Discovery of Novel Bactericides from Aspergillus alabamensis and Their Antibacterial Activity against Fish Pathogens

The emerging outbreak of bacterial diseases is a major challenge for the aquaculture industry. The development of new antibacterial agents from natural resources to curb fish bacterial diseases in aquaculture is becoming increasingly popular. In this study, eight new benzoic acid-containing alkaloids, asperalin A-F (1-6), asperalumazine A (7), and N-(3-acetamidopropyl)-3,4-dihydroxybenzamide (8), along with four known compounds (9-12) were isolated and identified from a seagrass-derived Aspergillus alabamensis. Their chemical structures were established on the basis of extensive spectroscopic analyses (including HRESIMS, 1D and 2D NMR spectroscopy), NMR computational methods, and electronic circular dichroism (ECD) calculations. Compounds 1-6 exhibited moderate or potent inhibitory activities against at least one fish pathogenic bacterium, among Edwardsiella ictalurid, Streptococcus iniae, and Streptococcus parauberis, and these compounds represent the first report of the coupling of dihydroquinolone alkaloids with benzoic acid derivatives. Compounds 3 and 4 showed strong activities against Staphylococcus aureus, S. iniae, and S. parauberis, with an MIC value of 10.1, 5.0, and 10.1 μM, respectively. Compound 5, an N-alkylated product of 4, exhibited the strongest inhibitory effects against S. iniae, with an MIC value of 2.2 μM. Notably, compound 6, as a new natural bactericide, showed moderate to potent inhibitory activity toward all strains tested, including one Gram-negative bacterium E. ictalurid (10.9 μM, MIC) and four Gram-positive bacteria S. iniae (43.6 μM, MIC), S. aureus (21.8 μM, MIC), S. parauberis (87.3 μM, MIC), and Bacillus subtilis (21.8 μM, MIC). Compound 7 represents the first example of a lumazine derivative directly coupled to a benzoic acid moiety by a hydroxymethyl group.