved in BmUFBP1-regulated BmBIP expression and BmNPV proliferation. These results suggest that BmUFBP1 facilitates BmNPV proliferation via ATF6-BIP signaling, and provide a potential molecular target for BmNPV prevention and silkworm breeding.

Discovery of new AMR drugs targeting modulators of antimicrobial activity using in vivo silkworm screening systems

Global concerns about drug-resistant bacteria have underscored the need for new antimicrobial drugs. Emerging strategies in drug discovery include considering the third factors that influence drug activity. These factors include host-derived elements, adjuvants, and drug combinations, which are crucial in regulating antimicrobial efficacy. Traditional in vivo assessments have relied on animal models to study drug absorption, distribution, metabolism, excretion, and toxicity (ADMET). Alternative models, such as silkworms, are being explored to overcome the ethical and financial barriers associated with mammalian models. The silkworm has been proven effective in evaluating ADMET and in highlighting the therapeutic potential enhanced by third factors. Host factors (either mammalian or non-mammalian) enhance the antimicrobial activity of antimicrobial agents such as lysocin E. Additionally, using D-cycloserine to potentiate vancomycin has successfully combated vancomycin-resistant infections in silkworms. Leveraging silkworms in drug discovery could establish a novel screening method incorporating interactions with third factors, whether host related or non-host-related, thus promising new pathways for identifying antimicrobial drugs with unique mechanisms of action.

Development of a silkworm infection model for evaluating the virulence of Mycobacterium intracellulare subspecies estimated using phylogenetic tree analysis based on core gene data

Non-tuberculous mycobacteria (NTM) cause skin infections, respiratory diseases, and disseminated infections. Mycobacterium avium and Mycobacterium intracellulare, which are slow grown Mycobacterium, are main agents of those NTM diseases. A silkworm infection model with Mycobacterium abscessus, a rapidly growing Mycobacterium species, was established to quantitatively evaluate its virulence within a short period. However, a silkworm infection model to quantitatively evaluate the virulence of M. intracellulare has not yet been developed. In this study, we determined the virulence of M. intracellulare subspecies within 4 days using a silkworm infection model. The subspecies of M. intracellulare strains used in this study were estimated by phylogenetic tree analysis using core gene data. The median lethal dose (LD50) values, which are the dose of a pathogen required to kill half of the silkworms in a group, were determined 4 days after infection. The LD50 value of M. intracellulare subsp. chimaera DSM44623 was higher than that of M. intracellulare subsp. intracellulare ATCC13950. These results suggest that the virulence of M. intracellulare subspecies can be compared using a silkworm model within 4 days.

Beyond the Charge: Interplay of Nanogels' Functional Group and Zeta-Potential for Antifungal Drug Delivery to Human Pathogenic Fungus Aspergillus Fumigatus

The ubiquitous mold Aspergillus fumigatus (A. fumigatus) is one of the main fungal pathogens causing invasive infections in immunocompromised humans. Conventional antifungal agents exhibit limited efficacy and often cause severe side effects. Nanoparticle-based antifungal delivery provides a promising alternative, which can increase local drug concentration; while, mitigating toxicity, thereby enhancing treatment efficacy. Previous research underscores the potential of poly(glycidol)-based nanogels (NG) with negative surface charge as carriers for delivering antifungals to A. fumigatus hyphae. In this study, NG is tailored with 2-carboxyethyl acrylate (CEA) or with phosphoric acid 2-hydroxyethyl acrylate (PHA). It is discovered that quenching with PHA clearly improves the adhesion of NG to hyphal surface and the internalization of NG into the hyphae under protein-rich conditions, surpassing the outcomes of non-quenched and CEA-quenched NG. This enhancement cannot be solely attributed to an increase in negative surface charge but appears to be contingent on the functional group of the quencher. Further, it is demonstrated that itraconazole-loaded, PHA-functionalized nanogels (NGxPHA-ITZ) show lower MIC in vitro and superior therapeutic effect in vivo against A. fumigatus compared to pure itraconazole. This confirms NGxPHA as a promising antifungal delivery system.

Induction of acute silkworm hemolymph melanization by Staphylococcus aureus treated with peptidoglycan-degrading enzymes

Staphylococcus aureus, a Gram-positive bacterium, causes inflammatory skin diseases, such as atopic dermatitis, and serious systemic diseases, such as sepsis. In the skin and nasal environment, peptidoglycan (PGN)-degrading enzymes, including lysozyme and lysostaphin, affects S. aureus PGN. However, the effects of PGN-degrading enzymes on the acute innate immune-inducing activity of S. aureus have not yet been investigated. In this study, we demonstrated that PGN-degrading enzymes induce acute silkworm hemolymph melanization by S. aureus. Insoluble fractions of S. aureus treated with lysozyme, lysostaphin, or both enzymes, were prepared. Melanization of the silkworm hemolymph caused by the injection of these insoluble fractions was higher than that of S. aureus without enzyme treatment. These results suggest that structural changes in S. aureus PGN caused by PGN-degrading enzymes affect the acute innate immune response in silkworms.

Acid-treated Staphylococcus aureus induces acute silkworm hemolymph melanization

The skin microbiome maintains healthy human skin, and disruption of the microbiome balance leads to inflammatory skin diseases such as folliculitis and atopic dermatitis. Staphylococcus aureus and Cutibacterium acnes are pathogenic bacteria that simultaneously inhabit the skin and cause inflammatory diseases of the skin through the activation of innate immune responses. Silkworms are useful invertebrate animal models for evaluating innate immune responses. In silkworms, phenoloxidase generates melanin as an indicator of innate immune activation upon the recognition of bacterial or fungal components. We hypothesized that S. aureus and C. acnes interact to increase the innate immunity-activating properties of S. aureus. In the present study, we showed that acidification is involved in the activation of silkworm hemolymph melanization by S. aureus. Autoclaved-killed S. aureus (S. aureus [AC]) alone does not greatly activate silkworm hemolymph melanization. On the other hand, applying S. aureus [AC] treated with C. acnes culture supernatant increased the silkworm hemolymph melanization. Adding C. acnes culture supernatant to the medium decreased the pH. S. aureus [AC] treated with propionic acid, acetic acid, or lactic acid induced higher silkworm hemolymph melanization activity than untreated S. aureus [AC]. S. aureus [AC] treated with hydrochloric acid also induced silkworm hemolymph melanization. The silkworm hemolymph melanization activity of S. aureus [AC] treated with hydrochloric acid was inhibited by protease treatment of S. aureus [AC]. These results suggest that acid treatment of S. aureus induces innate immune activation in silkworms and that S. aureus proteins are involved in the induction of innate immunity in silkworms.

Recent advances in the biosensors application for reviving infectious disease management in silkworm model: a new way to combat microbial pathogens

Silkworms are an essential economic insect but are susceptible to diseases during rearing, leading to yearly losses in cocoon production. While chemical control is currently the primary method to reduce disease incidences, its frequent use can result in loss of susceptibility to pathogens and, ultimately, antibiotic resistance. To effectively prevent or control disease, growers must accurately, sensitively, and quickly detect causal pathogens to determine the best management strategies. Accurate recognition of diseased silkworms can prevent pathogen transmission and reduce cocoon loss. Different pathogen detection methods have been developed to achieve this objective, but they need more precision, specificity, consistency, and promptness and are generally unsuitable for in-situ analysis. Therefore, detecting silkworm diseases under rearing conditions is still an unsolved problem. As a consequence of this, there is an enormous interest in the development of biosensing systems for the early and precise identification of pathogens. There is also significant room for improvement in translating novel biosensor techniques to identify silkworm pathogens. This study explores the types of silkworm diseases, their symptoms, and their causal microorganisms. Moreover, we compare the traditional approaches used in silkworm disease diagnostics along with the latest sensing technologies, with a precise emphasis on lateral flow assay-based biosensors that can detect and manage silkworm pathogens.

Alternative in-vivo models of mucormycosis

Mucormycosis is still regarded a rare fungal infection, but the high incidences of COVID-associated cases in India and other countries have shown its potential threat to large patient cohorts. In addition, infections by these fast-growing fungi are often fatal and cause disfigurement, badly affecting patients' lives. In advancing our understanding of pathogenicity factors involved in this disease, to enhance the diagnostic toolset and to evaluate novel treatment regimes, animal models are indispensable. As ethical and practical considerations typically favor the use of alternative model systems, this review provides an overview of alternative animal models employed for mucormycosis and discusses advantages and limitations of the respective model.

Comparing the virulence of four major clades of Candida auris strains using a silkworm infection model: Clade IV isolates had higher virulence than the other clades

Candida auris is an emerging fungal pathogen that is feared to spread of infection because of its propensity for multidrug resistance and high mortality rate. This pathogenic yeast is classified into four major clades by phylogenetic analyses, which are referred to the South Asia clade (clade I), East Asia clade (clade II), South Africa clade (clade III), and South America clade (clade IV), based on the location of the initial isolate. In this study, we evaluated the virulence of C. auris strains belonging to four major clades and the therapeutic effects of micafungin in a silkworm infection model. The highest mortality rate at 21 h after C. auris inoculation was observed for strains from clade IV (80% or more). In contrast, it was 20% or less in those from other clades. Antifungal susceptibility tests indicated resistance to fluconazole and sensitivity to echinocandins in the blood-derived strains. Micafungin prolonged the survival of blood-derived C. auris infected silkworms. These results suggest that the silkworm infection model is useful for evaluating the virulence of C. auris and determining its therapeutic effects.

A rabbit model of ear otitis established using the Malassezia pachydermatis strain C23 from dogs

Background and Aim

Fungal infections are a growing problem for both humans and animals due to the emergence of pathogenic strains resistant to modern antifungal treatments. To evaluate the efficacy of new antifungal drugs, it is essential to develop animal models that demonstrate typical responses to both the infection (pathogenesis and clinical course) and to the treatment, including adverse effects. In this study, we established a rabbit otitis model by infection of an aggressive multidrug-resistant strain from dogs, Malassezia pachydermatis C23, with no need for concomitant immunosuppression.

Materials and Methods

Twenty healthy adult male gray giant rabbits (1 year old, 5.5 kg) were inoculated once with M. pachydermatis C23 at 108 colony-forming units/mL. We observed the clinical signs of the disease and collected ear smears and blood samples every 5 days.

Results

The infection progressed rapidly and exhibited characteristic clinical signs without spontaneous recovery for at least 1 month. In fact, substantial deterioration was observed as evidenced by blood parameters.

Conclusion

This rabbit otitis model established using an aggressive drug-resistant fungus strain without immunosuppression could prove valuable for testing novel antifungal agents.

Ecdysteroid UDP-Glucosyltransferase Expression in Beauveria bassiana Increases Its Pathogenicity against Early Instar Silkworm Larvae

Beauveria bassiana (B. bassiana) is a broad-spectrum entomopathogenic fungus that can control pests in agriculture and forestry. In this study, encoding ecdysteroid uridine diphosphate glucosyltransferase gene (egt) was successfully screened in B. bassiana on the medium containing 500μg/mL G418 sulfate solution through the protoplast transformation method. This enzyme has the function of 20E (20-hydroxyecdysone) inactivation, thus increasing the mortality of the early instar larvae infected with B. bassiana. In this study, we transformed B. bassiana with the egt gene, which deactivates 20-hydroxyecdysone, a key hormone in insect development. The results showed that transgenic B. bassiana killed more silkworms of the 2nd instar larvae than the wild-type with a shorter LT50 time, which was reduced by approximately 20% (day 1 of the 2nd instar silkworm infection of B. bassiana) and 26.4% (day 2 of the 2nd instar silkworm infection of B. bassiana) compared to the wild-type, and also showed a higher mortality number before molting. The transgenic B. bassiana had a higher coverage of the body surface of silkworms compared to the wild type on the 3rd instar. In summary, improving entomopathogenic fungi using biological methods such as genetic engineering is feasible.

In vivo efficacy of pitavastatin combined with itraconazole against Aspergillus fumigatus in silkworm models

Azole resistance in Aspergillus fumigatus is a worldwide concern and new antifungal drugs are required to overcome this problem. Statin, a 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor, has been reported to suppress the growth of A. fumigatus, but little is known about its in vivo antifungal effect against A. fumigatus. In this study, we evaluated the in vivo efficacy of pitavastatin (PIT) combined with itraconazole (ITC) against azole-susceptible and azole-resistant strains with silkworm models. Prolongation of survival was confirmed in the combination-therapy (PIT and ITC) group compared to the no-treatment group in both azole-susceptible and azole-resistant strain models. Furthermore, when the azole-susceptible strain was used, the combination-therapy resulted in a higher survival rate than with ITC alone. Histopathological analysis of the silkworms revealed a reduction of the hyphal amount in both azole-susceptible and azole-resistant strain models. Quantitative evaluation of fungal DNA by qPCR in azole-susceptible strain models clarified the reduction of fungal burden in the combination-therapy group compared with the no-treatment group and ITC-alone group. These results indicate that the efficacy of PIT was enhanced when combined with ITC in vivo. As opposed to most statins, PIT has little drug-drug interaction with azoles in humans and can be used safely with ITC. This combination therapy may be a promising option as an effective treatment in clinical settings in the future. IMPORTANCE Azole resistance among A. fumigatus isolates has recently been increasingly recognized as a cause of treatment failure, and alternative antifungal therapies are required to overcome this problem. Our study shows the in vivo efficacy of PIT combined with ITC against A. fumigatus using silkworm models by several methods including evaluation of survival rates, histopathological analysis, and assessment of fungal burden. Contrary to most statins, PIT can be safely administered with azoles because of less drug-drug interactions, so this study should help us to verify how to make use of the drug in clinical settings in the future.

Hog1-mediated stress tolerance in the pathogenic fungus Trichosporon asahii

Trichosporon asahii is an opportunistic pathogenic fungus that causes severe and sometimes fatal infections in immunocompromised patients. Hog1, a mitogen-activated protein kinase, regulates the stress resistance of some pathogenic fungi, however its role in T. asahii has not been investigated. Here, we demonstrated that the hog1 gene-deficient T. asahii mutant is sensitive to high temperature, cell membrane stress, oxidative stress, and antifungal drugs. Growth of the hog1 gene-deficient T. asahii mutant was delayed at 40 °C. The hog1 gene-deficient T. asahii mutant also exhibited sensitivity to sodium dodecyl sulfate, hydrogen peroxide, menadione, methyl methanesulfonate, UV exposure, and antifungal drugs such as amphotericin B under a glucose-rich condition. Under a glucose-restricted condition, the hog1 gene-deficient mutant exhibited sensitivity to NaCl and KCl. The virulence of the hog1 gene-deficient mutant against silkworms was attenuated. Moreover, the viability of the hog1 gene-deficient mutant decreased in the silkworm hemolymph. These phenotypes were restored by re-introducing the hog1 gene into the gene-deficient mutant. Our findings suggest that Hog1 plays a critical role in regulating cellular stress responses in T. asahii.

Silkworm model of biofilm formation: In vivo evaluation of antimicrobial tolerance of a cross-kingdom dual-species (Escherichia coli and Candida albicans) biofilm on catheter material

Biofilms are formed by microorganisms and their products on the surface of materials such as medical devices. Biofilm formation protects microorganisms from antimicrobial agents. Bacteria and fungi often form dual-species biofilms on the surfaces of medical devices in clinical settings. An experimental system to evaluate in vivo biofilm formation by the pathogenic fungus Candida albicans was established using silkworms inserted with polyurethane fiber (PF), a catheter material. In the present study, we established an in vivo experimental system using silkworms to evaluate the antimicrobial tolerance of Escherichia coli in single- and dual-species biofilms formed on the surface of the PF. The injection of E. coli into the PF-inserted silkworms led to the formation of a biofilm by E. coli on the surface of the PF. E. coli in the biofilm exhibited tolerance to meropenem (MEPM). Furthermore, when E. coli and C. albicans were co-inoculated into the PF-inserted silkworms, a dual-species biofilm formed on the surface of the PF. E. coli in the dual-species biofilm with C. albicans was more tolerant to MEPM than E. coli in the single-species biofilm. These findings suggest the usefulness of an in vivo experimental system using PF-inserted silkworms to investigate the mechanisms of MEPM tolerance in E. coli in single- and dual-species biofilms.

Bombyx mori as a model for Niallia circulans pathogenicity

Increasing incidences of resistance to antibiotics by pathogenic bacteria is a worldwide concern and isolation of antibiotic-resistant strains of Niallia circulans (formerly known as Bacillus circulans), an opportunistic human pathogen, has been reported. Due to their lack of ethical constraints as well as their cost-effective rearing, invertebrates have been commonly used to study infection by bacteria pathogenic to humans. In this study, we demonstrate that a foodborne strain of N. circulans kills larvae of the silkworm, Bombyx mori within 48 h after hemolymph injection. The infected larvae turned black with an increase in the phenoloxidase (PO) activity in the hemolymph. Midgut injection of N. circulans resulted in the killing of larvae within 96 h. A significant increase in bacterial load was observed in the hemolymph 12 h after infection. The viable hemocyte number decreased to 48% within 12 h of injection. RT-qPCR analysis revealed that upon hemolymph infection with N. circulans the expression of the antimicrobial peptide (AMP) genes, Bmdefensin-B and Bmgloverin-3, were upregulated 2.5- and 1.8-fold, respectively, whereas 1.6-fold upregulation was observed for BmToll-2 in the larval fat body. Therapeutic effects of antibiotics like tetracycline, imipenem, ceftriaxone, ampicillin, and clindamycin were observed against N. circulans in the Bombyx larvae with varying efficacies. Results from this study suggest that larvae of B. mori can be used as infection models for screening therapeutics that are effective against N. circulans.

Quantitative evaluation of Mycobacterium abscessus clinical isolate virulence using a silkworm infection model

Mycobacterium abscessus causes chronic skin infections, lung diseases, and systemic or disseminated infections. Here we investigated whether the virulence of M. abscessus clinical isolates could be evaluated by calculating the median lethal dose (LD50) in a silkworm infection model. M. abscessus subsp. abscessus cells were injected into the silkworm hemolymph. When reared at 37˚C, the silkworms died within 2 days post-infection with M. abscessus subsp. abscessus. Viable cell numbers of M. abscessus increased in the hemolymph of silkworms injected with M. abscessus. Silkworms were not killed by injections with heat-killed M. abscessus cells. The administration of clarithromycin, an antibacterial drug used to treat the infection in humans, prolonged the survival time of silkworms injected with M. abscessus. The LD50 values of 7 clinical isolates in the silkworm infection model were differed by up to 9-fold. The Mb-17 isolate, which was identified as a virulent strain in the silkworm infection model, induced more detachment of human THP-1-derived macrophages during infection than the Mb-10 isolate. These findings suggest that the silkworm M. abscessus infection model can be used to quantitatively evaluate the virulence of M. abscessus clinical isolates in a short time period.

Cryptococcus neoformans MET5 Gene is not Essential for Virulence in the Silkworm Infection Model

The involvement of the MET5 gene in virulence of Cryptococcus neoformans was examined using the silkworm Bombyx mori infection model. In the virulence assay, the met5Δ mutant showed virulence not significantly different from the wild-type strain, suggesting that the MET5 gene is not essential for full virulence of C. neoformans. The effect of silkworm hemolymph on the survival of the met5Δ mutant was also tested. The C. neoformans met5Δ strain incubated in the silkworm hemolymph for five days remained viable, suggesting that silkworm hemolymph supports survival of the met5Δ strain.

Microbial polyketides and their roles in insect virulence: from genomics to biological functions

Covering: May 1966 up to January 2022Entomopathogenic microorganisms have potential for biological control of insect pests. Their main secondary metabolites include polyketides, nonribosomal peptides, and polyketide-nonribosomal peptide (PK-NRP) hybrids. Among these secondary metabolites, polyketides have mainly been studied for structural identification, pathway engineering, and for their contributions to medicine. However, little is known about the function of polyketides in insect virulence. This review focuses on the role of bacterial and fungal polyketides, as well as PK-NRP hybrids in insect infection and killing. We also discuss gene distribution and evolutional relationships among different microbial species. Further, the role of microbial polyketides and the hybrids in modulating insect-microbial symbiosis is also explored. Understanding the mechanisms of polyketides in insect pathogenesis, how compounds moderate the host-fungus interaction, and the distribution of PKS genes across different fungi and bacteria will facilitate the discovery and development of novel polyketide-derived bio-insecticides.

The Evaluation of the Biological Effects of Melanin by Using Silkworm as a Model Animal

Melanin has been reported to have potential applications in industries such as cosmetics and food due to its anti-UV and antioxidative qualities. However, the corresponding data on its safety evaluation or biological consequences are fairly limited; such data are critical given its widespread use. The effect of different concentrations (1, 2, 3, and 4%) of melanin on growth status (larvae length and weight, cocoon weight, and morphology), the microstructure of the various tissues (fat body, silk gland, and midgut), and silk properties was studied by using the silkworm (bombyx mori) as the model organism. The weight and length of silkworm larvae fed with melanin were lower than the control, indicating that melanin appears to have a negative effect on the growth status of silkworms; however, the histophysiology analysis indicates that the cell morphologies are not changed, the XRD and FTIR spectra indicate that the secondary and crystalline structures of silks are also well preserved, and the thermogravimetric analysis and tensile test indicate that the thermal stability and mechanical properties are well maintained and even improved to some extent. Generally, it indicates that melanin has a certain inhibitory effect on the growth of silkworm larva but causes no harm to the cell microstructures or silk properties; this demonstrates that the safety of melanin as a food addictive should be considered seriously. The increase of thermal stability and mechanical properties shows that melanin may be a good chemical modifier in textile industries.

The Response of the Estrogen-Related Receptor to 20-Hydroxyecdysone in Bombyx mori: Insight Into the Function of Estrogen-Related Receptor in Insect 20-Hydroxyecdysone Signaling Pathway

Estrogen-related receptor (ERR) is an orphan nuclear receptor that was first discovered in animals, and play an important role in metabolism, development, and reproduction. Despite extensive research on the function of ERR, its transcriptional regulation mechanism remains unclear. In this study, we obtained the upstream region of Bombyx mori ERR (BmERR) and confirmed the promoter activity of this region. Interestingly, we found that 10 and 50 nM 20-hydroxyecdysone (20E) up-regulated the transcriptional activity of BmERR promoter. In addition, eight putative ecdysone response elements (EcREs) were predicted in the upstream sequence of BmERR. Based on their positions, the upstream sequence of BmERR was truncated into different fragments. Finally, an EcRE-like sequence (5′-AGTGCAGTAAACTGT-3′) was identified. Electrophoretic mobility shift assay (EMSA) and cell transfection experiments confirmed that this motif specifically binds to the complex formed between ecdysone receptor (BmEcR) and the ultraspiracle (BmUSP), a key complex in the 20E signaling pathway. Interference of BmERR or BmEcR mRNA in the embryonic cells of Bombyx mori significantly affected the expression of BmEcR and BmUSP. Overall, these results suggested that an EcRE element was identified from BmERR, and this will help understanding the detailed regulatory mechanism of ERR in insects.

Silkworm model for Bacillus anthracis infection and virulence determination

Bacillus anthracis is an obligate pathogen and a causative agent of anthrax. Its major virulence factors are plasmid-coded; however, recent studies have revealed chromosome-encoded virulence factors, indicating that the current understanding of its virulence mechanism is elusive and needs further investigation. In this study, we established a silkworm (Bombyx mori) infection model of B. anthracis. We showed that silkworms were killed by B. anthracis Sterne and cured of the infection when administered with antibiotics. We quantitatively determined the lethal dose of the bacteria that kills 50% larvae and effective doses of antibiotics that cure 50% infected larvae. Furthermore, we demonstrated that B. anthracis mutants with disruption in virulence genes such as pagA, lef, and atxA had attenuated silkworm-killing ability and reduced colonization in silkworm hemolymph. The silkworm infection model established in this study can be utilized in large-scale infection experiments to identify novel virulence determinants and develop novel therapeutic options against B. anthracis infections.

Products of Sericulture and Their Hypoglycemic Action Evaluated by Using the Silkworm, Bombyx mori (Lepidoptera: Bombycidae), as a Model

Simple Summary

The use of invertebrates as animal models is gaining attention within the scientific community due to numerous advantages during the development of the experiments, low cost of rearing, and fewer ethical problems. The well-documented biology of the silkworm (Bombyx mori) makes this insect an ideal candidate to be used in different fields of research. In this study, we demonstrated the feasibility of using the silkworm to evaluate the hypoglycemic action of various products of sericulture included in the diet after promoting glucose or sucrose-induced hyperglycemia in silkworms. The postprandial antihyperglycemic activity of fibroin, sericin, and powder made from pupae of silkworms is confirmed. These natural products are therefore ideal candidates for the prevention and treatment of diabetes, obesity, and other lifestyle-related diseases.

Abstract

Sericulture generates different natural products with potential medical applications. Silk peptides, worms, or even pupae are commonly employed in traditional Asian medicine with a wide variety of purposes, and some scientific work has been focused on their antidiabetic properties. This work evaluates the postprandial antihyperglycemic activity of fibroin, sericin, and powder made from either larvae or pupae of silkworms, and Bombyx mori L. (Lepidoptera: Bombycidae), employing the silkworm itself as an animal model. The results indicate a reduction in the glucose levels in hemolymph after sucrose or glucose-induced hyperglycemia when these products are included in the diet of the worms.

Caenorhabditis elegans as an Infection Model for Pathogenic Mold and Dimorphic Fungi: Applications and Challenges

The threat burden from pathogenic fungi is universal and increasing with alarming high mortality and morbidity rates from invasive fungal infections. Understanding the virulence factors of these fungi, screening effective antifungal agents and exploring appropriate treatment approaches in in vivo modeling organisms are vital research projects for controlling mycoses. Caenorhabditis elegans has been proven to be a valuable tool in studies of most clinically relevant dimorphic fungi, helping to identify a number of virulence factors and immune-regulators and screen effective antifungal agents without cytotoxic effects. However, little has been achieved and reported with regard to pathogenic filamentous fungi (molds) in the nematode model. In this review, we have summarized the enormous breakthrough of applying a C. elegans infection model for dimorphic fungi studies and the very few reports for filamentous fungi. We have also identified and discussed the challenges in C. elegans-mold modeling applications as well as the possible approaches to conquer these challenges from our practical knowledge in C. elegans-Aspergillus fumigatus model.

SPINK7 Recognizes Fungi and Initiates Hemocyte-Mediated Immune Defense Against Fungal Infections

Serine protease inhibitors of Kazal-type (SPINKs) were widely identified in vertebrates and invertebrates, and played regulatory roles in digestion, coagulation, and fibrinolysis. In this study, we reported the important role of SPINK7 in regulating immune defense of silkworm, Bombyx mori. SPINK7 contains three Kazal domains and has 6 conserved cysteine residues in each domain. Quantitative real-time PCR analyses revealed that SPINK7 was exclusively expressed in hemocytes and was upregulated after infection with two fungi, Saccharomyces cerevisiae and Candida albicans. Enzyme activity inhibition test showed that SPINK7 significantly inhibited the activity of proteinase K from C. albicans. Additionally, SPINK7 inhibited the growth of three fungal spores, including S. cerevisiae, C. albicans, and Beauveria bassiana. The pathogen-associated molecular patterns (PAMP) binding assays suggested that SPINK7 could bind to β-D-glucan and agglutinate B. bassiana and C. albicans. In vitro assays were performed using SPINK7-coated agarose beads, and indicated that SPINK7 promoted encapsulation and melanization of agarose beads by B. mori hemocytes. Furthermore, co-localization studies using immunofluorescence revealed that SPINK7 induced hemocytes to aggregate and entrap the fungi spores of B. bassiana and C. albicans. Our study revealed that SPINK7 could recognize fungal PAMP and induce the aggregation, melanization, and encapsulation of hemocytes, and provided valuable clues for understanding the innate immunity and cellular immunity in insects.

Genetic differences between Japan and other countries in cyp51A polymorphisms of Aspergillus fumigatus

BACKGROUND

Mutations in cyp51A gene are known as main mechanisms of azole resistance in Aspergillus fumigatus, whereas azole-susceptible strains also carry cyp51A mutations (polymorphisms). The polymorphisms found in Europe mainly consist of two combinations of mutations, that is combinations of five single-nucleotide polymorphisms (SNPs) of cyp51A, referred to as cyp51A-5SNPs, and combinations of three SNPs of cyp51A, referred to as cyp51A-3SNPs. Few studies have compared the distributions of cyp51A polymorphisms between different regions.

OBJECTIVES

The aim of this study was to investigate the regional differences of cyp51A polymorphisms.

METHODS

We compared the proportions of cyp51A polymorphisms in clinical and environmental strains isolated in various countries, and analysed the strains phylogenetically using short tandem repeats (STRs) and whole-genome sequence (WGS).

RESULTS

Among the Japanese strains, 15 out of 98 (15.3%) clinical strains and 8 out of 95 (8.4%) environmental strains had cyp51A polymorphisms. A mutation of cyp51A^N248K was the most prevalent polymorphism in both clinical (n = 14, 14.3%) and environmental strains (n = 3, 3.2%). Only one environmental strain harboured cyp51A-5SNPs, which was reported to be the most prevalent in Europe. For phylogenetic analyses using STRs and WGS, 183 and 134 strains, respectively, were employed. They showed that most of the strains with cyp51A^N248K clustered in the clades different from those of the strains with cyp51A-5SNPs and cyp51A-3SNPs as well as from those with TR₃₄ /L98H mutations.

CONCLUSIONS

This study suggests that there are genetic differences between cyp51A polymorphisms of A. fumigatus in Japan and Europe.

Development of an efficient gene-targeting system for elucidating infection mechanisms of the fungal pathogen Trichosporon asahii

Trichosporon asahii is a pathogenic fungus that causes severe, deep-seated fungal infections in neutropenic patients. Elucidating the infection mechanisms of T. asahii based on genetic studies requires a specific gene-targeting system. Here, we established an efficient gene-targeting system in a highly pathogenic T. asahii strain identified using the silkworm infection model. By comparing the pathogenicity of T. asahii clinical isolates in a silkworm infection model, T. asahii MPU129 was identified as a highly pathogenic strain. Using an Agrobacterium tumefaciens-mediated gene transfer system, we obtained a T. asahii MPU129 mutant lacking the ku70 gene, which encodes the Ku70 protein involved in the non-homologous end-joining repair of DNA double-strand breaks. The ku70 gene-deficient mutant showed higher gene-targeting efficiency than the wild-type strain for constructing a mutant lacking the cnb1 gene, which encodes the beta-subunit of calcineurin. The cnb1 gene-deficient mutant showed reduced pathogenicity against silkworms compared with the parental strain. These results suggest that an efficient gene-targeting system in a highly pathogenic T. asahii strain is a useful tool for elucidating the molecular mechanisms of T. asahii infection.

Efficacy of Posaconazole against Rhizopus oryzae Infection in Silkworm

Rhizopus oryzae causes fatal invasive mucormycosis, especially in immunocompromised patients. Posaconazole is used to treat mucormycosis caused by R. oryzae, which is resistant to fluconazole and voriconazole. We evaluated the efficacy of posaconazole against R. oryzae in vivo using a silkworm infection model at 37℃, the human body temperature. The level of pathogenicity differed among the R. oryzae isolates, and posaconazole prolonged the survival of infected silkworms. Therefore, the silkworm infection model is suitable for investigating the virulence factors of R. oryzae and developing antifungal agents for mucormycosis.

Evaluating Candida albicans biofilm formation in silkworms

Candida albicans is a pathogenic fungus that causes deep mycosis in immunocompromised patients and forms a biofilm on catheter surfaces. Here we showed that C. albicans infection of silkworms led to biofilm formation on the surface of polyurethane fibers, a catheter substrate material, while inside the silkworm body. Silkworms inserted with polyurethane fibers survived for at least 48 hours. When silkworms inserted with polyurethane fibers were subsequently infected with C. albicans, biofilm formed on the surface of the polyurethane fiber within 24 hours in the silkworm body. These results suggest that silkworms can be used to evaluate C. albicans biofilm formation.

Evaluation of Antibacterial Drugs Using Silkworms Infected by Cutibacterium acnes

Cutibacterium acnes is a causative agent of inflammatory skin diseases and systemic infections. Systemic infections caused by C. acnes are difficult to treat, and the development of a systemic infection model for C. acnes would be useful for elucidating the mechanisms of infection and searching for therapeutic agents. In this study, we established a silkworm infection model as a new experimental system to evaluate the interaction between C. acnes and the host, and the efficacy of antibacterial drugs. Silkworms infected with C. acnes died when reared at 37 °C. The dose of injected bacterial cells required to kill half of the silkworms (LD₅₀) was determined under rearing conditions at 37 °C. The viable cell number of C. acnes was increased in the hemolymph and fat body of the infected silkworms. Silkworms injected with autoclaved C. acnes cells did not die during the study period. The survival time of silkworms injected with C. acnes was prolonged by the injection of antibacterial drugs such as tetracycline and clindamycin. These findings suggest that the silkworm C. acnes infection model can be used to evaluate host toxicity caused by C. acnes and the in vivo efficacy of antimicrobial drugs.

Pharmacokinetics of voriconazole and its alteration by Candida albicans infection in silkworms

Voriconazole (VRCZ) is a triazole antifungal agent used for the treatment and prophylaxis of invasive fungal infections. Therapeutic drug monitoring of VRCZ is widely applied clinically because of the large inter-individual variability that is generally observed in VRCZ exposure. The blood levels of VRCZ are increased during an underlying inflammatory reaction, which is associated with infections. Silkworms are useful experimental animals for evaluating the pharmacokinetics and toxicity of compounds. In this study, we investigated the pharmacokinetic parameters, such as elimination half-life, clearance, and distribution volume of VRCZ using silkworms. The pharmacokinetic parameters of VRCZ were determined based on the concentrations in silkworm hemolymph after injection of VRCZ. The elimination half-life of VRCZ in silkworms was found to be similar to that observed in humans. In addition, we assessed the impact of Candida albicans infection on VRCZ concentrations in a silkworm infection model. The VRCZ concentration at 12 h after injection in the Candida albicans-infected group was significantly higher than that in the non-infected group. In the silkworm infection model, we were able to reproduce the relationship between inflammation and VRCZ blood concentrations, as observed in humans. We demonstrate that silkworms can be an effective alternative model animal for studying the pharmacokinetics of VRCZ. We also show that silkworms can be used to indicate essential infection and inflammation-based pharmacokinetic variations in VRCZ, which is usually observed in the clinic.

Direct Visualization of Fungal Burden in Filamentous Fungus-Infected Silkworms

Invasive fungal infections (IFIs) are difficult to diagnose and to treat and, despite several available antifungal drugs, cause high mortality rates. In the past decades, the incidence of IFIs has continuously increased. More recently, SARS-CoV-2-associated lethal IFIs have been reported worldwide in critically ill patients. Combating IFIs requires a more profound understanding of fungal pathogenicity to facilitate the development of novel antifungal strategies. Animal models are indispensable for studying fungal infections and to develop new antifungals. However, using mammalian animal models faces various hurdles including ethical issues and high costs, which makes large-scale infection experiments extremely challenging. To overcome these limitations, we optimized an invertebrate model and introduced a simple calcofluor white (CW) staining protocol to macroscopically and microscopically monitor disease progression in silkworms (Bombyx mori) infected with the human pathogenic filamentous fungi Aspergillus fumigatus and Lichtheimia corymbifera. This advanced silkworm A. fumigatus infection model could validate knockout mutants with either attenuated, strongly attenuated or unchanged virulence. Finally, CW staining allowed us to efficiently visualize antifungal treatment outcomes in infected silkworms. Conclusively, we here present a powerful animal model combined with a straightforward staining protocol to expedite large-scale in vivo research of fungal pathogenicity and to investigate novel antifungal candidates.

Five Silkworm 30K Proteins Are Involved in the Cellular Immunity against Fungi

Simple Summary

The molecular mechanism of 30K proteins in anti-fungal immunity remains unclear. Here, we examined the mRNA levels of 30K proteins, including BmLP1, BmLP2, BmLP3, BmLP4, and BmLP7, and found that all of these proteins were significantly upregulated after injection of pathogen-associated molecular patterns to the fifth instar larvae, implying their involvement in immune response. The binding assay results showed that only BmLP1 and BmLP4 can bind to both fungal cells and silkworm hemocytes. In vitro, the encapsulation of hemocytes on day 5 of the fifth instar larval stage was promoted by the coating of agarose beads with recombinant BmLP1 and BmLP4. Therefore, these results demonstrate that 30K proteins are involved in the cellular immunity of silkworms by acting as pattern recognition molecules to directly recruit hemocytes to the fungal surface. We believe that our study makes a significant contribution to the literature because it provides insights into the 30K-mediated cellular immunity in silkworms.

Abstract

Background: 30K proteins are a major group of nutrient storage proteins in the silkworm hemolymph. Previous studies have shown that 30K proteins are involved in the anti-fungal immunity; however, the molecular mechanism involved in this immunity remains unclear. Methods: We investigated the transcriptional expression of five 30K proteins, including BmLP1, BmLP2, BmLP3, BmLP4, and BmLP7. The five recombinant 30K proteins were expressed in an Escherichia coli expression system, and used for binding assays with fungal cells and hemocytes. Results: The transcriptional expression showed that the five 30K proteins were significantly upregulated after injection of pathogen-associated molecular patterns to the fifth instar larvae, indicating the possibility of their involvement in immune response. The binding assay showed that only BmLP1 and BmLP4 can bind to both fungal cells and silkworm hemocytes. Furthermore, we found that BmLP1-coated and BmLP4-coated agarose beads promote encapsulation of hemocytes in vitro. The hemocyte encapsulation was blocked when the BmLP1-coated beads were preincubated with BmLP1 specific polyclonal antibodies. Conclusions: These results demonstrate that 30K proteins are involved in the cellular immunity of silkworms by acting as pattern recognition molecules to directly recruit hemocytes to the fungal surface.

Facilitating Drug Discovery in Human Disease Models Using Insects

Drugs are developed through basic studies and clinical trials. In basic studies, researchers seek drug candidates using in vitro evaluation systems and subsequently examine their effectiveness in animal experiments as in vivo evaluations. Drug candidates identified in basic studies are tested to determine whether they are effective against human diseases in clinical trials. However, most drug candidates identified in in vitro evaluation systems do not show therapeutic effects in animal experiments due to pharmacokinetics and toxicity problems in the in vivo evaluations. This review outlines drug discovery using insect disease models that allow us to perform in vivo screening. Since insects have various advantages as experimental animals such as low cost for rearing and few ethical concerns, researchers can perform large-scale in vivo screening to find drug candidates. Silkworms are insects frequently used for studies of drug efficacy, pharmacokinetics, and toxicity. Based on silkworm research, I describe the benefits of using insect disease models for drug discovery. The use of insect disease models for in vivo screening is expected to facilitate drug discovery.

In Vitro or In Vivo Models, the Next Frontier for Unraveling Interactions between Malassezia spp. and Hosts. How Much Do We Know?

Malassezia is a lipid-dependent genus of yeasts known for being an important part of the skin mycobiota. These yeasts have been associated with the development of skin disorders and cataloged as a causal agent of systemic infections under specific conditions, making them opportunistic pathogens. Little is known about the host-microbe interactions of Malassezia spp., and unraveling this implies the implementation of infection models. In this mini review, we present different models that have been implemented in fungal infections studies with greater attention to Malassezia spp. infections. These models range from in vitro (cell cultures and ex vivo tissue), to in vivo (murine models, rabbits, guinea pigs, insects, nematodes, and amoebas). We additionally highlight the alternative models that reduce the use of mammals as model organisms, which have been gaining importance in the study of fungal host-microbe interactions. This is due to the fact that these systems have been shown to have reliable results, which correlate with those obtained from mammalian models. Examples of alternative models are Caenorhabditis elegans, Drosophila melanogaster, Tenebrio molitor, and Galleria mellonella. These are invertebrates that have been implemented in the study of Malassezia spp. infections in order to identify differences in virulence between Malassezia species.

A novel silkworm infection model with fluorescence imaging using transgenic Trichosporon asahii expressing eGFP

Trichosporon asahii is a pathogenic fungus that causes deep mycosis in patients with neutropenia. Establishing an experimental animal model for quantitatively evaluating pathogenicity and developing a genetic recombination technology will help to elucidate the infection mechanism of T. asahii and promote the development of antifungal drugs. Here we established a silkworm infection model with a transgenic T. asahii strain expressing eGFP. Injecting T. asahii into silkworms eventually killed the silkworms. Moreover, the administration of antifungal agents, such as amphotericin B, fluconazole, and voriconazole, prolonged the survival time of silkworms infected with T. asahii. A transgenic T. asahii strain expressing eGFP was obtained using a gene recombination method with Agrobacterium tumefaciens. The T. asahii strain expressing eGFP showed hyphal formation in the silkworm hemolymph. Both hyphal growth and the inhibition of hyphal growth by the administration of antifungal agents were quantitatively estimated by monitoring fluorescence. Our findings suggest that a silkworm infection model using T. asahii expressing eGFP is useful for evaluating both the pathogenicity of T. asahii and the efficacy of antifungal drugs.

Therapeutic Effect of Antibiotics Against Escherichia coli O157:H7 in Silk Moth Larvae Animal Model

The increasing clinical incidence of antibiotic resistance in bacteria is a major global health care issue. Rampant use of antimicrobials is one of the major reasons of the dramatic rise in antibiotic-resistant bacterial strains. Suitable animal models are required to improve our understanding of bacterial pathogenicity, evolution and search for novel antibiotics. The larvae of the silk moth (commonly called silkworm), Bombyx mori, have been used as an animal model for testing the pathogenicity of a clinically isolated strain of enterohemorrhagic Escherichia coli O157:H7 upon injection through hemolymph. Here, we show that a foodborne E. coli O157:H7 strain can kill silkworm larvae upon injection through either hemolymph (blood) or midgut. Bacterial number in the hemolymph started to increase after 3 h of injection into hemolymph, while the number of viable circulating hemocytes decreased. Administration of four well-known antibiotics into the larval hemolymph up to 100 µg per larva showed therapeutic effect with varying efficacies against E. coli O157:H7 with ceftriaxone and imipenem showing better effect. Our findings indicate that silkworm larvae can be used as an animal model to screen for novel antibiotics that are effective against E. coli O157:H7.

Polysaccharides of a fermented food, natto, suppress sucrose-induced hyperglycemia in an in vivo evaluation system and inhibit glucose uptake by human intestinal cells

Natto is a well-known traditional Japanese food produced by fermenting soybeans with Bacillus subtilis var natto. Here we found that the water-soluble viscous fraction of natto inhibits sucrose- or glucose-induced hyperglycemia in silkworms. The water-soluble viscous fraction treated with DNase I, RNase A, and proteinase K, followed by phenol extraction also suppressed sucrose-induced hyperglycemia in silkworms. The enzyme-treated polysaccharide fraction of natto inhibits glucose uptake by Caco-2 cells, human intestinal epithelial cells. These findings suggest that the polysaccharide components of natto selected on the basis of their suppressive effects on sucrose-induced hyperglycemia in silkworms inhibit glucose uptake by human intestinal cells.

Induction of signal transduction pathways related to the pathogenicity of Cryptococcus neoformans in the host environment

Cryptococcus neoformans, a human pathogenic fungus, infects immunocompromised humans and causes serious diseases such as cerebral meningitis. C. neoformans controls the expression of virulence factors in response to the host environment via various signal transduction pathways. Understanding the molecular mechanisms involved in C. neoformans infection will contribute to the development of methods to prevent and treat C. neoformans-related diseases. C. neoformans produces virulence factors, such as a polysaccharide capsule and melanin, to escape host immunity. Several proteins of C. neoformans are reported to regulate production of the virulence factors. In this review, on the basis of studies using gene-deficient mutants of C. neoformans and animal infection models, we outline the signal transduction pathways involved in the regulation of virulence factors.

Suppressive effects of whey protein hydrolysate on sucrose-induced hyperglycemia in silkworms

Silkworms are useful for evaluating substances that suppress postprandial hyperglycemia by oral administration. In this study, orally administered whey protein hydrolysate (WPH), obtained by enzymatic treatment of whey protein, suppressed sucrose-induced hyperglycemia in silkworms in a dose-dependent manner. WPH also inhibited glucose-induced hyperglycemia in silkworms. These findings suggest that WPH contains a bioactive peptide that inhibits glucose uptake from the intestinal tract and thereby suppresses sucrose-induced hyperglycemia.

Additive effects of Kothala himbutu (Salacia reticulata) extract and a lactic acid bacterium (Enterococcus faecalis YM0831) for suppression of sucrose-induced hyperglycemia in an in vivo silkworm evaluation system

Using a silkworm evaluation system, we previously evaluated various substances that suppress postprandial hyperglycemia. Enterococcus faecalis YM0831, a lactic acid bacterium that inhibits glucose uptake by the human intestinal Caco-2 cell line, exhibited hyperglycemia-suppressing effects in the silkworm system. In the present study, we found that Kothala himbutu (Salacia reticulata) extract, a traditional medicine containing α-glucosidase inhibitors, suppressed sucrose-induced hyperglycemia in the silkworm system. Moreover, combined oral administration of lactic acid bacteria YM0831 with Kothala himbutu extract had stronger suppressive effects on sucrose-induced hyperglycemia than single administration of either component. These findings suggest that the silkworm system provides a simple way to evaluate the effects of supplements on the suppression of blood glucose level induced by sucrose ingestion.

Silkworm as an experimental animal for research on fungal infections

Silkworm, Bombyx mori, has various advantages as an experimental animal, such as the low cost for rearing and fewer ethical problems. Models utilizing silkworms of infection with pathogenic bacteria have been established for identification of genes encoding virulence factors by large-scale in vivo screening. In this review, we describe recent progress in the study of silkworm infection models for elucidating the mechanisms of fungi infection. Silkworm infection models have been established for Candida albicans, Candida tropicalis, Candida glabrata and Cryptococcus neoformans, which are yeast type fungi, and Aspergillus fumigatus, Arthroderma vanbreuseghemii, Arthroderma benhamiae, Microsporum canis, Trichophyton rubrum, and Rhizopus oryzae, which are filamentous fungi. Novel genes encoding virulence factors in C. albicans and C. glabrata have been identified by using the silkworm infection models. We here outline the benefits of using silkworm infection models and a strategy for identifying the genes responsible for pathogenicity of microorganisms such as fungi. © 2019 The Authors. Microbiology and Immunology Published by The Societies and John Wiley & Sons Australia, Ltd.