الفهرس 
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Abstract
The current study was designed to investigate therapeutic trials of the most common bacterial diseases associated with mass mortalities in Oreochromis niloticus at Al-manzala fish farms in Dakahlia governorate, Egypt, during disease outbreaks in the summer season of 2019. A total number of 200 diseased and freshly dead Oreochromis niloticus (Nile tilapia) that exhibited signs of hemorrhagic septicemia and exposed to clinical, postmortem, bacteriological examinations that revealed detection of 286 bacterial isolates and all isolates were G -ve rod shape bacteria. The most prevalent species among all bacterial groups was Aeromonas spp (63.64%) with reference to histopathological alterations. Also, Molecular investigation on A. hydrophila (The highest prevalence of bacterial isolates) was carried out by PCR amplification with A. hydrophila 16s rRNA specific primer for confirmatory identification, pathogenicity test and antibiogram susceptibility of the main isolated bacterial spp. were carried out and showed that the MAR index values of identified A. hydrophila was ranged from 0.2 to 0.6 with average (0.32). Also, physiochemical analysis of water samples that collected from infected fish farms showed degree of fluctuation among various water measurements. Furthermore, we adopted ecofriendly trials by dietary supplementation of Nile tilapia with yucca extract and probiotic to control and treat the most common bacterial infection based on our results as a substitute of antibiotics. A. hydrophila showed significant improvement of CAT, SOD, GSH levels in G2, G4 and G5 compared to control G1 (P<0.05). Moreover, MDA activity exhibited significant decrease in all treated groups compared to G1. The mortality rate G4 showed no mortalitiesin G4 throughout the experiment. slight to moderate histopathological changes were recorded in internal organs tissues of G4 and G5 compared to those of G1.