الفهرس 
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Abstract
A total number of 300 cultured Oreochromis niloticus with an average body weight of 40±5 g was obtained from fish hatchery at fish farming and technology institute, Suez Canal University, Ismailia governorate. They were divided into ten groups each group subdivided into 2 subgroups (replicates, each 15 fish/ aquarium). Groups (I) and (VI) were fed on a basal diet (control), Groups (II) and (VII) were fed on a basal diet containing 5g Spirulina platensis/kg diet and groups (III) and (VIII) were fed on a basal diet containing 10g Spirulina platensis/kg diet and groups (IV) and (IX) were fed on diet containing 1 g S. cerevisiae /kg diet and groups (V) and (X) were fed on diet containing 1 g Bacillus subtilis /kg diet. A challenge test at 6 weeks were applied by the intraperitoneal injection of a highly virulent pathogenic Pseudomonas fluorescens strain at a dose of 0.2 ml saline with a concentration of (1.5×108 CFU/ml) and the mortalities were estimated post challenge for 7 days. Growth performance were estimated after 1 and 2 months. At the end of the experiment (8 weeks), hematological parameters. liver enzymes, total protein, kidney functions and glucose level were estimated. Fish fed on probiotics and spirulina have a higher growth rate, feed conversion ratio and specific growth rate than control group. Also, they have improved immunity, lysozymes activity, hematological parameter, total protein, kidney function, liver enzyme and glucose levels. Spirulina platensis at a dose of 10 g/kg diet gives the best results in all challenged and non-challenged groups than control ones.