Assessment of antibacterial (Vibrio alginolyticus and Exiguobacterium qingdaonense) activity of seaweed collected from St. Martin’s Island

Seaweed with unique pharmacological effects including antibacterial have been increasingly studied, and assessed for the antimicrobial activities against viral and bacterial infection in aquaculture units. In this study, four seaweeds (Hypnea spinella, Padina australis, Chnoospora implexa, Sargassum carpophyllum) collected from St. Martin's Island, Bangladesh, were studied to evaluate their antibacterial activity against a gram-negative bacteria Vibrio alginolyticus, and a gram-positive bacteria Exiguobacterium qingdaonense isolated from Mud crab. Crude extracts of each seaweed were prepared using five solvents (water, methanol, ethanol, ethyl acetate, hexane). The extracts were tested for antibacterial activity against bacteria at a dose of 5 mg/disc. At different doses, six commercial antibiotics (Ampicillin, Chloramphenicol, Ciprofloxacin, Tetracycline, Erythromycin, Penicillin G) were also examined. There was no zone detected for the V. alginolyticus plates with seaweed extracts while V. alginolyticus showed susceptibility to antibiotics ciprofloxacin and tetracycline. On the other hand, there was zone detected in the E. qingdaonense plates containing the solvents methanol, ethanol, ethyl acetate, hexane, and E. qingdaonense had susceptibility to all antibiotics. Since the ethanol extract of P. australis showed the largest inhibitory zone (11 mm) for the E. qingdaonense growth, a concentration analysis ranging from 1-10 mg/disc was performed; there was a strong, positive linear relationship between inhibition zone and concentrations. The IC₅₀ of this extract was then assessed using the broth microdilution method, ranging from 2.5-0.078125 mg/ml concentrations, and 0.625 mg/ml was determined as the IC₅₀ value. The in vivo experiment of the toxicity of seaweed extracts at 2 doses (0.5 & 1 mg/ml) on brine shrimp demonstrated that average mortality rate for all extracts in our investigation was less than 50%; putting these extracts forward as applicable in crustacean aquaculture units. Altogether, the results of this study have pointed out the applicability of ethanol extract of P. australis seaweed in controlling the growth of bacterium E. qingdaonense; nevertheless, more meticulous scientific studies are essential to reveal the inhibitory effects of these seaweeds on other bacteria pathogenic to fish and shellfishes, and to explore the applicability of these seaweeds in sustainable aquaculture production.

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