الفهرس 
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Abstract
S. agalactiae is highly invasive pathogen which has the ability to infect a broad range of hosts causing neonatal meningitis and sepsis in human, mastitis in cattle and septicemia in fish. Streptococcosis caused by S. agalactiae has become an important cause of massive fish mortality in many marine and fresh water fish species up to 50-70% especially in intensive system resulting in several economic losses in aquaculture industries globally. S. agalactiae is the main pathogen of tilapia associated with fatal outcomes in tilapia production. In this study, 300 hundred of diseased Nile tilapia fish were collected from different fish farms (100) and retailed shops (200) located at Dakahlia and Damietta Governorates, Egypt, for microbiological analysis. The suspected isolates of S. agalactiae were confirmed using Polymerase Chain Reaction (PCR) as it is the most accurate, sensitive and fast method. In this study, 21/300 (7%) isolates of S. agalactiae were recorded, 13/100 (13%) of isolates from farms and 8/200 (4%) from markets. In our study, three virulence genes were determined using multiplex PCR to predict the virulence of S. agalactiae isolates. the fbsA gene was detected in (100%) of isolates, cfb gene in (76%) of isolates and pbp1A/ponA gene in (52%) of isolates. Sequencing of S. agalactiae strains was applied by Elim biopharmaceuticals (USA). The nucleo¬tide sequence of the S. agalactiae strains was deposited in GenBank under accession no. OL335944. In this study, the sensitivity of S. agalactiae isolates against ten antimicrobial agents showed that the isolates were highly resistant to ampicillin (95%), erythromycin (95%), trimethoprim/sulfamethoxazole (76%), cefotaxime (76%), ceftriaxone (72%), tetracycline (66%). The isolates were highly susceptible to imipenem (90%), gentamicin (38%). The isolates were intermediate susceptible to ciprofloxacin (43%). MDR was observed in 90.5% of isolates tested.