Molecular characterization of Edwardsiella tarda bacteria causing severe mortalities in cultured Oreochromis niloticus fish with treatment trials

The present study was carried out to isolate Edwardsiella tarda (E. tarda) from cultured Oreochromis niloticus (O. niloticus) and that identified by both Biochemical tests (API 20 E) and PCR. A total of 2 E. tarda isolates were isolated from 50 cultured O. niloticus fish collected randomly from the ponds of private fish farms at Kafr El Sheikh Governorate, Egypt. The clinical picture of the collected fish exhibited loss of escape reflex; skin darkness; bilateral exophthalmia with corneal opacity and ulcers varied in their degrees, inflammation, congestion, hemorrhage and enlargement of most internal organs were apparent in postmortem examination. The isolated E. tarda was screened for presence of 3 virulence genes (esrB, gyrB and gadB genes) using multiplex PCR and the results showed the amplification of the concerned gene at molecular size esrB (311 bp), gadB (583) in one strain and gyrB (415) in the other strain. Histopathological examination for haemobiotic organs liver, spleen and kidney as well as gills revealed necrosis of most internal organs, inflammatory reaction, associated with hemosiderosis. In vitro antibiotic sensitivity pattern of E. tarda isolates was conducted by disc diffusion method for five antibiotic discs where, the isolate was found to be sensitive against florfenicol, Ciprofloxacine and sulphadimethexine and resistant to oxytetracycline and ampicillin. Isolated E. tarda was used for experimental infection of healthy fish; typical symptoms in naturally infected fish appear in experimentally infected fish. Treatment trials for experimental infected fish reveal effectiveness of florfenicol, Ciprofloxacine and sulphadimethexine.