الفهرس 
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Abstract
The present study was planned for Isolation of Vibrio species from cultured marine fish collected from different localities with phenotypic and molecular characterization of isolates species using gene-based PCR amplification, Multiplex PCR using species-specific primers adopted further molecular identification. In addition, antimicrobial susceptibilities for the examined Vibrio species against some commonly used antimicrobial agents in aquaculture in Egypt, as well as virulence determinants, including trh, tdh and tlh genes, finally study pathogenicity characterization of the isolated strains. Therefore, 220 of both apparently healthy and moribund sea bass (Dicentrarchus Labrax) were collected between April 2020 to February 2021 from cultured marine fish farms in Kafr El sheikh, Behaira, and Alexandria Governorates, and Fayoum. The following results were summarized into the following:
Clinical examination of freshly dead seabass revealed that, corneal opacity, hemorrhages, and erosions on opercula, gills, around the mouth, and on the fin basis, vibrio exhibiting eye lesions including exophthalmia, corneal edema, hemorrhagic eyes, and ulceration.
The postmortem examination of infected fishes revealed that, pale gill, congested internal organs, petechial hemorrhage of viscera, enlarged congested spleen, enlarged hemorrhagic liver, and yellowish liquefactive fluid in the intestine.
Prevalence of vibrio species in diseased seabass was 1.8% (4/220). The occurrence of Vibrio alginolyticus in examined seabass was 25% from both liver and spleen while the occurrence of Vibrio parahaemolyticus was 25% in both liver and kidney.
The four biochemically identified vibrio species were identified by VITEK system.
Representative isolates were confirmed as V. parahaemolyticus V. alginolyticus by 16S rRNA gene sequence. Regarding to virulence genes, isolated V. parahaemolyticus strains harbored recA and tlh genes at molecular weight 793 and 449 bp, respectively. V. alginolyticus isolates was positive to trh at 250 bp.
V. alginolyticus isolates NW1 and NW2 with the known 16S rRNA gene sequences of Vibrio spp. isolates stored on GenBank databases.
Antimicrobial susceptibility testing of four isolates of biochemically identified V. parahaemolyticus and V. alginolyticus isolates revealed that the isolates were sensitive to chloramphenicol, ciprofloxacin, Doxycycline HCL and Ampicillin and were resistant to Amoxycillin, Amoxiclav, Nitrofurantoin and Azithromycin.
Pathogenicity of V. alginolyticus and V. parahemolyticus against seabasses revealed that vibrio alginolyticus with trh virulence gene is the most virulent with mortalities reached 100%, followed by vibrio alginolyticus with the unknown virulent gene with mortalities reached 80%. While Vibrio parahaemolyticus with different virulence genes with mortalities reached to 70 and 50 % after the end of experiment.