Immunomodulatory effect of sodium alginate enriched diet in kelp grouper Epinephelus brneus against Streptococcus iniae.

The effect of diets containing sodium alginate at 0, 0.5, 1.0, and 2.0 g kg⁻¹ following challenge with Streptococcus iniae in kelp grouper Epinephelus bruneus were assessed with reference to survival rate and innate immune parameters such as alternative complement, lysozyme, natural haemagglutination, respiratory burst, superoxide dismutase, and phagocytic activities on week 1, 2, and 4. Fish fed with sodium alginate containing diet at 1.0 and 2.0 g kg⁻¹ after being challenged with S. iniae had higher survival rates of 75% and 60%, respectively than those fed with control diet (0 g kg⁻¹). With any enriched diet the percentage of macrophages significantly decreased from week 1-4, while the percentage of neutrophils and lymphocytes significantly increased. The alternate complement activity, natural haemagglutination, and phagocytic activities of infected fish fed with sodium alginate containing diet at 1.0 g kg⁻¹ on week 2 and 1.0 and 2.0 g kg⁻¹ diets on week 4 were significantly higher when compared to the control. The lysozyme, respiratory bursts, and superoxide dismutase activities of fish fed with enriched diets at 1.0 and 2.0 g kg⁻¹ were significantly increased on week 2 and 4. We therefore recommend that at 1.0 or 2.0 g kg⁻¹ dietary administration of sodium alginate can enhance innate immunity and disease resistance in kelp grouper against S. iniae.