Evaluation of dietary Heat-killed Lactobacillus plantarum strain L-137 supplementation on growth performance, immunity and stress resistance of Nile tilapia (Oreochromis niloticus)

Four feeding trials were carried out to investigate the effect of immunobiotic (Heat-killed Lactobacillus plantarum strain L-137-HK L-137) supplement in diets on growth, immune responses and stress resistance of Nile tilapia (Oreochromis niloticus). For the growth evaluation, three different feeding regimes allocated for tilapia juveniles including 0, 10, 20 and 50 ppm concentrations of HK L-137 for the first 50-day feeding period, and then fish were continuously fed diets with reduced HK L-137 levels of 0, 2, 4 and 4 ppm, respectively for 70 days, and finally the last feeding period prolonged for 30 days without HK L-137 in diets. In terms of the immunity test, fish were exposed to ammonium chloride and Streptococcus agalactiae to investigate the immune responses and stress resistance. Fish fed HK L-137 containing diets showed significantly higher growth compared to those fed the control diet. Phagocyte and lysozyme activity of fish fed HK L-137 diets were significantly higher than those of the control for 50-day feeding period and a similar situation obtained at day 75 for fish fed 4 ppm HK L-137-containing diets compared to those of the remaining diets (0 and 2 ppm HK L-137). Stress resistance in fish to ammonia indicated that fish fed HK L-137 supplemented diets performed a lower increment of cortisol concentration compared to those fed the control diet. Interestingly, the lowest cumulative mortality rate (78.33%) obtained in fish fed 20 ppm HK L-137 while the poorest values observed in group fed control diet (96.67%) after exposure to S. agalactiae for 15 days. The present study revealed that the appropriate doses of HK L-137 ranged from 20 to 50 ppm in diets provided the notable positive effects enhancing not only the growth performances but also immunity responses and stress resistance of Nile tilapia.