Research on bioactive secondary metabolite profile of Septofusidium berolinense and biotranformation of cycloartane type saponins by endophytic fungi

Natural products offer unique chemical structures with a wide variety of biological activities that have found multiple uses in many therapeutic areas. It is generally accepted that natural products are the future of drug discovery due to their unique structural diversity, specific biological interactions with molecular targets resulted from a long evolutionary process and still largely unexplored natural product resources. Filamentous fungi have been one of the most studied whole cell systems for microbial natural product isolation and biotransformation of natural products. Among them, plant associated endophytic and pathogenic fungi have received attention as biocatalysts in modification of complex natural products, due to their ability to produce special enzymes and chemicals for colonization of plant tissues. This study consists of two different parts. The first part includes isolation, structure elucidation and bioactivity screening studies of the fungus Septofusidium berolinense. Further investigation on this strain resulted in the isolation of a new (GE-01) and a known (GE-02) compound. The compounds exhibited broad-spectrum antibacterial activity against all clinical test strains [methicillin-resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa, and vancomycin-resistant Enterococcus faecium (VREF)] with MIC values in the range of 20-40 µg/ml. Cytotoxicities of the compounds were evaluated against five human cancer cell lines (CaCo-2, Hela, MCF7, U87MG and PC3) using MTT assay. GE-02 demonstrated notable cytotoxic activity against all cell lines with IC50 values in the range of 5.2-22.3 µg/ml, whereas compound GE-01 showed weak activity toward the two cell lines CaCo-2 and Hela. The second part consists of biotransformation studies performed on Astragalus cycloartanes by endophytic fungi isolated from Astragalus species. The endophytic fungi isolated from different parts of Astragalus condensatus and Astragalus angustiflorus were characterized by molecular phylogenetic analysis based on their ITS sequences. The biotransformation studies performed on cycloastragenol (CA) and its derivatives [astragenol (AG), cyclocanthogenol (SKG) and 20(27)-nor CA], which have recently become important due to their anti-aging effects and are only present in Astragalus genus, by using endophtic fungi afforded 27 biotransformed products, including 19 new metabolites. Their structures were established by the extensive use of 1D-, 2D-NMR, HRTOFMS and LC-MS analysis.