Abstract
The probiotic activity of Lactobacillus acidophilus bacteria as a dietary supplement was estimated by its effect on the immune response of Nile tilapia (Oreochromis niloticus) and its protective effect against challenge infections The present study predestined the effects of different Lactobacillus acidophilus dietary levels as feed supplement on growth performance, biochemical parameters, antioxidant activity and protection of Oreochromis niloticus against Enterococcus faecalis that previously isolated from cultured Oreochromis niloticus from Kafr Elsheikh Governorate fish farms that identified by biochemical tests and API20 E .Six types of bacteria were found, Enterococcus faecalis by 25%, Aeromonas hydrophilla by 20.9%, Pseudomonas.auriginousa. by 18.6% Staphylococcus aureus20..9%., Escherichia coli by11.6% and proteus vulgaris by 2.3% were isolated from 60 apparently healthy and naturally infected Oreochromis niloticus fish collected from Kafr El Sheikh Governorate fish farms . Fish were allotted into 5 groups, 1st and 2nd group fed on basal diet and 3rd ,4th and 5th fed on basal diet supplemented by different levels of dietary Lactobacillus acidophilus 1.5 × 108, 3 × 108 and 6 × 108 CFU /g respectively for 10 weeks. Our results showed that dietary administration of different levels of L. acidophilus improved weight gain rate, relative growth rate, average body weight and feed conversion ratio compared to fish fed unsupplemented diet. Also showed the superior results in biochemical parameters (significant increase (P < 0.05) in lysosomal activity, bactericidal activity, WBCs, antioxidant enzymes activities and significant decrease (P < 0.05) in cholesterol, cortisol and glucose and the appropriate inclusion is 6 × 108 CFU /g. It could be concluded that using Lactobacillus acidophilus good due to their high effective growth promoting, antibacterial and positive impact on innate immunity and antioxidant enzymes activities and had hypocholesteremic and hypoglycemic effect. This study highly recommends the use of Lactobacillus acidophilus as therapeutic agents in dealing with Enterococcus faecalis infection in fish culture. On our study Lactobacillus acidophilus can act as probiotics to improve immune responses ,growth, and fish welfare.

Key words: : Lactobacillus Acidophillus, growth performance, immune status, disease resistance, Nile Tilapia. : Lactobacillus Acidophillus, growth performance, immune status, disease resistance, Nile Tilapia.