الفهرس 
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Abstract
Sequencing of virulence gene(s) of Aeromonas hydrophila isolated from diseased fish Aeromonas hydrophila 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 A. hydrophila in diseased Nile tilapia fish, genetic characterization of the virulence encoding genes (act, aerA, alt, and ast genes), and antibiotic susceptibility. Out of the 500 diseased Nile tilapia fish samples, 70% (350/500) Aeromonas species were isolated. from which 53.4% (187/350) of Aeromonas hydrophila strains identified. The prevalence of A. hydrophila strains in organs; kidneys were the dominant affected organ (11.6%), followed by liver (9.8%), spleen (7%), intestine (5.4%), and gills (3.6%). The results of virulotyping displayed the presence of act, and aerA genes in a high percentage of 40%, followed by alt gene (30%), but ast gene was not detected (0%) in A. hydrophila strains. DNA sequence analysis of virulence-associated genes (act, aerA, and alt genes) of one A. hydrophila strain was identical to the virulence genes related to several A. hydrophila strains found in different countries. These virulence associated-genes of A. hydrophila strain act, aerA, and alt genes were identical to several A. hydrophila strains with a similarity of 91.4%, 93.1%, and 97.7%, respectively, and the sequences were recorded in GenBank under the accession number (MW086870, MW086871, and MW086872) for act, aerA, and alt genes, respectively. The phylogenetic tree showed the genetic relationship with related species. Finally, The current study showed high resistance of A. hydrophila strains to chloramphenicol 67.4% , followed by amikacin 51.9% and gentamicin 47.1% with intermediate resistance against ceftriaxone 32.6%, followed by tetracycline 28.9%, and cefotaxime 28.3%, however high susceptibility was displayed toward meropenem 90.9%, followed by ciprofloxacin 84.2%, amoxicillin-clavulanic acid 73.3% and trimethoprim-sulfamethoxazole 64.2%. The multidrug-resistant A. hydrophila strains observed in 69.0% of strains with six resistance patterns. Furthermore, the present investigation showed that the existence of virulence determinants (act, aerA, alt) in different A. hydrophila strains exhibited various antimicrobial resistance patterns. The antimicrobial pattern of A. hydrophila strains under investigation revealed that the MAR index values ranged from 0.2-0.8, which might be due to massive usage, high risk of antibiotic exposure during treatment and maladministration of growth promoters.