Effects of Dietary Host-Derived Bacillus–Fructo-Oligosaccharide Formulations on Growth Performance and Thermal Challenge Responses in Juvenile Olive Flounder (Paralichthys olivaceus)

This study evaluated the effects of dietary host-derived Bacillus strains combined with fructo-oligosaccharide (FOS) on growth performance, basal physiological status, intestinal morphology, and thermal stress responses in juvenile olive flounder (Paralichthys olivaceus). A total of 486 fish with an initial body weight of 7.26 ± 0.04 g were randomly distributed into 27 tanks and fed nine experimental diets for nine weeks. The diets consisted of a basal control, an FOS-only diet, three single-strain synbiotic diets containing FOS and one host-derived Bacillus strain (B. sonorensis, B. subtilis, or B. velezensis), and four multi-strain synbiotic diets containing FOS and combinations of two or three strains. Probiotics were included at 1 × 107 CFU g−1 diet, and FOS was supplemented at 5 g kg−1 diet. After the feeding trial, no significant dietary effects were observed on growth performance, somatic indices, whole-body proximate composition, plasma biochemical parameters, antioxidant enzyme activities, immune-related indicators, stress-related biomarkers, or intestinal morphology. Fish were subsequently subjected to lethal and acute high-temperature challenges to evaluate thermal stress tolerance and associated physiological responses. In the lethal temperature challenge, fish fed the multi-strain diets FOS + B. sonorensis + B. velezensis and FOS + B. sonorensis + B. subtilis + B. velezensis showed numerically higher survival than the other groups; however, these differences were not statistically significant. Following acute heat exposure, dietary treatments did not significantly affect plasma metabolites, and most heat-shock- and energy-metabolism-related genes were not differentially expressed among treatments. Hepatic AMPKβ expression showed a significant dietary treatment effect, with higher expression in the BCF and ACF groups than in the ABF group. Overall, host-derived synbiotic supplementation did not significantly enhance growth performance or basal physiological responses under the present experimental conditions. However, some multi-strain combinations showed a non-significant tendency toward higher survival under lethal thermal stress. Further studies incorporating gut microbiome profiling, metabolomic analysis, and alternative dietary conditions are required to clarify whether host-derived Bacillus–FOS synbiotic formulations can influence thermal stress resilience in olive flounder.