Silkworm fungal infection model for identification of virulence genes in pathogenic fungus and screening of novel antifungal drugs

TrCla4 promotes actin polymerization at the hyphal tip and mycelial growth in Trichophyton rubrum

IMPORTANCE

Superficial fungal infections, such as athlete's foot, affect more than 10% of the world's population and have a significant impact on quality of life. Despite the fact that treatment-resistant fungi are a concern, there are just a few antifungal drug targets accessible, as opposed to the wide range of therapeutic targets found in bacterial infections. As a result, additional alternatives are sought. In this study, we generated a PAK TrCla4 deletion strain (∆Trcla4) of Trichophyton rubrum. The ∆Trcla4 strain exhibited deficiencies in mycelial growth, hyphal morphology, and polarized actin localization at the hyphal tip. IPA-3 and FRAX486, small chemical inhibitors of mammalian PAK, were discovered to limit fungal mycelial proliferation. According to our findings, fungal PAKs are interesting therapeutic targets for the development of new antifungal medicines.

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.

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.

Panorama of Bacterial Infections Caused by Epidemic Resistant Strains

Antimicrobial resistance (AMR) represents a critical obstacle to public health worldwide, due to the high incidence of strains resistant to available antibiotic therapies. In recent years, there has been a significant increase in the prevalence of resistant epidemic strains, associated with this, public health authorities have been alarmed about a possible scenario of uncontrolled dissemination of these microorganisms and the difficulty in interrupting their transmission, as nosocomial pathogens with resistance profiles previously considered sporadic. They become frequent bacteria in the community. In addition, therapy for infections caused by these pathogens is based on broad-spectrum antibiotic therapy, which favors an increase in the tolerance of remaining bacterial cells and is commonly associated with a poor prognosis. In this review, we present the current status of epidemic strains of methicillin-resistant Staphylococcus aureus (MRSA), Vancomycin-resistant Enterococcus (VRE), MDR Mycobacterium tuberculosis, extended-spectrum β-lactamase-producing Enterobacterales (ESBL), Klebsiella pneumoniae carbapenemase (KPC), and—New Delhi Metallo-beta-lactamase-producing Pseudomonas aeruginosa (NDM).

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.

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.

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.

Establishment of a gnotobiotic silkworm model

Gnotobiotic animals are useful for investigation of the effects of specific lactic acid bacteria on individual animals. Here we report that lactic acid bacteria colonize and proliferate in the intestinal tract of germ-free silkworms. When silkworms hatching from formalin-treated eggs were reared to fifth-instar larvae with an artificial diet containing antibiotics, bacteria and fungi were not observed in their intestines. An antibiotic-free diet supplemented with viable lactic acid bacteria, such as Enterococcus faecalis 0831-07, Lactococcus lactis 11/19-B1, or Leuconostoc carnosum #7-2, was fed to the germ-free silkworms for 1 day. After feeding the larvae on a diet without lactic acid bacteria for 5 days, each type of lactic acid bacterium was found in the intestine. Moreover, an increase in the number of Enterococcus faecalis 0831-07 was observed in the intestine 2-5 days after ingestion. These findings suggest that we successfully established a method to construct a gnotobiotic silkworm model.

Galactomannan antigenemia as a biomarker for therapeutic response of invasive aspergillosis: implications for clinical trial design and patient care

INTRODUCTION

Clinical trials for invasive pulmonary aspergillosis, a potentially lethal mold infection, are complex investigations that require protracted time and extensive resources, delaying the development of new antifungal agents for this important disease. Areas covered: In this paper, the authors examine a novel approach to study invasive pulmonary aspergillosis in humans, with a focus on the potentials and pitfalls of surrogate end points such as galactomannan antigenemia to evaluate therapeutic response to novel compounds. Expert commentary: The authors believe the use of serum galactomannan as a primary end point in clinical trials may allow for development studies that could be accomplished with fewer resources. It is widely appreciated that serum galactomannan values strongly correlate with outcome of invasive aspergillosis in patients with hematologic cancer and may be applicable to other at-risk patient populations.

[A Silkworm Infection Model to Evaluate Antifungal Drugs for Cryptococcosis]

The development of effective drugs against fungal diseases involves performing infection experiments in animals to evaluate candidate therapeutic compounds. Cryptococcus neoformans is a pathogenic fungus that causes deep mycosis, resulting in respiratory illness and meningitis. Here we describe a silkworm system established to evaluate the safety and efficacy of therapeutic drugs against infection by Cryptococcus neoformans and the advantages of this system over other animal models. The silkworm assay system has two major advantages: 1) silkworms are less expensive to rear and their use is less problematic than that of mammals in terms of animal welfare, and 2) in vivo screenings for identifying candidate drugs can be easily performed using a large number of silkworms. The pharmacokinetics of compounds are consistent between silkworms and mammals. Moreover, the ED₅₀ values of antibiotics are concordant between mammalian and silkworm infection models. Furthermore, the body size of silkworms makes them easy to handle in experimental procedures compared with other invertebrate infectious experimental systems, and accurate amounts of pathogens and chemicals can be injected fairly easily. These advantages of silkworms as a host animal make them useful for screening candidate drugs for cryptococcosis.

An invertebrate infection model for evaluating anti-fungal agents against dermatophytosis

Animal models of pathogenic infection are needed to evaluate candidate compounds for the development of anti-infectious drugs. Dermatophytes are pathogenic fungi that cause several infectious diseases. We established a silkworm dermatophyte infection model to evaluate anti-fungal drugs. Injection of conidia of the dermatophyte Arthroderma vanbreuseghemii into silkworms was lethal. A. vanbreuseghemii conidia germinated in liquid culture were more potent against silkworms than non-germinated conidia. Germinated conidia of other dermatophytes, Arthroderma benhamiae, Trichophyton rubrum, and Microsporum canis, also killed silkworms. Injection of heat-treated germinated A. vanbreuseghemii conidia did not kill silkworms, suggesting that only viable fungi are virulent. Injecting terbinafine or itraconazole, oral drugs used clinically to treat dermatophytosis, into the silkworm midgut had therapeutic effects against infection with germinated A. vanbreuseghemii conidia. When silkworms were injected with A. vanbreuseghemii expressing enhanced green fluorescent protein (eGFP), mycelial growth of the fungus was observed in the fat body and midgut. Injection of terbinafine into the silkworm midgut, which corresponds to oral administration in humans, inhibited the growth of A. vanbreuseghemii expressing eGFP in the fat body. These findings suggest that the silkworm infection model with eGFP-expressing dermatophytes is useful for evaluating the therapeutic activity of orally administered anti-fungal agents against dermatophytes.

Silkworm: A Promising Model Organism in Life Science

As an important economic insect, silkworm Bombyx mori (L.) (Lepidoptera: Bombycidae) has numerous advantages in life science, such as low breeding cost, large progeny size, short generation time, and clear genetic background. Additionally, there are rich genetic resources associated with silkworms. The completion of the silkworm genome has further accelerated it to be a modern model organism in life science. Genomic studies showed that some silkworm genes are highly homologous to certain genes related to human hereditary disease and, therefore, are a candidate model for studying human disease. In this article, we provided a review of silkworm as an important model in various research areas, including human disease, screening of antimicrobial agents, environmental safety monitoring, and antitumor studies. In addition, the application potentiality of silkworm model in life sciences was discussed.