Two rare quinone-type metabolites from the fungus Septofusidium berolinense and their biological activities

Bioactive natural products can be produced by almost all types of living organisms, and especially soil bacteria and fungi are rich sources of these natural products.^{1, 2} Among them, fungi are remarkable organisms with unique biochemical pathways that produce a wide range of natural products such as penicillin, cyclosporin, statins, aflatoxins, trichothecenes and ergot alkaloids.^{3, 4} In the group of microscopic fungi, fungi imperfecti, the ascomycetes and several other filamentous and endophytic fungal species are the most frequent producers. Although fungi have proven to be source of diverse secondary metabolites, only a small portion of these have been cultivated and screened for new bioactive compounds. Due to the increasing demand for new bioactive natural products, the use of fungal diversity in the search for new bioactive molecules is crucial.⁵

In our ongoing research on bioactive secondary metabolites from filamentous fungi, a fungal strain identified as Septofusidium berolinense was isolated from a soil sample that was collected from Ayvacık, Çanakkale, Turkey, in 2006. Further investigation of this strain resulted in the isolation of a new metabolite (1), and a known compound (2) reported by Ymele-Leki et al.⁶ in 2012. Herein, we report the isolation and structure elucidation of these secondary metabolites from S. berolinense. In addition, antimicrobial and cytotoxic activities of the compounds were evaluated. The compounds exhibited broad-spectrum antibacterial activity against clinical strains of methicillin-resistant Staphylococcus aureus (MRSA, ATCC 43300), Escherichia coli O157:H7 (RSKK 234), Pseudomonas aeruginosa (ATCC 27853) and vancomycin-resistant Enterococcus faecium (VREF, DSM 13590). Cytotoxicities of the isolated compounds were evaluated against five human cancer cell lines; CaCo-2, HeLa, MCF7, U87MG and PC3 as well as one non-tumorigenic cell line (Vero), using the MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2Htetrazolium bromide) assay. Compound 2 showed more potent cytotoxicity than compound 1 against the entire range of cell lines. This is the first report on bioactive secondary metabolites produced by the fungus S. berolinense.

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