الفهرس 
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Abstract
Edwardsiella tarda is a major waterborne pathogen, which induces various diseases in freshwater fish with the capability for zoonotic potential. This study was applied to investigate the prevalence of E. tarda in diseased Nile tilapia fish, genetic characterization of the virulence encoding genes (pvsA, qseC, cds1, and edw1 genes), biofilm formation and antibiotic susceptibility. Out of the 250 diseased Nile tilapia fish samples, the infection rate of E. tarda in the examined fish was 6% (15/250). The distribution of E. tarda among the examined fish samples was 12 (4.8%) in gills followed by liver samples 9 (3.6%), kidneys 7 (2.8%), skin and intestine 6 (2.4% each). A PCR approach based on the use of the gyrB gene that considered as a taxonomic marker for the detection of E. tarda in diseased fish, was used to confirm E. tarda isolates. In this study, gyrB gene was successfully amplified in 40 morphologically and biochemically identified isolates. Also, the results of virulotyping displayed the presence of edw1, and cds1 genes in a high percentage of 75%, and 70% respectively followed by qseC gene (42.5%), but pvsA gene was harbored by one isolate (2.5%) in E. tarda strains. DNA sequence analysis of the marker gene (gyrB) and one virulence-associated gene (edw1) from purified PCR products of one Edwardsiella tarda strain SS8 isolated from the gills of diseased Nile tilapia fish was identical to the virulence genes related to several E. tarda strains found in gene bank from different countries. These genes of E. tarda strain gyrB and edw1 were identical to several E. tarda strains with a similarity of (85.6-100%), and (96-100%) respectively, and the sequences were recorded in GenBank under the accession number of MW911830 and MW911829 for gyrB, and edw1 genes, respectively. The phylogenetic tree showed the genetic relationship with related species. Out of 40 E. tarda isolates, (82.5%) were positive for biofilm development, with 54.5% being weak, 18.2% being moderate, and 27.3% being strong biofilm producers, While 17.5% did not develop biofilms. Finally, The current study showed a remarkable resistance to ampicillin, amoxicillin, clindamycin, and cefuroxime (100% each) followed by penicillin (92.5 %), amikacin (85%), streptomycin (45%), neomycin (40%), and cefoperazone (40%). While, they were highly sensitive to ciprofloxacin (87.5%). The antimicrobial pattern of E. tarda strains under investigation revealed that the MAR index values ranged from 0.5 to 1, which might be due to massive usage, high risk of antibiotic exposure during treatment and maladministration of growth promoters.