Synergistic Effects of Fructooligosaccharide and Bacillus subtilis Dietary Supplementation on Growth, Innate Immune Responses, and Protection against Streptococcus agalactiae in Juvenile Nile Tilapia (Oreochromis niloticus)

T he primary objective of this study was to examine the impact of combining fructooligosaccharide (FOS) and Bacillus subtilis as the synbiotic effect on the growth, innate immunity functions, and protection against Streptococcus agalactiae in tilapia (Oreochromis niloticus). During the 56-day feeding trial, tilapia with an average weight of 24.5 ± 1.6 g were fed four different types of diets, including a control diet (G1) and three other diets (G2, G3, G4) supplemented with varying levels of fructooligosaccharides (FOS) and Bacillus subtilis at doses of 1 g FOS /kg feed + 1x109 CFU B. subtilis /g feed (G2), 3 g FOS /kg feed + 3x109 CFU B. subtilis /g feed (G3), and 5 g FOS /kg feed + 5x109 CFU B. subtilis /g feed (G4), respectively. Quantitative Reverse Transcription PCR (qRT-PCR) was utilized to quantify the expression levels of immune-related genes in the liver tissue of tilapia at the end of the feeding trial. The immune-related genes evaluated comprised complement C3, IL-1β, IL-8, TNF-α, IFN-γ, and hsp70. Following the experimental feeding, tilapia were intraperitoneally injected with S. agalactiae. The results showed that the synbiotic feeding did not significantly impact the growth performances or survival of the fish (p > 0.05). However, tilapia fed with G3 and G4 diets exhibited significantly higher lysozyme and respiratory burst activities (p < 0.05). Tilapia fed with synbiotic additives showed significant modulation of immune-related genes and slightly higher survival rates after the challenge with S. agalactiae compared to the control group. These findings suggest that synbiotic supplementation may enhance the innate immune responses of tilapia against S. agalactiae infection.