Molecular Characterization Of Edwardsiella Species Isolated From African Catfish (Clarias Gariepinus) And Nile Tilapia (Oreochromis Niloticus) In Wakisodistrict, Uganda

ABSTRACT

 Edwardsiella tarda is an opportunistic pathogenic bacterium that causes Edwardsiellosis in cultured and wild fish. It is among the most important bacterial diseases causing severe economic losses in fish worldwide. This study was carried out to determine the occurrence and characterize E. tarda in cultured Nile tilapia and African catfish from selected fish farms in Wakiso district, Uganda. A total of 111 fish samples were collected from 17 fish farms between September 2016 and February 2017. Clinical and post mortem examination of sampled fish was done followed by bacteriological examination of the collected internal organs (spleen, kidney and liver), gills and skin swabs. Identification of the bacterium was done using conventional biochemical tests, API 20E kits and sequencing of 16S rRNA. Phylogenetic analysis was done by Neighbor-Joining method in MEGA 7.0.2 against the 16SrRNA gene sequences retrieved from the GenBank. The isolate was screened for presence of selected virulence genes by polymerase chain reaction (PCR). From this study, one isolate from O. niloticus was identified and confirmed to be E. tarda by the 16S rRNA sequencing. The isolate gave an identity rate of 99.9% to other members of E. tarda on comparison with known 16S rRNA sequences in the GenBank database. In phylogenetic analysis, the isolate did not cluster with any of the E. tarda isolates suggesting a distant relationship with the isolates whose sequences were included in this study. Six virulence genes were detected in the isolate that enhance bacterial survival and pathogenesis in the host including; CitC, muk, gadB, katB, esaV and fimA. Although the study confirmed only one E. tarda isolate, the isolate tested positive for several virulence genes indicating their potential to cause disease in fish and since the bacterium is of public iii health importance, awareness should be created amongst fish farmers and stakeholders to take precaution to avoid disease outbreak.