Study on Secondary Metabolites and Antibacterial Activity of Endophytic Aspergillus in Nine Fragrant Insects
The secondary metabolites of microorganisms are the source of various bioactive compounds such as antibiotics, and have significant medicinal value. In recent years, they have also become a hot topic in natural medicinal chemistry research. Insect symbiotic fungi refer to fungi that have spent all or part of their life cycle in the host insect without causing significant infection symptoms in the host. They can decompose organic components to provide nutrients for the host, help the host resist foreign microbial invasion, and trigger the host’s immune response. In the long-term evolution process, fungi have developed a series of methods to adapt to the intestinal environment and form stable symbiotic relationships with their hosts. They have certain peculiarities in host selection and colonization.
Jiuxiang Bug, commonly known as “Stinky Butt Bug”, is a type of insect traditional Chinese medicine with a long history of medicinal use recorded in pharmacopoeias. It is also a delicious food in some areas. The Compendium of Materia Medica records that Jiuxiangchong is mainly used for treating stagnation of qi in the diaphragm and epigastric region, deficiency of spleen and kidney, and strengthening of yang. It can be used for kidney deficiency, impotence, soreness and weakness of the waist and knees, and bloating and pain in gastritis. Research has shown that the nourishing effect of nine spice worms may be related to their abundant vitamins, trace elements, phospholipids, amino acids, proteins, lipids, and N-acetyldopamine substances. In addition, the extract of nine spice worms has antibacterial effects on Staphylococcus aureus, Salmonella typhi, Salmonella paratyphoid A, and Shigella flexneri, and new antimicrobial peptides have also been found in their hemolymph. These antibacterial active substances may be related to the treatment of gastritis with nine spice worms. The antibacterial effect of Jiuxiang insect suggests that its endophytic fungi may also produce secondary metabolites with antibacterial activity. At present, there is a lack of relevant reports on the endophytic fungi and their metabolites of the nine spice beetle. This study selected the nine spice insect as the research object, isolated a strain of Aspergillus from its intestine, and used rice solid culture medium for large-scale fermentation. The secondary metabolites were isolated, purified, and structurally identified, and their antibacterial activity was tested in order to obtain metabolites with good antibacterial activity and enrich the chemical composition and biological activity research of insect endophytes.

 

In this study, silica gel, MCI, gel column chromatography and semi preparative liquid methods were used to separate and purify the secondary metabolites of endophytic Aspergillus septentrionalis. Six compounds were isolated from them, including one new compound and five known compounds, all of which were polyketides. According to the structural type, they can be further divided into pyranones, diphenyl ethers and butyrolactones. Compounds 1 and 2 were isolated from Aspergillus for the first time. Diphenyl ether compound 4 exhibits certain antibacterial activity, and literature also reports that such compounds have good antibacterial activity, especially with stronger halogen substitution effects. Therefore, it is possible to isolate and purify similar compounds from other parts of the bacterial extract, among which there are many compounds with outstanding activity that are worth exploring. Literature reports that quercetin (3) has good antioxidant and anti browning biological activities, which can be studied for related biological activities. Aspergillus fungi have been reported to synthesize various types of bioactive substances, but their secondary metabolites are non essential substances with relatively low yields. In the later stage, large-scale fermentation, improved culture media, induced biosynthesis, and genomic modification techniques can be used to further explore the true secondary metabolites of Aspergillus fungi, in order to obtain more good active metabolites and provide reference for the rational development and utilization of entomopathogenic fungal resources.