Evaluations of Bacillus spp. as dietary additives on growth performance, innate immunity and disease resistance of olive flounder (Paralichthys olivaceus) against Streptococcus iniae and as water additives

• Supplemental effects of Bacillus spp. were investigated in diets for olive flounder.
• B. subtilis supplementation significantly enhanced fish growth performance.
• Innate immunity of fish was improved by administration of probiotics.
• B. subtilis and B. licheniformis enhanced disease resistance of olive flounder.
• B. subtilis reduced water ammonia concentration and increased fish survival.

Two studies were conducted to investigate the effects of dietary supplementation of three Bacillus spp. on growth performance, innate immunity and disease resistance of olive flounder against Streptococcus iniae and to examine the efficacy of Bacillus spp. on water quality reducing ammonia concentration and increasing fish survival. In the first experiment, a basal diet was prepared and regarded as a control and three other diets were prepared by supplementing one of three Bacillus spp. (B. subtilis, B. pumilus or B. licheniformis) at the dietary level of 0.5%. Triplicate groups of fish (initial body weight, 25 g) were fed one of the four diets for eight weeks. Fish fed with B. subtilis containing diet showed significantly higher growth performance and feed efficiency than fish fed with the control diet. Also, significantly higher survival rates were observed in fish fed B. subtilis or B. pumilus containing diets during the feeding trial. Phagocytic activity was significantly increased in fish fed the probiotic containing diets and higher superoxide dismutase activity was observed in fish fed B. pumilus or B. licheniformis when compared to the control. However, serum lysozyme and myeloperoxidase activities did not significantly differ compared to the control. After a challenge with S. iniae, fish fed B. subtilis or B. licheniformis showed significantly higher survival rates compared to fish fed the control diet. In the second experiment, fish (30.1 g) were fed a commercial diet for three weeks and then they were distributed to nine tanks at a density of 30 fish per tank (3 replicates per treatment). Fish were fasted for 5 days in static water system, while one of two probiotics, B. subtilis or B. licheniformis was treated every 24 h into the rearing water at the concentration of 104 CFU ml− 1. During the trial, significantly decreased ammonia concentration and fish mortality were observed by the supplementation of B. subtilis into the rearing water. The results in these studies indicate that B. subtilis has beneficial effects on growth performance, innate immunity and disease resistance of olive flounder and quality of the rearing water.