Abstract
Over 1,000 compounds, including ecteinascidin-743 and didemnin B, have been isolated from ascidians, with most having bioactive properties such as antimicrobial, antitumor, and enzyme-inhibiting activities. In recent years, direct and indirect evidence has shown that some bioactive compounds isolated from ascidians are not produced by ascidians themselves but by their symbiotic microorganisms. Isolated culturable bacteria associated with ascidians and investigating their potential bioactivity are an important approach for discovering novel compounds. In this study, a total of 269 bacteria were isolated from the ascidian Styela clava collected from the coast of Weihai in the north of the Yellow Sea, China. Phylogenetic relationships among 183 isolates were determined using their 16S rRNA gene sequences. Isolates were tested for antimicrobial activity against seven indicator strains, and an antiproliferative activity assay was performed to test for inhibition of human hepatocellular carcinoma Bel 7402 and human cervical carcinoma HeLa cell proliferation. Our results showed that the isolates belonged to 26 genera from 18 families in four phyla (Firmicutes, Actinobacteria, Proteobacteria, and Bacteroidetes). Bacillus and Streptomyces were the most dominant genera; 146 strains had potent antimicrobial activities and inhibited at least one of the indicator strains. Crude extracts from 29 strains showed antiproliferative activity against Bel 7402 cells with IC50 values below 500 μg·mL−1, and 53 strains showed antiproliferative activity against HeLa cells, with IC50 values less than 500 μg·mL−1. Our results suggest that culturable bacteria associated with the ascidian Styela clava may be a promising source of novel bioactive compounds.
Highlights
Ascidians are sessile marine filter-feeding invertebrates belonging to the phylum Chordata
A total of 125 strains were isolated from S. clava collected from Xiaoshi Island, and another 144 strains were isolated from S. clava collected from Poyu Town
45 strains (16.73% strains) displayed activities against the human pathogenic fungus C. albicans, with two strains, HQA030 (Actinoalloteichus sp.) and HQA819 (Streptomyces sp.), both having inhibition zone diameters of more than 22 mm. These results suggested that some culturable strains associated with S. clava, especially actinobacterial strains, Table 3: Antimicrobial activities of the isolated strains associated with ascidian Styela clava
Summary
Ascidians (sea squirts) are sessile marine filter-feeding invertebrates belonging to the phylum Chordata. Studies on ascidians can be traced back to 1847 when their blood was observed to change color following exposure to air Investigation of this unusual phenomenon led to the isolation of a series of hydroquinoid compounds called tunichromes from the blood of several species of ascidians [1]. It was not until 1974 that the first ascidian-derived bioactive metabolite, geranyl hydroquinone, was isolated from an Aplidium species. Over 1,000 compounds have been isolated from ascidians, the majority of which have bioactive properties that include antimicrobial, antitumor, and antimalarial activities [3]. Some of these compounds have been used as clinical drugs and preclinical leads [4]
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