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Metabolites from the Fungal Endophyte Aspergillus austroafricanus in Axenic Culture and in Fungal–Bacterial Mixed Cultures

Ebrahim, Weaam, El-Neketi, Mona, Lewald, Laura-Isabell, Orfali, Raha S., Lin, Wenhan, Rehberg, Nidja, Kalscheuer, Rainer, Daletos, Georgios, Proksch, Peter
Journal of natural products 2016 v.79 no.4 pp. 914-922
Aspergillus, Bacillus subtilis, Eichhornia crassipes, Staphylococcus aureus, Streptomyces lividans, antibacterial properties, anticarcinogenic activity, aquatic plants, axenic culture, chromatography, cytotoxicity, derivatization, diphenyl ethers, endophytes, ethyl acetate, fungi, leaves, lymphoma, median effective concentration, metabolites, mice, minimum inhibitory concentration, mixed culture, nuclear magnetic resonance spectroscopy, rice, sesquiterpenoids, xanthone
The endophytic fungus Aspergillus austroafricanus isolated from leaves of the aquatic plant Eichhornia crassipes was fermented axenically on solid rice medium as well as in mixed cultures with Bacillus subtilis or with Streptomyces lividans. Chromatographic analysis of EtOAc extract of axenic cultures afforded two new metabolites, namely, the xanthone dimer austradixanthone (1) and the sesquiterpene (+)-austrosene (2), along with five known compounds (3–7). Austradixanthone (1) represents the first highly oxygenated heterodimeric xanthone derivative. When A. austroafricanus was grown in mixed cultures with B. subtilis or with S. lividans, several diphenyl ethers (8–11) including the new austramide (8) were induced up to 29-fold. The structures of new compounds were unambiguously elucidated using 1D- and 2D-NMR spectroscopy, HRESIMS, and chemical derivatization. Compound 7 exhibited weak cytotoxicity against the murine lymphoma L5178Y cell line (EC₅₀ is 12.6 μM). In addition, compounds 9 and 10, which were enhanced in mixed fungal/bacterial cultures, proved to be active against Staphylococcus aureus (ATCC 700699) with minimal inhibitory concentrations (MICs) of 25 μM each (6.6 μg/mL), whereas compound 11 revealed moderate antibacterial activity against B. subtilis 168 trpC2 with an MIC value of 34.8 μM (8 μg/mL).