الفهرس 
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Abstract
This study was carried out on 120 of apparently healthy O. niloticus, with an average body weight (55 ± 5 g) were obtained from integrated fish farm at Al-Manzala, Al-Dakhalia Governorate, Egypt. To evaluate the effect of commercially available probiotics as bacterial mixture probiotic (AquaStar®) and autolyzed-dried Saccharomyces cerevisiae (Levabon®) as feed additives on the immune response and intestinal histology of Nile tilapia, O. niloticus. Fish were stocked in 10 glass aquaria measuring (80 Χ 50 Χ 40 cm) for two weeks as acclimatization period, during which they fed on a basal experimental diet containing 30% protein and offered to fish 2 times daily at 3% of their body weight provided in equal rations at 09:00 Am and 13:00 Pm. After the acclimatization period fish were divided into 5 groups each group contains 24 fish as two replicate per each treatment: At 2nd and 4th weeks of the experimental period two blood samples were taken from each treatment. The first one collected in EDTA tubes to prevent blood coagulation and used for determination of TLC, DLC and RBCs. The second blood sample was collected into clean and dry 1.5 ml Eppendorf tubes devoid of anticoagulant and left to clot at room temperature, then centrifugated at 3000 rpm for 10 minutes for collection of clear serum sample used for the biochemical analysis of serum total protein, albumin and globulin also for Lysozyme activity. In addition to tissue sample were taken also at the mid and end point of the experiment from the mid-intestine and immediately fixed in 10% formalin for study the intestinal morphological changes indicating the immune response of probiotics. The obtained results could be summarized in the following: The results indicated an enhanced immune response of Saccharomyces cerevisiae probiotic dietary supplementation than the bacterial mixture probiotic and the control group. This enhanced immune response were represented by the significant (P≤ 0.05) elevation of RBCs count, TLC, lymphocyte percentage, heterophils percentage, monocytes percentage, eosinophils percentage and basophiles percentage and so enhancing the cellular immune response. The dietary supplemented groups with the bacterial mixture probiotic didn’t significantly affected on the RBCs, TLC and DLC when compared with the control group during the first and the second phases of the experiment. Enhancing the humoral immune response occur through increasing the total protein, albumin, globulins and decrease A/G ratio of the blood sera of O. niloticus fed probiotics supplemented diets of bacterial mixture probiotic than the Saccharomyces cerevisiae supplemented groups and the control group. Saccharomyces cerevisiae supplemented group in a dose 1 g/kg showed significant (P≤ 0.05) increasing in total serum protein, albumin and globulin when compared to the control group but not more than bacterial mixture probiotic, also in a dose 0.5 g/kg showed increasing but near to the control group. Saccharomyces cerevisiae as probiotic in the dose 1g/ kg enhanced the humoral immune response through the significant (P≤ 0.05) increasing in the serum lysozyme activity than the commercial bacterial mixture probiotic. The bacterial mixture dietary supplemented groups showed an increasing in lysozyme concentration however the difference was not statistically significant (P≤ 0.05) from the control group in the two phases of the experiment. Fish groups fed on Saccharomyces cerevisiae supplemented diet, showed intestinal colonization of yeast cells onto the epithelium of intestinal villi with an increase in the microvilli length and density confirming the immune response. Histological examinations confirm the immune response for groups fed on bacterial mixture probiotic where it revealed modulation of the intestinal microflora and influenced the host intestinal morphology confirming the immune response.