الفهرس 
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Abstract
A total number of 120 Nile-tilapia fish (Oreochromis niloticus) were divided into four equal groups; the first one was served as control fed on basal non-treated diet. The second group fed on diet supplemented with Bacillus subtilis (0.5 gm/kg standard fish diets) as probiotic diet. The third group fed on diet supplemented with allicin (1gm/kg standard fish diets) as prebiotic diet. While the fourth group fed on diet supplemented with mixture of Bacillus subtilis and allicin as symbiotic diet (1.5gm/kg standard fish diets). The number of Aeromonas hydrophila and Listeria monocytogenes isolated from internal and external organs of experimental fish (gills, muscles and liver) every two weeks for two months were significantly lower in all treated groups than control. The lowest record was in the diet supplemented with synbiotics. The growth performance was significantly higher in all treatments than in the control group and the highest record was in the group of synbiotic supplemented diet. The hematocrit values, nitroblue tetrazolium (NBT) and lysozyme activities were increased significantly in all treated groups than the control and the highest record was in the group of synbiotic supplemented diet. The percent level of protection among the three treated groups after challenge infection using A. hydrophila (0.5 ml of culture suspension of pathogen containing 107 bacteria ml-1) was higher than control and the highest record was in the group of synbiotic supplemented diet. The results of the present study support the use of Synbiotic (Bacillus subtilis and Allicin)at a rate (1.5gm/kg standard fish diets) supplementation as immunostimulants and growth promoter in common fish diets.