الفهرس 
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Abstract
Fish diseases are the result of interaction between host, pathogen and environmental conditions. The need to better understand the contribution of each element on the development of fish diseases has always been a priority to improve the fish aquaculture practices. In this work we evaluated the water parameters, bacterial fish pathogens and the potential relationships between them with special focus on the most prevalent bacterial pathogen in five different fish farms at Fayoum Governorate, Egypt. Water samples were analyzed and heavy metals levels were determined. Fish samples were examined for bacterial infections and its histopathological alterations in infected fish. Assessment of the most virulence of most prevalent bacterial strain has been studied via molecular detection of genetic determinants. The most prevalent bacterial spiecies were; Aeromonas hydrophila 18.8, A.sobria 13.2%, Ps. putida 14%, Ps.fluresnence 10% and Flavobacterium spp. 8.4% . Positive linear correlation between heavy metal levels and bacterial infections were detected with correlation coefficient r (Fe = 0.74, Zn =0.63, Ni = 0.58). Aeromonas hydrophila showed resistance to commonly used antibiotics Oxytetracyclin and Amoxicillin, however less commonly used antibiotics (trimethoprim– sulfamethoxazole, Ciprofloxacin, Kanamycin, novobiocin, and chloramphenicol) proved to be effective. PCR detection of virulence factors showed the presence of Act, Ast,Alt and Ela virulence genes in the isolated strains. Aerolysin phylogeny was studied in order to infer structural similarity between homologous toxins; it showed great similarity between toxins from Aeromonas and Vibrio. The drug discovery efforts revealed important potential natural compounds that can act as inhibitors for the aerolysin toxin that can further be studied clinically.