Synergistic Antibacterial Activity of Ecklonia cava extract against Fish Pathogen Streptococcus parauberis

Abstract

The in vitro antibacterial activity of Ecklonia cava extracts and its synergistic effect in commercial antibiotics were tested against fish pathogens Streptococcus parauberis isolated from olive flounder Paralichthys olivaceus. In this study, disc diffusion assay and minimum inhibitory concentration (MIC) were used to describe the antibacterial activity of E. cava extracts. E. cava was fraction using different solvent-soluble extracts to further elucidate its antibacterial effect. Fraction extract of E. cava using ethyl acetate (EtOAc) and hexane (Hexane) exhibit a broad spectrum and highest inhibition among the tested seaweed species against S. parauberis. Disc diffusion assay showed potent antibacterial effect of 12 mm and 11 mm inhibition zones (EtOAc and Hexane) respectively. Minimum inhibitory concentration (MIC) exhibit desirable concentration of EtOAc and Hex extract (512 µg/mL and 256 µg/mL) to completely inhibit the growth S. parauberis. Results also revealed that tested S. parauberis strains exhibit resistance strain against oxytetracycline and erythromycin. The highest inhibition were 7.0 mm and 9.0 mm in both oxytetracycline and erythromycin, respectively. The MIC data shows that the lowest concentration to inhibit the growth of S. parauberis was ranges from 512 µg/mL to 1,024 µg/mL. The FIC index between E. cava extract and oxytetracycline were ranges from 0.302 to 0.353 which showed synergistic activity in all isolates except S. parauberis (KSP7 and KSP47) which exhibit additive activity of 0.560 and 0.703, respectively. Synergistic activity was also observed in combination of E. cava extract and erythromycin in all isolates of S. parauberis which ranges from 0.304 to 0.360. Synergism was observed at a concentration of 16 to 128 µg/mL in E. cava extract, 4 to 32 µg/mL in oxytetracycline and 4 to 64 µg/mL in erythromycin. This results benchmark the potential antibacterial properties of EtOAc and Hex extract of E. cava against pathogenic S. parauberis and appears useful in the development of combination therapy in some commercial antibiotic agents used in aquaculture. To our extent this is the first report on the efficacy of E. cava extract against fish pathogenic bacteria S. parauberis.