Evaluation of a Dairy‐Yeast Prebiotic in the Diet of Juvenile Nile Tilapia, Oreochromis niloticus

Two feeding trials were conducted to evaluate the effects of dietary supplementation with the prebiotic GroBiotic‐A on juvenile Nile tilapia, Oreochromis niloticus. Juvenile fish in both trials received a basal diet composed of practical and semipurified ingredients or the basal diet supplemented with GroBiotic‐A at either 1 or 2% of dry weight. Trial 1 (8 wk), was conducted with juvenile fish initially averaging 4.3 g/fish in which their performance indices, nonspecific humoral and cellular immune responses, and intestinal microbiota were characterized. In that trial, only survival was significantly affected by dietary treatment with the greatest survival (98.7%) observed for fish fed the diet containing 2% GroBiotic‐A and significantly (P < 0.05) lower survival (86.9%) for fish fed the diet containing 1% GroBiotic‐A (GBA) and intermediate survival (90.7%) for fish fed the basal diet. Trial 2 (5 wk), was conducted with larger juveniles initially averaging 6.5 g/fish to provide a more in‐depth assessment of cellular immune responses, intestinal morphometric characteristics, and disease resistance of tilapia fed the three experimental diets. Fish fed the diet with 2% GroBiotic‐A exhibited significantly (P < 0.05) increased production of extracellular superoxide anion by macrophages and a distinct microbial community within the intestine compared to fish fed the other diets. No significant differences were seen in survival after controlled exposure to Streptococcus iniae although fish fed the diet with 2% GBA tended to experience reduced mortality (12.5%) compared to those fed the basal diet (35%). Thus, based on the results from both trials, supplementation of 2% GroBiotic‐A generally improved survival, altered gut microbiota, and enhanced extracellular superoxide anion production of Nile tilapia.