The aim of this research is to explore the biological and chemical diversity of bacteria associated with a marine flatworm Paraplanocera sp., and to discover the bioactive metabolites from culturable strains. A total of 141 strains of bacteria including 45 strains of actinomycetes and 96 strains of other bacteria were isolated, identified and fermented on a small scale. Bioactive screening (antibacterial and cytotoxic activities) and chemical screening (ultra-performance liquid chromatography–mass spectrometry (UPLC-MS)) yielded several target bacterial strains. Among these strains, the ethyl acetate (EA) crude extract of Streptomyces sp. XY-FW47 fermentation broth showed strong antibacterial activity against methicillin-resistant Staphylococcus aureus ATCC43300 (MRSA ATCC43300) and potent cytotoxic effects on HeLa cells. The UPLC-MS spectral analysis of the crude extract indicated that the strain XY-FW47 could produce a series of geldanamycins (GMs). One new geldanamycin (GM) analog, 4,5-dihydro-17-O-demethylgeldanamycin (1), and three known GMs (2–4) were obtained. All of these compounds were tested for antibacterial, cytotoxic, and antifungal activities, yet only GM (3) showed potent cytotoxic (HeLa cells, EC50 = 1.12 μg/mL) and antifungal (Setosphaeria turcica MIC = 2.40 μg/mL) activities. Their structure–activity relationship (SAR) was also preliminarily discussed in this study.

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A Gram-negative, rod-shaped and slightly halophilic bacterium, strain XY-R17T, was isolated from the surface sediment of Mai Po Inner Deep Bay Ramsar Site birdwatch in Hong Kong. The 16S rRNA gene sequence of strain XY-R17T exhibited 96.5 % similarity to that of the type strain of Altererythrobacter atlanticus. Optimal growth occurred at pH 7.0-7.5, 3-8 % (w/v) NaCl and at 30 °C. The major respiratory quinone was ubiquinone-10. The principal fatty acids were summed feature 8 (C18 : 1ω7c and/or C18 : 1ω6c), summed feature 3 (C16 : 1ω7c and/or C16 : 1ω6c), 11-methyl C18 : 1ω7c and C17 : 1ω6c .The polar lipid profile consisted of the major compounds sphingoglycolipid, phosphatidylethanolamine, phosphatidylglycerol, diphosphatidylglycerol, phosphatidylcholine and two unknown lipids. The DNA G+C content was 64.7 mol%. Based on its phenotypic properties, chemotaxonomic characteristics and phylogenetic distinctiveness, strain XY-R17T should be designated as a novel species of the genus Altererythrobacter. Therefore, the name Altererythrobacter salegens sp. nov. (type strain XY-R17T=KCTC 52267T=MCCC 1K01500T) is proposed.

A Gram-positive, non-motile and coccoid strain, designated XY-FW106, was isolated from a marine flatworm identified to be Planocera sp. The 16S rRNA gene sequence of this pink organism was consistent with membership of the genus Deinococcus, with high sequence similarity to Deinococcus aetherius ST0316 (94.7%). The optimum growth temperature range of the strain XY-FW106 was found to be 25-30 °C and optimum growth occurs at pH 7.2-7.4 without NaCl. The strain XY-FW106 was found to contain unidentified glycolipids, unidentified phosphoglycolipids, unidentified phospholipids and unidentified lipids, which differed from those of closely related species. Menaquinone MK-8 was identified as the major respiratory quinone and the predominant cellular fatty acids were found to be Summed Feature 3 (C ω7c/C ω6c), C, iso-C, and Summed Feature 8 (C ω7c/C ω6c). The DNA G+C content was determined to be 70.2 mol%. The biochemical and chemotaxonomic data together suggest that the strain represents a new species for which the name Deinococcus planocerae sp. nov. is proposed. The type strain is XY-FW106 (=MCCC 1K01499=KCTC 33809).