| Cancer, which has been the No. 2 killer, continues to be one of the most difficult life-threatening diseases to treat, but drugs derived from natural products could and would have helped to ameliorate the situation. Since the discovery of penicillin from Penicillium notatum in the 1940s, terrestrial microorganisms, especially Streptomyces, have drawn attention for their ability to produce an immense number and variety of bioactive secondary metabolites. Given that the rate of discovery of new biologically active compounds from common soil microorganisms has been falling, more recently marine microorganisms came into the focus of research as the source of several interesting new structures. The oceans, covering over 70% of earth's surface, consist of a diverse of microflora which reside in environments of extreme variations in pressure, salinity and temperature, and have developed unique metabolic and physiological capabilities that offer the potential for the production of metabolites with unique structure and significant bioactivity. Therefore, an ongoing program was carried out to screen cytotoxic compounds from marine microorganisms by our group. In this paper, 59 samples were collected from the coastal areas around Hainan Island, thereof, 1409 marine microorgansisms, including 962 bacteria, 168 fungi and 279 actinomycetes, were isolated by adopting 10 different media (IM, GS, SC, GA, Ml, YE, GN, HV, CZ and MT ) and 5 pretreating methods ( heat,.SDS, Phenol, chitin-enriching, YE-enriching). The results still showed that it would greatly facilitate the isolation of actinomycetes when adopting the pretreatments such as heat and SDS, and using the media such as GS, SC, and YE.We compare the relationship on the screening results of marine microorganisms with cytotoxicity by the MTT assay and anti-Pyricularia grisea spores assay, to ascertain if the anti-Pyricularia grisea assay can be used to screen for antitumor agents. The results showed that if the conidia assay is taken as screening model for antitumor agents in the primary screening, the false positive rate will be up to 26.26%, which is so high that will greatly increase futile workload, and more than a half of supernatant broth with bioactive antitumor activity will be leaked out. By the MTTassay, 208 strains with cytotoxicity were selected from the total of 1409 microrganisms, and among them, the positive rate of bacteria was 12.75%, actinomycetes 22.10%, fungi 14.29%.The physiological and biochemical properties of strains 050642, 060386 and 060524, discerned with strong cytotoxicity against human leukemia K562 cell line (ID50>1000), were primarily investigated, the results showed that all the 3 strains had stable heridity on producing cytotoxic substances, and also they displayed potentially significant values on the natural bioactive metabolites.From the fermentation broth of Streptomyces sp. 060524, which exhibited strong cytotoxicity, 13 compounds were isolated by silica gel chromatography, Sephadex LH-20 and C18 chromatography methods and in combination of bioassay-guided fractionation, and identified by the extensive spectroscopic analyses (MS, NMR, DEPT as well as NOED techniques). Among them, there are 5 compounds with low cytotoxicity against K562 cell line (20 jig/ml < IC5o < 50 ug/ml), such as daidzein, genistein, 3'-hydroxydaidzein, 3'-hydroxygenistein and benzaldehyde; 2 compounds with middle cytotoxicity (10 ug/ml < IC5o < 20 ug/ml ), as represented by l-H-3-indole carboxylic acid and l-H-3-indole carboxylate; 2 compounds with strong cytotoxicity (ICso |
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