Abstract
Sporothrix schenckii, Sporothrix brasiliensis, and Sporothrix globosa are etiological agents of sporotrichosis, a human subcutaneous mycosis. Although the protocols to evaluate Sporothrix virulence in animal models are well described, the cell preparation before inoculation is not standardized, and several culturing media are used to grow yeast-like cells. Here, we found that carbon or nitrogen limitation during fungal cell preparation negatively impacted the ability of S. schenckii and S. brasiliensis to kill Galleria mellonella larvae, but not S. globosa. The fungal growth conditions associated with the short median survival of animals were accompanied by increased hemocyte countings, phenoloxidase activity, and cytotoxicity. The fungal growth under carbon or nitrogen limitation also affected the cell wall composition of both S. schenckii and S. brasiliensis and showed increased exposure of β-1,3-glucan at the cell surface, while those growing conditions had a minimal impact on the S. globosa wall, which had higher levels of this polysaccharide exposed on the wall regardless of the culture condition. This polysaccharide exposure was linked to the increased ability of insect hemocytes to uptake fungal cells, suggesting that this is one of the mechanisms behind the lower virulence of S. globosa or cells from the other species grown in carbon or nitrogen limitation.
Highlights
Superficial, subcutaneous, and deep-seated mycoses are a frequent human health problem worldwide, and the morbidity and mortality rates associated with systemic infections are continuously increasing, despite the availability of antifungal drugs and different treatment schemes [1,2].Sporotrichosis is a subcutaneous infection distributed worldwide that affects humans and other mammals, domestic species such as cats and less frequently dogs [3,4,5]
When the ability to kill the animal population of the same species grown in different conditions was analyzed, we found that the survival curves of animals inoculated with S. schenckii yeast-like was analyzed, we found that the survival curves of animals inoculated with S. schenckii yeast-like cells grown in YPD or brain–heart infusion (BHI) were similar to each other (p = 0.14; Figure 2A), killing faster the animal cells grown in YPD or BHI were similar to each other (p = 0.14; Figure 2A), killing faster the animal population than cells grown in YP or YNB (p < 0.05; Figure 2A)
A search within the available scientific literature found that conidia and yeast-like cells are the most frequent cell morphologies used for animal inoculation, but we only included the latter in this study, as conidia were shown to generate erratic killing curves upon inoculation in G. mellonella larvae, and the results were not comparable with those generated in mice [32]
Summary
Superficial, subcutaneous, and deep-seated mycoses are a frequent human health problem worldwide, and the morbidity and mortality rates associated with systemic infections are continuously increasing, despite the availability of antifungal drugs and different treatment schemes [1,2]. Even though many cases can be self-limited and treated with conventional antifungal therapies, some can become systemic infections with high mortality rates; in particular, if the host is immunocompromised [3,5] This disease is caused by members of the Sporothrix genus, a taxonomic group that includes environmental species as well as pathogens of insects and mammals [3,6]. S. globosa has been found as an etiological agent of human sporotrichosis, but it is a species with low molecular diversity, whose main prevalence is in Asia and to a lesser extent in America [8,12,13] Both genomic and phenotypical analyses of these species have highlighted that they have species-specific traits that might contribute to explain the pathogenicity of these species, the clinical presentation of the infections, geographical distributions, and responses to treatments [12,14,15,16,17,18]. We assessed the effect of the culture media on the fungal cell wall composition and the ability of insect hemocytes to uptake fungal cells
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