الفهرس 
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Abstract
The purpose of this study was conducted to:1- Isolation and identification of some strains of Enterobacteriaceae that affect fish, including tilapia, catfish and mullet species.2_ Doing sensitivity tests for some isolated strains to investigate the extent of their sensitivity to various antibiotics.3- Knowing the extent of the virulence of Edwardsilia Tarda microbe isolated from the examined samples.4- Conducting the genetic sequencing of two virulent genes of Ed. tarda and compare it with the International Gene Bank.The study was conducted on 150 fish samples (60 Nile tilapia, 60 catfish, and 30 mullet fishes) that exhibited pathological symptoms in the Qalyubia governorate and were subjected to physical and bacteriological examination in the samples of internal organs represented by liver, kidneys, gills and spleen separately.Whereas, 102 isolates were isolated from lactose fermenter strains (42, 44, and 16 in catfish, tilapia and mullet, respectively). While, 73 isolates were non-lactose- fermenter (39, 21, and 13 in catfish, tilapia and mullet, respectively).By conducting sensitivity tests to find out the effectiveness of the different antibiotics, it was found that most of the isolated bacteria were highly resistant to oxacillin, ampicillin and sulfa trimethoprim, while gentamicin and enrofloxacin were the most effective on the isolated microbes. By conducting the bacteriological and biochemical classification of the lactose fermenters, they revealed presence of Escherichia coli, Enterobacter, Klebsiella and Citrobacter species in different proportions in the examined samples. Whereas, the biochemical classification of non- lactose-fermenting isolates revealed the presence of Salmonella, Edwardsella, Proteus, and Actinobacter species in different proportions in the examined samples.Serological classification of E. coli isolates showed the presence of E. coli O159:H7, O158:H4, O63:H2, and O27:H2 in different proportions in the examined samples. Whereas, the results of the serological classification of salmonella isolates showed isolation of S. Essen, S. Enteritides, and S. Saintpaul in the samples of catfish and mullet only.The virulence tests were based on eight Ed. tarda isolates, showed the following: 1- Five isolates showed strong biofilm formation ability.2- The results of the genetic analysis of the cds1, edw1, and qseC genes showed their presence in all the tested isolates, while the pvsA gene was present in five out of eight isolates by incidence of 62.5%.3 The results of the genetic sequencing of the edwI and qseC genes were confirmed for their presence in Ed. tarda isolated from different regions and samples worldwide, compared to the International Genebank.