Bioactivity of Red Seaweed Gracilaria arcuata against Aeromonas hydrophila and Vibrio sp.

Background: New antibacterial agents are urgently needed due to the increasing number of resistance of microorganisms to available antibiotics. Seaweeds are known as a source of bioactive compounds including antibacterial agents. Our research focuses on one of abundance Indonesian's red seaweed Gracilaria arcuata. Objectives: The objectives of this study were to screen the bioactivity of G. arcuata against fish pathogenic bacteria (Aeromonas hydrophila and Vibrio sp.) and to identify antibacterial compounds. Biological activity evaluation showed inhibition against the fish pathogenic bacteria under bioautography, MIC, and MBC assay.In addition, we examined the potential of G. arcuata as an agarophyte. Methods: G. arcuata was extracted with ethyl acetate. Bioassay guided fractionation was applied to find active fraction. Fractionation was done on silica gel on column chromatography with eluents 100% chloroform to 100% methanol. Active compounds were identified using GC-MS and dereplication using MarinLit. Result: The bioautography assay showed that 3 fractions were identified with antibacterial activity and one fraction is only active against A. hydrophila. The value of MIC for extract was 2.5 μg/μl and 1.25 μg/μl for active fraction. The compounds were identified as Hexadecanoic acid and sterol compounds such as Cholest-5-en-3-ol, Stigmast-5-en-3-B-ol (24S), Ergost-5,7-dien-3-ol (3β), Ergost- 5-en-3-ol. FT-IR analysis of water fraction showed that G. arcuata is not agarophyte type. The signals of sulfated β-D-galactose and sulfated vibration (S=O and SO2) that concluded that G. arcuata to be predicted as sulfated polysaccharide. Conclusion: This research indicates a great potential of G. arcuata as an antibacterial agent and produce sulfated polysaccaride.