A new bacterial strain, designated 452T, was isolated from the rhizosphere soil of the mangrove Avicennia marina in China. As determined, its cell wall peptidoglycan contained LL-diaminopimelic acid; MK-9(H8) and MK-9(H6) were the major isoprenoid quinones; and iso-C16:0 (31.3%), anteiso-C15:0 (16.9%), and iso-C15:0 (12.5%) were the major cellular fatty acids (>10.0%). Phylogenetic analysis based on the 16S rRNA gene sequence revealed that strain 452T formed a distinct lineage in the clade of the genus Streptomyces, and was closely related to S. coerulescens DSM 40146T (99.6% sequence identity), S. bellus DSM 40185T (99.5%), and S. coeruleorubidus DSM 41172T (99.3%). The DNA-DNA relatedness between strain 452T and these type strains ranged between 29.3 and 42.3%. Based on the phenotypic, chemotaxonomic, and phylogenetic features, the strain 452T is considered to represent a novel species of the genus Streptomyces, for which the name Streptomyces nigra sp. nov. is proposed. The type strain is 452T (=KCTC 39960T = MCCC 1K03346T). Further, strain 452T extracts exhibited a pronounced antitumor activity against human cancer cell lines A549, HCT-116, and HepG2, but not against normal human colon cells CCD-18Co. Active substances in the fermentation broth of strain 452T were isolated by bioassay-guided analysis, and then purified using a macroporous resin, silica gel, sephadex LX-20 column, and semi-preparative high-performance liquid chromatography (HPLC). Eight proline-containing diketopiperazines, namely, cyclo(Pro-Ala), cyclo(Pro-Gly), cyclo(Pro-Phe), cyclo(Pro-Met), cyclo(Pro-Val), cyclo(Pro-Leu), cyclo(Pro-Tyr), and cyclo(L-Leu-trans-4-hydroxy-L-Pro), were identified by electrospray ionization mass spectrometry (MS) and nuclear magnetic resonance (NMR). The compounds displayed different levels of cytotoxicity. The highest cytotoxicity was exhibited by cyclo(Pro-Ala) and cyclo(Pro-Met) against A549 cells, and cyclo(Phe-Pro) and cyclo(Pro-Ala) against HCT-116 cells, with average IC50 values equal to 18.5, 27.3, 32.3, and 47.6 μg/mL, respectively. The diversity of diketopiperazines and other chemicals produced by 452T was further investigated using gas chromatography (GC)-MS and liquid chromatography (LC)-MS. The analysis revealed 16 types of metabolites with antitumor activity and 16 other types of diketopiperazines. Hence, extracts of the newly identified strain may be used a starting material for the development of antitumor agents.

The mangrove ecosystem is a rich resource for the discovery of actinomycetes with potential applications in pharmaceutical science. Besides the genus Streptomyces, Micromonospora is also a source of new bioactive agents. We screened Micromonospora from the rhizosphere soil of mangrove plants in Fujian province, China, and 51 strains were obtained. Among them, the extracts of 12 isolates inhibited the growth of human lung carcinoma A549 cells. Strain 110B exhibited better cytotoxic activity, and its bioactive constituents were investigated. Consequently, three new isoflavonoid glycosides, daidzein-4′-(2-deoxy-α-l-fucopyranoside) (1), daidzein-7-(2-deoxy-α-l-fucopyranoside) (2), and daidzein-4′,7-di-(2-deoxy-α-l-fucopyranoside) (3) were isolated from the fermentation broth of strain 110B. The structures of the new compounds were determined by spectroscopic methods, including 1D and 2D nuclear magnetic resonance (NMR) and high-resolution electrospray ionization mass spectrometry (HR-ESIMS). The result of medium-changing experiments implicated that these new compounds were microbial biotransformation products of strain M. aurantiaca 110B. The three compounds displayed moderate cytotoxic activity to the human lung carcinoma cell line A549, hepatocellular liver carcinoma cell line HepG2, and the human colon tumor cell line HCT116, whereas none of them showed antifungal or antibacterial activities.

No abstract