Potential of indigenous Bacillus spp. as probiotic feed supplements in an extruded low-fish-meal diet for juvenile olive flounder, Paralichthys olivaceus

A 12-week feeding trial was designed to assess the probiotic potential of indigenousBacillus amyloliquefaciensand/orBacillus subtilissingly or in combination withBacillus licheniformisin an extruded feed for olive flounder (Paralichthys olivaceus) juveniles. A high fish meal (FM) diet (control) and a low-FM diet containing an alternative protein blend (30% FM replacement, FM30) were formulated. Three other experimental diets were prepared by inclusion ofB. amyloliquefaciens(BA),B. subtilis(BS), or a mixture ofB. amyloliquefaciens,B. subtilis, andB. licheniformis(BASL) into FM30 diet, with a final concentration of 10(6)CFU/g diet. Results indicated that the FM30 diet was well tolerated by flounder, and the overall performance was not affected by dietary treatments. Lysozyme activity and total immunoglobulin level were significantly reduced in flounders when fed with the FM30 diet compared with the BASL and BA diets, respectively. TheBacillusadditives neither enriched the relative abundance of the correspondingBacillusspp. in the relevant gut microbiota of olive flounder nor modulated the presumptive gene functions of the gut microbiome. Despite the absence of growth-promoting effect, the tested probiotics could still be economically viable for use as immunostimulants in commercial flounder diets with partial FM replacement.