Strain-Specific Effects of Bacillus subtilis, Enterococcus faecium, and Pediococcus pentosaceus Supplementation on Growth Performance, Immunity, and Disease Resistance in Olive Flounder (Paralichthys olivaceus)

Olive flounder (Paralichthys olivaceus), a key aquaculture species in East Asia, is prone to stress and bacterial diseases under intensive farming. Antibiotics are often used to control these problems, but their overuse promotes resistance and threatens sustainability. To provide safer alternatives, this study evaluated the strain-specific effects of dietary probiotics on growth, immunity, and disease resistance in olive flounder. A five-week feeding trial was conducted to evaluate the effects of three isolates—Bacillus subtilis, Enterococcus faecium, and Pediococcus pentosaceus—on growth, blood biochemistry, immune responses, and resistance against Edwardsiella tarda. Each strain was incorporated individually into a basal diet. After the feeding trial, probiotic supplementation improved growth performance. P. pentosaceus significantly increased final body weight and other growth indices (p < 0.05), while E. faecium yielded the lowest feed conversion ratio. Plasma glucose was markedly reduced in the E. faecium and P. pentosaceus groups (p < 0.001), whereas other biochemical indices remained stable. Phagocytic activity was significantly increased in the B. subtilis (p < 0.05) and P. pentosaceus (p < 0.01) groups, while lysozyme activity was significantly elevated in the E. faecium (p < 0.01) and P. pentosaceus (p < 0.05) groups. Following the E. tarda challenge, survival improved in all probiotic-fed groups (22.5–28.9%) compared with the control (11.5%). These findings demonstrate complementary, strain-specific benefits: P. pentosaceus enhanced growth, E. faecium improved feed efficiency and disease resistance, and B. subtilis stimulated immune responses. Validation under farm conditions and exploration of multi-strain formulations are warranted to optimize probiotic use in olive flounder aquaculture.