Antibacterial compounds in the intestinal fungus Alternaria tenuissima SCSIO41701 of tilapia
The tilapia streptococcal disease caused by Streptococcus agalatae and Streptococcus iniae has caused serious economic losses to the tilapia farming industry. At present, tilapia farming mainly uses antibiotics and other chemical drugs to prevent and treat streptococcal disease, but prolonged use of broad-spectrum antibiotics can induce drug-resistant strains.
Alternaria fungi are widely present in nature, with over 300 identified species of saprophytic, endophytic, and pathogenic fungi. Many structurally novel active compounds have been discovered in fungi of the genus Alternaria, mainly including pyranone, terpenes, sterols, aromatic polyketones, anthraquinones, alkaloids, peptides, etc. Some compounds have biological activities such as enzyme inhibition, cytotoxicity, antifungal, antioxidant, and promotion of root growth in monocotyledonous and dicotyledonous plants. Among them, Alternarian toxins such as Alternariol, Alternariol methyl ether, and tentoxin have significant toxicity. Alternaria tenuissima is a common environmental fungus related to A There is relatively little research on the secondary metabolites of tenuissima. In our study, Pan et al. obtained A. from deep-sea sediment sources Novel spirocyclic hydrogenated anthraquinone and nitrogen-containing anthraquinone compounds were isolated from tenuissima DEFSCOS013.
In order to search for new antibiotics, we used Streptococcus agalactiae and Streptococcus iniae as indicator bacteria to screen a batch of fungi isolated and identified from the intestinal tract of tilapia for antibacterial activity. Among them, we screened a strain of A. tenuissima SCSIO41701, which has significant inhibitory activity on the growth of Streptococcus agalactiae and Streptococcus iniae. This study used activity tracking method to investigate the active compounds in the fermentation products of A. tenuissima SCSIO41701 rice medium that inhibit the growth of Streptococcus agalactiae and Streptococcus iniae in tilapia

In this study, we identified through antibacterial activity tracking that the main active compound in the fungus A. tenuissima SCSIO41701 that inhibits the growth of Streptococcus agalactiae and Streptococcus iniae in tilapia is high content aternariol (1). Through HPLC analysis and thin-layer plate analysis, we found that components Fr.6~Fr.10 contain a large amount of compounds 1 and 2, with compound 1 being the main peak. It is speculated that the components Fr.6~Fr.10 with better antibacterial activity mainly exert antibacterial effects through compound 1. The reason for the cross distribution of compounds 1 and 2 in these components is due to the high content of compounds 1 and 2 being slightly soluble in the dichloromethane methanol solvent system used for washing when the crude extract is separated by a normal phase silica gel column. Compounds 1-6 all belong to the class of benzopyranolide compounds. Comparing their structures and anti streptococcal activity against tilapia, it is speculated that the presence of a conjugated system between the hydroxyl group on the C-5 benzene ring and C7~C10a plays an important role in their antibacterial activity.
Compounds 1 and 2 are common streptomycin toxins that can be detected in foods such as grains, fruits, rapeseed, sunflower seeds, olives, etc. Compounds 1 and 2 both have cytotoxic activity and can induce genetic mutations, leading to genotoxic breaks of DNA strands in vitro and inhibiting topoisomerases I and II α. Additionally, compound 2 can induce mitochondrial mutations in human colon cancer cells. In addition, compound 1 has good activity in inhibiting Staphylococcus aureus, Streptococcus lactiae, Escherichia coli, and Pseudomonas aeruginosa, with MIC values of 8.82 μ g/mL. It also strongly inhibits the activity of plant pathogenic fungi such as apple rot, watermelon wilt, tomato gray mold, and pepper phytophthora, with MIC values of 15.63, 7.82, 1.96, and 3.91 μ g/mL; Compound 2 also has antibacterial effects on Bacillus subtilis, Pseudomonas aeruginosa, Ralstonia lactis, Pseudomonas aeruginosa, and Pseudomonas aeruginosa, with MIC50 values of 28.83, 34.29, 27.08, 29.21, and 30.06 μ g/mL, respectively. In this study, we found for the first time that compound 1 has strong activity in killing tilapia pathogenic bacteria Streptococcus agalactiae and Streptococcus dolphin, and has a high yield in strain A. tenuissima SCSIO41701. However, considering the strong toxicity of this compound as a fungal toxin, further research is needed to determine whether it can be developed for the prevention and treatment of tilapia streptococcal disease.