Modulatory Effect of Three Intestinally Isolated Dietary Probiotic Bacteria in Low Fish Meal Diets on Growth, Feed Utilization, Serum Biochemistry, Innate Immunity, Gut Health and Resistance Against Bacterial Challenge in Juvenile Olive Flounder <em>Paralichthys olivaceus</em>
Authors: Choi Wonsuk , Moniruzzaman Mohammad , Bae Jinho , Lee Seunghan , Min Taesun , Lee Seunghyung , Bai Sungchul C. 
Published: April 2024

Probiotics play an important role as immunostimulants in improving growth and health status in fish. However, little is known on the effects of probiotics when fish offered a low fish meal diet. This study evaluated the effects of three probiotic bacteria isolated from the fish intestine to replace antibiotics in a reduced fish meal diet fed to the juvenile olive flounder Paralichthys olivaceus. A basal diet without feed additives was used as a control (CON) and five other diets were formulated by supplementation with Bacillus subtilis WB60 (BSWB60) at 1×108 CFU/g, B. subtilis SJ10 (BSSJ10) at 1×108 CFU/g, Enterococcus faecium SH30 (EFSH30) at 1×107 CFU/g, amoxicillin (AMO) at 4 g/kg and oxytetracycline (OTC) at 4 g/kg of the basal diet. Triplicate groups of 25 fish averaging 12.1±0.09 g (mean±SD) were fed one of the six experimental diet groups for 8 weeks. Then, 45 fish from each group were intraperitoneally injected with the pathogenic bacteria, Edwardsiella tarda, and mortality was recorded for 14 days. At the end of the feeding trial, weight gain, specific growth rate and feed efficiency of fish fed the BSWB60 diet were significantly greater than those of fish fed the CON, OTC and AMO diets (P &amp;lt; 0.05). Furthermore, feeding efficiency and protein efficiency ratio of fish fed the BSWB60 diet were significantly greater than those of fish fed the CON, OTC and AMO diets. However, there were no significant differences in survival, hepatosomatic indices, viscerosomatic indices and condition factors among the fish fed the experimental diets. The whole-body proximate composition of the fish was unaffected by the dietary treatments. The serum biochemistry parameters, such as aspartate aminotransferase levels were significantly greater in fish fed the BSSJ10 diet than in those fed the BSWB60 diet. However, serum alanine aminotransferase, glucose and total protein levels were not significantly affected by the experimental diets. The lysozyme activity of fish fed the BSSJ10, BSWB60 and EFSH30 diets was significantly greater than that of the CON, OTC and AMO diets. The myeloperoxidase activity of fish fed the BSWB60 diet was significantly greater than that of the fish fed the CON and AMO diets. The flounder growth hormone levels of fish fed the BSSJ10, BSWB60, EFSH30 and AMO diets were significantly greater than that of the fish fed the CON diet. The interleukin 1β (IL-1β) gene expression levels in fish fed the BSSJ10 and EFSH30 diets were significantly greater than those in fish fed the CON, OTC and AMO diets. The interleukin 10 (IL-10) gene expression levels in fish fed the BSSJ10, BSWB60, EFSH30 and OTC diets were significantly greater than those of fish fed the CON and AMO diets. Intestinal histology revealed that the average villus length of fish fed the BSSJ10, BSWB60, and EFSH30 diets was significantly greater than that of fish fed the CON, OTC and AMO diets. The cumulative survival rates of fish fed the EFSH30 and BSWB60 diets were significantly greater than those of fish fed the CON diet after the 15th day of the challenge test. Overall, the results of the present study demonstrated that the supplementation of autochthonous bacteria, B. subtilis (108 CFU/g diet) or E. faecium (107 CFU/g diet) in the diet could be ideal probiotics to replace antibiotics based on growth performance, the serum biochemical composition, immune responses and disease resistance in olive flounder fed a low fish meal diet..