Chemical constituents of marine Streptomyces sp. rssa1 from the South China Sea
Marine actinomycetes are a class of valuable medicinal microbial resources that can produce a variety of bioactive substances with diverse structural types, such as macrolides, alkaloids, peptides, quinones, etc. They have great potential in the exploration of antibacterial compounds. Schinke et al. found that 69% of antibacterial compounds derived from marine bacteria between 2010 and 2015 were derived from marine actinomycetes. Due to the unique nature of the marine environment, marine actinomycetes differ from terrestrial actinomycetes in physiological, biochemical, and genetic background characteristics, making it highly promising to search for active molecules from marine actinomycetes.

In recent years, our research team has found some active molecules with novel structure from the symbiotic and epiphytic actinomycetes of marine organisms in the South China Sea. Recently, we isolated and screened a marine streptomyces sp. rssa1 strain with antibacterial activity from coral samples collected from the Xisha Islands, Hainan, China, but its antibacterial substances are not clear. In order to explore the active ingredients produced by this strain, this article first used rice solid culture medium for solid fermentation, systematically studied the chemical composition of its fermentation products, and screened its activity.

Actinobacteria are a class of valuable medicinal microorganisms, and nearly two-thirds of the natural antibiotics discovered so far are derived from actinomycetes, especially Streptomyces. This article isolated and identified 10 compounds from the metabolites of Streptomyces sp. rssa1 derived from coral. Among them, compounds 1 and 2 were tetracyclines A and B, respectively, and compound 3 was a benzonaphthoquinone compound called kanglemycin M. Activity screening showed that it had cytotoxic activity against RAW 264.7 macrophages, further enriching the natural product library of marine actinomycetes in China. Compound 4 is a diketopiperazine compound, which is a type of diketopiperazine. Li et al. reported that the compound has nematode killing activity, with a 48 hour mortality rate of 40% against the nematode (Panagrellus redivivus). This article is the first to discover compound 4 with nematode killing activity from marine Streptomyces, indicating that the bacterium may have value in the prevention and control of crop root knot nematode diseases. It was also found that the compound has weak inhibitory activity against Vibrio alginolyticus, providing clues for the development of drugs for the prevention and control of aquatic Vibrio diseases. Compounds 5-10 are common indole or benzene containing simple compounds. Among them, compound 7, first discovered by Jin et al. from the plant Salsola, has strong α – amylase inhibitory activity, with an inhibition rate of 44% at a concentration of 0.556mg/mL.

Polyene macrolides are a class of highly effective and broad-spectrum antifungal antibiotics, such as natamycin, which can inhibit mold and yeast, and are used as natural food preservatives and antibacterial additives due to their good physicochemical stability. The compounds 1 and 2 isolated in this study were identified as tetracycline A and B, respectively. Both compounds have a lactone ring skeleton composed of 25 carbon atoms, with 4 conjugated double bonds and multiple hydroxyl groups on the ring. The 15th hydroxyl group is connected to an amino sugar through a glycosidic bond. The currently discovered tetracycline producing bacteria are all Streptomyces actinomycetes, first discovered by Dornberger et al. from Streptomyces noursei, and they have strong inhibitory effects on agricultural pathogenic fungi such as alfalfa root rot fungus and rice blast fungus. However, due to the unstable chemical structure of tetracycline A and B under sunlight, they are sensitive to ultraviolet radiation, which limits their direct application in agricultural production. Zhang et al. found that tetracycline A and B have good bactericidal effects on Aspergillus flavus at lower concentrations, with the lowest bactericidal concentrations being 3.13 and 12.56 μ g/mL, respectively. Due to their low toxicity and no side effects on mammals, polyene macrolides have the potential to become antifungal agents for food and feed. This article for the first time discovered tetracycline A and tetracycline B, which are highly effective antifungal compounds, from the Streptomyces strain rssa1 derived from coral in the South China Sea. As many studies have shown that they have good application prospects in the fields of medicine and agricultural disease control, this bacterium may be an important strain resource for developing efficient, low toxicity drugs and green biocontrol agents. This discovery also provides important clues for searching for actinomycete resources with medicinal and agricultural value from tropical marine organisms in China.