Diversity and Anti-Infectious Components of Cultivable Rhizosphere Fungi Derived from Three Species of Astragalus Plants in Northwestern Yunnan, China

Astragalus, a group of legume plants, has a pronounced rhizosphere effect. Many species ofAstragaluswith limited resource reserves are distributed in the high-altitude area of northern Yunnan, China. Although some of these plants have high medicinal value, the recognition of them is still at a low level. The aim of this research is to explore the species diversity of cultivable rhizofungi derived fromAstragalus acaulis,A. forrestiiandA. ernestiigrowing in a special high–cold environment of northwest Yunnan and discover anti-infective components from these fungi. A total of 93 fungal strains belonging to 38 species in 18 genera were isolated and identified. Antibacterial and antimalarial screening yielded 10 target strains. Among them, the ethyl acetate crude extract of the fermented substrate of the rhizofungusAspergillus calidoustusAA12 derived from the plantA. acaulisshowed broad-spectrum antibacterial activity and the best antimalarial activity. Further chemical investigation led to the first discovery of seven compounds from the speciesA. calidoustus, including sesterterpine 6-epi-ophiobolin G; three sesquiterpenes, penicisochroman A, pergillin and 7-methyl-2-(1-methylethylethlidene)-furo [3,2-H]isoquinoline-3-one; and three polyketides, trypacidin, 1,2-seco-trypacidin and questin. Among them, the compound 6-epi-ophiobolin G exhibited moderate to strong antibacterial activity against six Gram-positive pathogens with the minimum inhibitory concentration (MIC) ranging from 25 to 6.25 μg/mL and a prominent inhibitory effect on the biofilm ofStreptococcus agalactiaeat an MIC value of 3.125 μg/mL. This compound also displayed potent antimalarial activity againstPlasmodium falciparumstrains 3D7 and chloroquine-resistant Dd2 at the half-maximal inhibitory concentration (IC50) values of 3.319 and 4.340 µmol/L at 72 h, respectively. This study contributed to our understanding of the cultivable rhizofungi from characteristicAstragalusplants in special high–cold environments and further increased the library of fungi available for natural anti-infectious product screening.