الفهرس 
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Abstract
A total of 300 marine fish of two different species represented as (150 tilapia zillii and 150 mugil capito) were freshly collected randomly from different markets in Ismailia governorate during different seasons, and subjected to full clinical, postmortem and bacteriological studies. The common clinical signs were darkness in skin, hemorrhage in base of fins, eyes & different parts on the body, abdominal distention, congestion in gills and increase in mucous secretion. The postmortem findings were white serous fluid in abdominal cavity and sometimes tinged with blood, pale or congested liver, kidney and spleen. This study indicated that the prevalence of bacterial pathogens among naturally infected Tilapia zillii and Mugil capito were Aeromonas hydrophila 91 isolates (39.39%), Vibrio alginolyticus 67 isolates (29%), Pseudomoas fluorescens 42 isolates (18.18%), Vibrio fluvialis 17 isolates (7.4%), Pseudomonas aeruginosa 14 (6.06%). Ciprofloxacin was found to be the most effective antibiotic against A. hydrophila while ciprofloxacin and rifampicine were more effective against P. fluorescens, also ciprofloxacin and amikacin were more effective against P. aeruginosa, while V. alginolyticus was highly sensitive to ciprofloxacin. Ciprofloxacin and nalidixic acid were more effective against V. fluvialis. Based on morphological and biochemical characteristics, a number of colonies representing all recovered Aeromonas were chosen for identification by conventional PCR and multiplex PCR. 16S rRNA gene, virulence genes (alt, ast, aer) and antimicrobial resistant genes (tetA, sul1, pse1) were found in 100% of the isolates, and act gene was not detected in any of the isolates. Results of this study indicated that polymerase chain reaction is very reliable method for identification of A. hydrophila isolates.