Diversity, antibacterial and phytotoxic activities of intestinal fungi from Epitheca bimaculata

Insect gut fungi, as specialized microorganisms, are a significant source of bioactive compounds. However, there is currently no systematic research on the diversity of gut fungi in Epitheca bimaculata and their bioactive secondary metabolites. A total of 54 strains of gut fungi were isolated and purified from the gut of E. bimaculata using 12 different isolation media. The identification results revealed that these fungal strains were distributed across seven classes (Agaricomycetes, Cystobasidiomycetes, Eurotiomycetes, Dothideomycetes, Sordariomycetes, Saccharomycetes, and Zygomycetes) in 17 genera. The dominant genera were Irpex, Cladosporium, Penicillium, Mucor, and Talaromyces, with isolation frequencies of 14.81%, 12.96%, 12.96%, 11.11%, and 9.25%, respectively. Antibacterial tests showed that six strains extracts exhibited inhibitory activity against at least one of the tested bacteria (Staphylococcus aureus, Micrococcus tetragenus, Escherichia coli, and Pseudomonas syringae pv. actinidiae). Phytotoxic tests indicated that strains QTU-39, QTU-22, QTU-9, QTU-41, QTU-37, QTU-28, and QTU-25 showed strong phytotoxic activity against Echinochloa crusgalli with the inhibition rate of exceeding 93.5%. Seven monomer compounds, including citrinin (1), emodin (2), citreorosein (3), 8-hydroxy-6-methyl-9-oxo-9 H-xanthene-1-carboxylic acid methyl ester (4), ergosterol (5), rubratoxin B (6), and erythrol (7), and two compounds, including flufuran (8) and 4-N-butylpyridine-2-carboxylic acid (9) were isolated from Penicillium sp. QTU-25 and Pestalotiopsis sp. QTU-28, respectively. Among these, compound 1 exhibited strong antibacterial activity against four pathogenic bacteria (S. aureus, M. tetragenus, E. coli, and P. syringae pv. actinidiae), with the IZD of 20.0, 18.0, 22.3, 24.1 mm, which were equal to those of positive gentamicin sulfate with the IZD of 25.7, 22.7, 27.6, 24.6 mm, respectively. Compound 9 also exhibited strong antibacterial activity against S. aureus, M. tetragenus, E. coli, and P. syringae pv. actinidiae, with the IZD of 14.3, 17.3, 13.3, and 21.1 mm, respectively. Furthermore, compounds 1 and 6 exhibited strong phytotoxic activity against E. crusgalli and Abutilon theophrasti with an inhibition rate of 97.4% and 87.4% at a concentration of 100 µg/mL, respectively. In conclusion, the fungi isolated from the gut of E. bimaculata exhibited significant microbial diversity, representing a promising natural source of antibacterial and herbicidal compounds.