Fungal endophytes: isolation, identification and assessment of bioactive potential of their natural products.

Abstract

Fungal endophytes produce a broad variety of bioactive compounds with potential to address some of the unmet human needs. Medicinal plants have an important role to play in the search for new strains of endophytes fungi, as it is possible that their beneficial characteristics are as a result of the metabolites produced by their endophytic community. However, inspite of this potential as repositories of bioactive compounds, the fungal endophytes of African medicinal plants remain largely underexplored. This thesis reports on studies that were conducted to bioprospect for endophytic fungi with antioxidant and antimicrobial activity hosted by the plants Warburgia salutaris, Annona senegalensis, Kigelia africana and Vitex payos used in Zimbabwean traditional medicine. The surface sterilization technique was used to isolate the endophytic fungi that were identified by ribosomal DNA sequencing of the nuclear ribosomal internal transcribed spacer region. Crude extracts obtained from the fermentation of the isolated endophytic fungi were screened for antimicrobial activity using the agar diffusion method and evaluated for total antioxidant activity using a commercial kit that used the single electron transfer mechanism. Fourier-transform infrared spectroscopy (FT-IR) and Gas Chromatography – Mass Spectrometry (GC-MS) were used to provide a snapshot of the metabolites present in the endophyte fungi extracts. A total of 33 endophytic fungi were isolated from the medicinal plants and the fungal endophyte colonisation rates varied by plant species and plant tissue. The isolated fungi across the different plant species and tissue types were found to be dominated by members of the phylum Ascomycota. The endophytic fungi Penicillium chloroleucon was isolated from all the plant species except for Cladosporium uredinicola and Myrothecium gramineum (both isolated from Kigelia africana) which had an inhibitory effect against Escherichia coli (ATCC1056). Whilst Epicoccum sorghinum isolated from Annona senegalensis exhibited the most potent antioxidant activity, a significant number of the screened endophytic fungi from the different plant species were also found to have some antioxidant activity. The total phenolic content was found to have a positive correlational relationship with total antioxidant activity of the screened endophytic fungi crude extracts. The endophytic fungi were shown to produce a diverse range of metabolites including phenolic and polyphenolic compounds through FT-IR and GC-MS analysis. The isolate Cladosporium uredinicola has potential as a source of antimicrobial compounds whilst the isolate Epicoccum sorghinum has potential as a source of natural antioxidant. Antioxidant activity is a common phenomenon in the studied endophytic fungi and the fungal endophytes of the medicinal plants of Zimbabwe have potential as sources of bioactive compounds. Keywords: Bioprospecting, antioxidant, antimicrobial, endophytic fungi, medicinal plants, Zimbabwe.