Effects of Hot-Water Extract from Sargassum sp. on Antibacterial Activity, Non-specific Immunity and TBARs Production on Asian Seabass (Lates calcarifer, Bloch)

Abstract

Hot-water extract of Sargassum sp., a marine macro-algae, was examined for its effects on non-specific immune responses and lipid peroxidation (TBARs) production in Asian seabass (Lates calcarifer,Bloch) upon bacterial infection.  Antibacterial activity of the extract was tested in vitro against four common pathogens of Asian seabass, namely Streptococcus iniae, Vibrio alginolyticus, V. parahaemolyticus, and V. vulnificus, using micro-dilution broth technique. Thereafter, the in vivo humoral non-specific immune stimulatory effects and TBARs production upon S. iniae infection were investigated. Asian seabass with average weight of 167.68±34.75 g were divided into four groups (15 fish per group). Group 1 acted as the negative control, whereas Group 2 was treated with Sargassum sp. extract at a dose of 50 mg kg^-1 of fish, Group 3 was infected with S. iniae (2 x 10³ CFUfish^-1), and Group 4 received both the extract and S. iniae. At 2, 24 and 48 hours after treatment, blood was withdrawn from the caudal vein of 5 fish in each group to examine haematocrit, lysozyme and alternative complement activities, while their livers were rapidly removed to determine TBARs. No antibacterial activity of Sargassum sp. extract was observed in vitro. However, the S. iniae injection caused an increase in serum TBARs (P<0.05) in fish which did not receive the seaweed extract, indicating induced oxidative stress. Serum alternative complement and lysozyme levels in groups receiving the seaweed extract became significantly higher than those of their corresponding controls after 24 and 48 hours post-treatment, respectively. The present study indicates that the hot-water extract from Sargassum sp. Enhances non-specific immune responses and suppresses lipid peroxidation in Asian seabass.