Overview on the Most Prevailing Bacterial Diseases Infecting Oreochromis niloticus at Aswan Fish Hatchery, Egypt

| Information about bacterial diseases from aquaculture systems at Aswan Governorate, Egypt is scarce and old, making the interpretation, prevention and treatment of their episodes very difficult. This study aimed to shed light on the most predominant bacterial infections in Oreochromis niloticus from Sahary fish hatchery; with special focus on isolation, identification, and characterization of pathogenic bacteria in this cultured fish. During an epizootic; from April to May 2018, one hundred clinically diseased Oreochromis niloticus were collected alive from the hatchery. Diseased fish showed hemorrhagic patches distributed all over the body, detachment of scales, fin erosions, distended abdomen and corneal opacity with exophthalmia in some cases. Collected aseptic tissue samples were cultured over different selective differential media for phenotypic characterization. Moreover, experimental infection using the most prevalent pathogenic bacteria (Aeromonas hydrophila) was conducted. Biochemical tests, revealed the presence of Aeromonas hydrophila, Aeromonas sobria, Pseudomonas fluorescens, Enterococcus faecalis, Staphylococcus aureus and Streptococcus agalactiae with prevalence of 56%, 45%, 33%, 11%, 13% and 14%, respectively. Molecular identification, antimicrobial susceptibility, multiple antibiotic resistance (MAR index) and histopathological changes induced by Aeromonas hydrophila were investigated. Aeromonas hydrophila molecularly revealed an amplified product of 356 bp size using the 16S rRNA gene and its pathogenicity is confirmed by both infectivity experiments and histopathological examination. Aeromonas hydrophila is sensitive to Gentamicin, Ciprofloxacin, Amikacin and Nalidixic acid recording average MAR index of 0.534 which has been indicative of isolates associated with a high risk of proceeding from environments with usual antimicrobials use. Aeromonas hydrophila is completely resistant to Penicillin and Erythromycin.