Silkworm as an experimental animal for research on fungal infections

Abstract

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.

Keywords: human pathogenic fungus; infectious disease; silkworm; virulence factor.

Figure 1

Injection of solution into silkworm. (a) A fifth instar silkworm fed with an artificial diet for one day. (b) Red ink is injected into the silkworm's hemolymph. (c) Red ink has diffused into the silkworm's hemolymph and its legs are stained red (intra‐hemolymph injection). (d) When red ink has been injected into the silkworm's intestinal tract, it stays in the intestinal tract and the silkworm's legs do not stain red (intra‐midgut injection).

Figure 2

Identification of virulence factors of pathogens using a silkworm infection model. (a) Method for evaluating avirulent mutants from a gene‐deficient mutant library. The wild type strain of a pathogen or gene‐deficient mutants are injected into silkworms and the number that survive measured. (b) Strategies for clarifying the infectious systems of pathogenic microorganisms. A gene‐deficient mutant library of a pathogenic microorganism is prepared and new genes necessary for pathogenicity to silkworms identified. Furthermore, the pathogenicity of the identified mutants against mice is confirmed. Genetic analyses of the identified pathogenic genes and biochemical analysis of the proteins (gene products) are carried out.

Figure 3

Visualization of infection in organs of silkworm using a dermatophyte expressing eGFP. (a) Method for evaluation of hyphal growth of dermatophytes in organs of silkworm by fluorescent imaging. (b) Fluorescence microscope images of midgut of silkworm infected with A. vanbreuseghemii expressing eGFP . Figure 3b was reproduced from Ishii et al., (8) with permission.