Characterization and antimicrobial activity of tropical basidiomycetes fungi and their secondary metabolites

Abstract

Microbial infections remain to be a major global public health challenge. In Kenya, these infections cause significant morbidity and mortality. Their devastating effects are majorly attributed to antimicrobial drug resistance. Nonetheless, the biodiversity and ecology of tropical Basidiomycetes fungi is an untapped source of potential bioactive compounds. Therefore, the current study aimed to isolate and test crude extracts and antimicrobial compounds from Basidiomycetes against selected microbial pathogens. The fungal basidiocarps were collected from Mt. Elgon National Park forest and the spores cultured in Yeast Malt Agar media (YMG), amended with antibiotics. Subsequent sub-culturing led to the establishment of pure cultures of 12 fungal strains. Their identity was determined using phenotypic- and ITS rDNA molecular-based techniques. They belonged to genera; Inonotus, Fomitiporia, Ganoderma, Perenniporia, Favolaschia, Hexagonia, Skeletocutis, Polyporus, Antrodia and Echinochaete. Mycelial cultures were thereafter fermented and metabolites extracted using ethyl acetate. However, Fomitiporia sp (KE/16-163) was further subjected to large scale cultivation and the crude extracts fractionated and purified using HPLC (high performance liquid chromatography), owing to it’s fast growth rate and initial antagonism. Notably, five previously undescribed pregnenolone-type steroids (assigned trivial names Aethiopinolones A-E) were successfully characterized from the fungus using 1-D and 2-D NMR (nuclear magnetic resonance) and HREIMS (high resolution electron impact mass spectroscopy) data. Antimicrobial assays of fungal crude extracts and pure compounds were performed against Bacillus subtilis, Escherichia coli, Mucor plumbeus and Candida albicans. 9 of the 12 tested fungal strains displayed considerable antimicrobial activity. Notably, Echinochaete sp (KE/16-198) and Favolaschia sp (KE/16-152) demonstrated high activities against B. subtilis and C. tenuis at minimum inhibitory concentration (MIC) values of 4.69 μg/ml and <2.34 μg/ml respectively. Further, the cytotoxicity of the novel compounds obtained was determined using a panel of seven mammalian cell lines (mouse fibroblast cells, HeLa cells, epidermoid carcinoma cells, human alveolar adenocarcinoma basal epithelial cells, prostate cancer cells, ovarian cancer cells and breast cancer cells). Interestingly, compounds 1, 3, 4 and 5 exhibited moderate to high (8-19μg/ml) cytotoxic effects against most cancer cell lines whereas, compound 2 was non-cytotoxic. Thus, the results reveal that Basidiomycetes fungi are a reservoir of fungal metabolites which could be exploited as sources of natural products of pharmaceutical importance.