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

Silkworm model of bacterial infection facilitates the identification of lysocin E, a potent, ultra-rapid bactericidal antibiotic

The development of novel antimicrobial agents is required to solve the problem of antimicrobial resistance. We established a quantitative method for evaluating the therapeutic efficacy of antimicrobial agents in a silkworm bacterial infection model. Pharmacokinetic factors are present in the silkworm as well as in mice, and evaluating the therapeutic efficacy of antimicrobial agents is possible in a silkworm infection model, comparable to that in a mammalian model. This silkworm model was used to screen for novel antimicrobial agents with therapeutic efficacy as an indicator. As a result, a new antibiotic, lysocin E, was discovered. Lysocin E has a completely different mechanism of action from existing antimicrobial agents, and its potent bactericidal activity leads to remarkable therapeutic efficacy in a mouse model. In this review, I describe the features of the silkworm model that have contributed to the discovery of lysocin E and its mechanisms of action.

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.

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.

Trichosporon asahii PLA2 Gene Enhances Drug Resistance to Azoles by Improving Drug Efflux and Biofilm Formation

Trichosporon asahii is an opportunistic pathogen that can cause severe or even fatal infections in patients with low immune function. sPLA2 plays different roles in different fungi and is also related to fungal drug resistance. However, the mechanism underlying its drug resistance to azoles has not yet been reported in T. asahii. Therefore, we investigated the drug resistance of T. asahii PLA2 (TaPLA2) by constructing overexpressing mutant strains (TaPLA2^OE). TaPLA2^OE was generated by homologous recombination of the recombinant vector pEGFP-N1-TaPLA2, induced by the CMV promoter, with Agrobacterium tumefaciens. The structure of the protein was found to be typical of sPLA2, and it belongs to the phospholipase A2_3 superfamily. TaPLA2^OE enhanced antifungal drug resistance by upregulating the expression of effector genes and increasing the number of arthrospores to promote biofilm formation. TaPLA2^OE was highly sensitive to sodium dodecyl sulfate and Congo red, indicating impaired cell wall integrity due to downregulation of chitin synthesis or degradation genes, which can indirectly affect fungal resistance. In conclusion, TaPLA2 overexpression enhanced the resistance to azoles of T. asahii by enhancing drug efflux and biofilm formation and upregulating HOG-MAPK pathway genes; therefore, it has promising research prospects.

Agrobacterium tumefaciens-Mediated Gene Transfer in a Major Human Skin Commensal Fungus, Malassezia globosa

Although the fungal microbiome in human skin mainly comprises lipophilic yeasts, including Malassezia species, these microorganisms can cause various dermatitis conditions, including pityriasis versicolor, seborrheic dermatitis, folliculitis, and atopic dermatitis, depending on the host condition. Both Malassezia globosa and Malassezia restricta are major species implicated in Malassezia-related dermatitis. However, the pathogenicity of these microorganisms has not been revealed at the genetic level owing to the lack of a genetic recombination system. Therefore, we developed a gene recombination system for M. globosa using Agrobacterium tumefaciens-mediated gene transfer of the target gene FKB1, which encodes the FKBP12 protein that binds the calcineurin inhibitor tacrolimus. The wild-type strain of M. globosa was sensitive to tacrolimus, whereas the FKB1 deletion mutant was resistant to tacrolimus. Reintroduction of FKB1 into the FKB1 deletion mutant restored wild-type levels of susceptibility to tacrolimus. Moreover, an FKB1-eGFP fusion gene was generated and expression of this fusion protein was observed in the cytoplasm. This newly developed gene recombination system for M. globosa will help further our understanding of the pathogenesis of M. globosa-related dermatitis at the genetic level.

A critical role of calcineurin in stress responses, hyphal formation, and virulence of the pathogenic fungus Trichosporon asahii

Trichosporon asahii is a conditional pathogenic fungus that causes severe and sometimes fatal infections in immunocompromised patients. While calcineurin, an essential component of a calcium-dependent signaling pathway, is known to regulate stress resistance and virulence of some pathogenic fungi, its role in T. asahii has not been investigated. Here, we demonstrated that calcineurin gene-deficient T. asahii mutants are sensitive to high temperature as well as cell-membrane and cell-wall stress, and exhibit decreased hyphal formation and virulence against silkworms. Growth of T. asahii mutants deficient in genes encoding subunits of calcineurin, cna1 and cnb1, was delayed at 40 °C. The cna1 and cnb1 gene-deficient mutants also showed sensitivity to sodium dodecyl sulfate, Congo red, dithiothreitol, and tunicamycin. On the other hand, these mutants exhibited no sensitivity to caffeine, sorbitol, monensin, CaCl₂, LiCl, NaCl, amphotericin B, fluconazole, or voriconazole. The ratio of hyphal formation in the cna1 and cnb1 gene-deficient mutants was decreased. Moreover, the virulence of the cna1 and cnb1 gene-deficient mutants against silkworms was attenuated. These phenotypes were restored by re-introducing each respective gene into the gene-deficient mutants. Our findings suggest that calcineurin has a role in regulating the cellular stress response and virulence of T. asahii.

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.

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.

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.