Background. The growing resistance of microorganisms to antimicrobial agents is a pressing public health issue. Bioprospecting efforts have mainly focused on well-known environments such as soil and animal gut in search for microorganisms with antibiotic production or antimicrobial activity, or terrestrial ecosystems for endemic plants with bioactive compounds. However, microbial communities thriving in stressed environments such as hot springs, are potential sources of metabolites that can be screened for antimicrobial activity. There is a need for research on bioprospecting of fungi as potential sources of antimicrobials.

Objectives. The study aimed to test the antimicrobial activity of endophytic and rhizospheric fungi associated with soft fern (Christella spp.) and Cinderella weed (Synederella nodiflora) inhabiting a hot spring in Los Baños, Laguna, Philippines.

Methods. A total of 23 endophytic and rhizospheric fungi isolated from soft fern and Cinderella weed were purified and phenotypically identified. These isolates were subjected to agar well diffusion and agar plug diffusion methods as preliminary assays for antimicrobial activity against Bacillus subtilis var. spizizenii (ATCC® 6633™), Staphylococcus  aureus (ATCC® 25923™), four multi-antibiotic resistant Escherichia coli (OT11, OT16, OT18, OT22), and Cladosporium  cladosporioides. Based on the results of the preliminary screening, ethyl-acetate extracts of selected fungal isolates were subjected to broth microdilution assay to determine the minimum inhibitory concentrations (MICs) for antibacterial activity, as well as poisoned food technique to determine the percent mycelial inhibition for antifungal activity. The nearest phylogenetic affiliations of fungal isolates with higher antimicrobial activities were determined.

Results. Ten rhizospheric fungal isolates from Cinderella weed and seven rhizospheric and six endophytic fungal isolates from soft fern were phenotypically identified as Aspergillus, Coniothyrium, Fusarium, Penicillium, Talaromyces,  and Trichoderma species. Ethyl acetate extracts from endophytic fungal isolates UL1 (Trichoderma sp.) and UL2 (Trichoderma sp.) and rhizospheric fungal isolates UR1 (Trichoderma sp.) and UR3 (Trichoderma sp.) showed activity against the test bacteria at 128-256 ?g/mL concentrations. Isolates UL1, UL2, and UR3, which exhibited higher antibacterial activities, were sequenced and confirmed to be most phylogenetically related to Trichoderma  virens. Eleven fungal isolates belonging to Aspergillus spp., Coniothyrium spp., Fusarium spp., Penicillium spp., and Talaromyces spp. demonstrated antagonism against C. cladosporioides. The rhizospheric fungal isolate FCRU4 (Talaromyces sp.), from where ethyl acetate extracts were recovered for testing mycelial inhibition, was confirmed to be most phylogenetically related to Talaromyces islandicus.

Conclusion. Endophytic and rhizospheric fungi asso ciated with Cinderella weed (Synedrella nodiflora) and soft fern (Christella sp.) from a hot spring in Los Baños, Laguna, Philippines have antimicrobial activity.