Novel Antimicrobial Cyclic Depsipeptides from a Terrestrial Streptomyces sp. 13F003

Although the need for novel antibiotics is increasing to treat multidrug-resistant pathogens, it is getting increasingly difficult to discover novel antimicrobial compounds possessing new structural entities. Investigation of unexplored sites is one of the effective ways to find novel secondary metabolites. Even though microbiological studies revealed Ulleung island, a small volcanic island in the Korean East Sea, contains richly diverse microorganisms, there are few reports on investigation of secondary metabolites from microorganisms in Ulleung island. Therefore, we isolated more than two hundred actinomycete strains from soil samples collected from Ulleung island and cultured them on a small scale. One strain Streptomyces sp. 13F003 whose EA extract exhibited the presence of the peptide molecules on LC/MS screening was chosen for further investigation. An extract of culture broth was fractionated by vacuum liquid chromatography and HPLC to yield two pure compounds. The structures of two compounds were assigned as novel cyclic depsipeptides on the basis of spectroscopic data. The absolute configurations were determined by NOESY, modified Mosher’s method, Marfey’s analysis, GITC, PGME derivatizations and Snatzke’s method. Two novel compounds exhibited antimicrobial activities against several pathogens on preliminary tests using disc diffusion assays.

Keywords : Actinomycetes, Streptomyces, Ulleung island, depsipeptides