Fungal strain matters: colony growth and bioactivity of the European medicinal polypores Fomes fomentarius, Fomitopsis pinicola and Piptoporus betulinus

Polypores have been applied in traditional Chinese medicine up to the present day, and are becoming more and more popular worldwide. They show a wide range of bioactivities including anti-cancer, anti-inflammatory, antiviral and immuno-enhancing effects. Their secondary metabolites have been the focus of many studies, but the importance of fungal strain for bioactivity and metabolite production has not been investigated so far for these Basidiomycetes. Therefore, we screened several strains from three medicinal polypore species from traditional European medicine: Fomes fomentarius, Fomitopsis pinicola and Piptoporus betulinus. A total of 22 strains were compared concerning their growth rates, optimum growth temperatures, as well as antimicrobial and antifungal properties of ethanolic fruit body extracts. The morphological identification of strains was confirmed based on rDNA ITS phylogenetic analyses. Our results showed that species delimitation is critical due to the presence of several distinct lineages, e.g. within the Fomes fomentarius species complex. Fungal strains within one lineage showed distinct differences in optimum growth temperatures, in secondary metabolite production, and accordingly, in their bioactivities. In general, F. pinicola and P. betulinus extracts exerted distinct antibiotic activities against Bacillus subtilis and Staphylococcus aureus at minimum inhibitory concentrations (MIC) ranging from 31-125 μg mL ⁻¹; The antifungal activities of all three polypores against Aspergillus flavus, A. fumigatus, Absidia orchidis and Candida krusei were often strain-specific, ranging from 125-1000 μg mL ⁻¹. Our results highlight that a reliable species identification, followed by an extensive screening for a ‘best strain’ is an essential prerequisite for the proper identification of bioactive material.