Antimicrobial Secondary Metabolites from the Marine-Derived Fungus Aspergillus sp. M28
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ANTIMICROBIAL SECONDARY METABOLITES FROM THE MARINE-DERIVED FUNGUS Aspergillus sp. M28
Le Thi Hong Minh,1 Nguyen Thi Hue,1 Nguyen Thuy Linh,1 Truong Bich Ngan,1,2 Vu Thi Quyen,1 Nguyen Mai Anh,1 Cao Duc Tuan,3 Chau Van Minh,1 Pham Van Cuong,1,2 and Doan Thi Mai Huong1,2*
Microbial sources have been the main contributor to discoveries of antibacterial compounds over the years. Most of these organisms come from the terrestrial environment, accounting for only a small portion of the microbial diversity. Therefore, discoveries of well-known compounds have been repeated at high frequency, considerably slowing down the drug development process. On the other hand, the marine ecosystems are less explored, and marine microorganisms have the potential to produce structurally unique bioactive compounds that cannot be found in the terrestrial ecosystems [1]. In recent years, a considerable number of secondary metabolites have been isolated from the marine-derived fungi, such as polyketides, alkaloids, peptides, lactones, terpenoids, and steroids [2]. In particular, two genera, Aspergillus and Penicillium, are the most frequently documented from the marine environment [3]. Aspergillus is well known as a plant pathogenic fungus as well as causing respiration diseases in humans. In addition, it generates multiple toxins, such as aflatoxin and ochratoxin A. However, it also plays an important role in the production of enzymes that catalyze starch, hemicellulose, cellulose, and other polysaccharides. Many Aspergillus species have been isolated recently from marine environment, and most studies in Aspergillus have focused on bioactive compounds [4]. Aspergillus spp. produces a series of secondary metabolites that are structurally diverse and significant in terms of biological activity such as anticancer, antimicrobial, antifungal, antiviral, anti-inflammatory, antioxidant, antibacterial, and cytotoxic [3–5]. In search of bioactive metabolites from marine-derived fungus, nine metabolites (1–9) were isolated from the fermentation broth of the fungus Aspergillus sp. (strain M28), which was isolated from sediment collected at a depth of 14 m from the Co To – Thanh Lan Island in Vietnam. The EtOAc extract of the M28 fermentation exhibited antimicrobial activity against three gram-positive bacteria (Enterococcus faecalis ATCC 29212, Staphylococcus aureus ATCC 25923, and Bacillus cereus ATCC 13245) and a yeast strain (Candida albicans ATCC 10231). The M28 was identified using 18S rRNA gene sequence analysis (GenBank registration code: MN272360). It displayed over 99% similarity with several 18S rRNA gene sequences registered in the GenBank database. The isolation, structural identification, and antimicrobial activity of compounds 1–9 are presented here. The compound 3,4-dihydroxy-6,7-dimethylquinoline-2-carboxylic acid (1) was isolated for the first time from a marine-derived fungus. The structures of 1–9 were identified based on their MS, NMR, and CD data and comparison with the literature as 3,4-dihydroxy-6,7-dimethylquinoline-2-carbox
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