An aqueous extract from Sargassum sp. enhances the immune response and resistance against Streptococcus iniae in the Asian sea bass (Lates calcarifer Bloch)

Abstract

The effects of the hot water extract from Sargassum sp. on the growth performance, immune responses, oxidative stress, and resistance of Asian sea bass (Lates calcarifer) to Streptococcus iniae were investigated. Four groups of fish were fed on the basal diet containing the extract at 0, 0.5, 1.0, and 2.0 g kg−1 diet for 30 days. Thereafter, the fish from each group were divided into equal halves. The first half was injected intraperitoneally with 0.85 % NaCl, and the second half was injected with S. iniae (2 × 103 cfu fish−1). At the end of the 30 days feeding trial, there were no significant differences in final body weight, weight gain, feed conversion ratio, and hepatosomatic index among four groups. Plasma protein, total immunoglobulin (Ig), and lysozyme messenger RNA (mRNA) levels in fish fed on diets containing 1.0 and 2.0 g kg−1 of Sargassum sp. extract, however, became increased in a non-dose-dependent manner. When fish were exposed to the bacteria, at 24 h, there were significantly (p < 0.05) higher levels of hematocrit, red blood cell and white blood cell, Ig, and serum lysozyme in fish fed on diet containing 2.0 g kg−1 of Sargassum sp. extract than those of the fish fed the control diet, and the highest survival rate was also observed in this group. In addition, fish receiving the seaweed extract were able to suppress lipid peroxidation especially at 24-h post S. iniae challenge. These findings thus suggested that Sargassum sp. extract can be used as an immunostimulant in Asian sea bass.