Studies on Antibacterial Secondary Metabolites from Marine-Derived Streptomyces sp. MS101

Abstract

Microorganisms such as fungi and bacteria secrete antibiotics to kill of inhibit the growth of other microorganisms. Genus Streptomyces, belonging to Gram-positive bacteria, comprises the main portion of microorganism which produces natural antibiotics. To investigate bioactive compounds from marine actinomycetes, 373 isolates were obtained from near-shore sediment sample collected in Micronesia in December 2011. A marine actinomycete with antibacterial activity was adopted for this study. By 16S rRNA gene sequence analysis, this strain was identified as a novel strain of Streptomyces genus. It named as Streptomyces sp. MS101. The isolate, labeled as strain MS101, was cultured in 6 normal actinomycete liquid media. Significant antibacterial activity was observed in GTYB medium. Therefore, Streptomyces sp. MS101 was fermented in large scale (98 L) in GTYB media at 28℃ with shaking at 130 rpm for 2 weeks. The culture broth was concentrated, extracted with MeOH, and sequentially fractionated with organic solvents. From the EtOAc fraction, compound 1-4 were isolated by variety chromatographic methods, such as flash column chromatography, open column chromatography, and high performance liquid chromatography. Structure elucidation of compound 1-4 was achieved by mass spectrometry and nuclear magnetic resonance spectroscopy. On the basis of the combined spectroscopic analysis, compound 1-4 were elucidated as Benzenacetamide, Sch 538415, Deoxynybomycin, and Cyclo(L-Pro-L-Phe), respectively. These compounds were already reported previously by other researchers. To determine the bioactivity of the purified compounds, minimum inhibitory concentration (MIC) assay and enzyme inhibition assay were performed. In the antimicrobial activity assays, compounds 2 and 3 showed significant antibacterial activities against both Gram-positive and Gram-negative bacteria with MIC values in the range from 0.20 to 6.25 μg/mL. However, compound 1-4 did not show antifungal activity. Inhibitory activity toward several target enzymes related to antimicrobial activities (isocitrate lyase, sortase A) was evaluated. Compounds 1, 3, and 4 have weak inhibitory activities to isocitrate lyase. Compound 2 has weak inhibitory activities to sortase A of Streptococcus mutans. These findings indicate that the relatively under investigated marine-derived Streptomyces have great potential for the discovery of bioactive small molecules.

Keywords: marine actinomycete, Streptomyces, natural product, antibacterial activity, enzyme inhibition

Student Number: 2012-23379