Molecular characterization of Edwardsiella species isolated from Oreochromis niloticus and Clarias gariepinus in Wakiso District, Uganda

Abstract

Abstract. Nantongo M, Mkupasi EM, Byarugaba DK. 2019. Molecular characterization of Edwardsiella species isolated from Oreochromis niloticus and Clarias gariepinus in Wakiso District, Uganda. Bioteknologi 16: 11-20. Edwardsiella tarda (Ewing & McWhorter, 1965) is well-known as an opportunistic pathogenic bacterium causing Edwardsiellosis in the host body of cultured and wild fish. The disease is among the most important bacterial diseases causing severe economic losses in fish worldwide. Here, we determined the occurrence and performed characterization of E. tarda in cultured Nile tilapia and African catfish from selected 17 fish farms in Wakiso District, Uganda. Fish samples were collected in the duration between September 2016 and February 2017. Bacteriological examination of the internal organs (kidney, liver, spleen) was followed by clinical and post-mortem examination of the overall fish body. The bacterium was identified using conventional biochemical tests, API 20E kits, and sequencing of 16S rRNA. Phylogenetic analysis was done by the Neighbor-Joining method in MEGA 7.0.2 against the 16S rRNA gene sequences retrieved from the GenBank. The isolate was further screened for the presence of selected virulence genes by polymerase chain reaction (PCR). One isolate from Oreochromis niloticus (Linnaeus, 1758) was confirmed to be E. tarda by the 16S rRNA sequencing. The isolate gave 99.9% identity to other members of E. tarda compared to known 16S rRNA sequences in the GenBank database. In the phylogenetic analysis, the isolate did not cluster with any of the E. tarda isolates, indicating a distant relationship with the isolates whose sequences were included in this study. Six virulence genes that enhance bacterial survival and host pathogenesis were identified in the isolate, including; CitC, muk, gadB, katB, esaV and fima. This study confirmed one positive E. tarda isolates. Nevertheless, the isolate possessed several virulence genes indicating their potential to cause disease in fish. Because the bacterium is of public health importance, awareness should be created amongst fish farmers and stakeholders to take precautions to avoid disease outbreaks.