In Vitro Antibacterial Activity of Eisenia bicyclis Extract against Fish Pathogenic Bacteria

Abstract

The present study discovered antibacterial activity of the Phaeophyceae Eisenia bicyclis methanolic extract and its organic soluble fractions against Gram-negatives fish pathogenic bacteria such as Edwardsella tarda, Photobacterium demselae and Vibrio harveyi. Extract showed a significant bactericidal effect when tested by standardized disc diffusion assay, minimum inhibitory concentration (MIC), and fractional inhibitory concentration (FIC). E. bicyclis was found to be a source of an alternative medicine and potency regenerator for erythromycin (ERY) as a non-effective antibiotic widely used in aquaculture. The largest diameter of the inhibition zone was 12 mm at 1 mg/disc and 20 mm at 5 mg/disc by ethyl acetate soluble fraction (EtOAC) and MIC ranged from 128 to 1,024 µg/mL. Furthermore, the combination of old fashion erythromycin and EtOAc lowered MIC dosage of the resistant strains and resulted in a synergistic way. Contrary, the combination of oxytetracycline (OTC) and EtOAc mainly resulted in antagonism. The present findings suggest that EtOAc of E. bicyclis restore the antibacterial activity of erythromycin, a 50S inhibitor antibiotic, and it can be providing a clue to fighting to spread antibiotic resistance in aquaculture.