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Amphibians (kingdom: Animalia, phylum: Chordata) are ectothermic tetrapod vertebrates that can live both in water and on land. As an essential component of our ecosystem, they serve humanity ecologically, medically and culturally. Despite their importance, their population has declined dramatically over the past three decades. One of the main reasons for the alarming population decline is chytridiomycosis. The causative agents of such deadly mycosis are chytrid fungi, mainly Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). However, some amphibians are not affected by the disease and this phenomenon is attributed to the presence of certain bacteria (anti-Bd bacteria) found in the skin microbiome of amphibians. Therefore, the present work deals with chemical aspects that could play a significant role during both the fungal infection and the bacterial defense processes.
To achieve this goal, the secondary metabolites produced by the attacking chytrid fungi, on the one hand, and by the defending bacteria of the skin microbiome, on the other hand, were intensively studied. These analyses were mainly performed by genomic-, mass spectroscopic- and NMR-spectroscopic-based methods. The chemical-physical experiments were further complemented with biological assays (of the strains, crude extracts and pure substances), in which the brine shrimp toxicity, antifungal and chitinase assays were decisive.
Genomic and metabolomic analyses of the fungus Bd revealed that it has a low potential to produce secondary metabolites. In agreement with this observation, the corresponding crude extract was also found to be inactive in the brine shrimp toxicity assay. These results indicate that secondary metabolites obviously play only a minor role in the infection process. In contrast, numerous secondary metabolites were successfully isolated from the four investigated anti-Bd bacteria. In total, six novel lipopeptides and four already known lipopeptides (pseudodesmin A, tensin, virginiafactin A and B) were fully characterized. In the bioassays, particularly pseudodesmin A, tensin and virginiafactin B showed moderate antifungal activity (MIC: 12.5 - 50 μg/mL) against both chytrid fungi. Seven other anti-Bd bacteria of the genera Stenotrophomonas, Janthinobacterium, Duganella, and Paenibacillus showed the ability to produce chitinases. This suggests that anti-Bd bacteria may use a combination of enzymatic and chemical "weapons" for the defense against chytrid fungi. |
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