Molecular Characterization and Antibiotic Susceptibility of Edwardsiella tarda isolated from Farmed Nile Tilapia and African Catfish from Wakiso, Uganda

Abstract

Abstract. This study was conducted to isolate and characterize Edwardsiella tarda (E. tarda) and
assess its antimicrobial susceptibility. The bacterium was isolated in Wakiso District, Uganda, from
symptomatic and asymptomatic Nile tilapia and African catfish raised in earthen ponds, tanks and
cages between September 2016 and February 2017. The bacterium was then identified using
conventional biochemical tests and API 20E test kits and characterized by sequencing 16S rRNA
gene. The antibiotic susceptibility of 16 drugs was established using the Kirby BeurDisc diffusion
method. Eight E. tarda isolates were identified using conventional biochemical tests but only one
isolate was confirmed to be E. tarda by PCR. Phylogenetic analysis indicated a distant relationship
with other 16S rRNA gene sequences retrieved from the GenBank. Six virulence genes (CitC, muk,
gadB, katB, esaV, and fimA) that enhance bacterial survival and pathogenesis in the host were
detected. The isolate registered low levels of antibiotic resistance as it was resistant only to Oxacillin,
Vancomycin and Penicillin, to which it is intrinsically resistant. This implies low antibiotic usage in
aquaculture in the area. Despite its low occurrence, presence of virulent genes in E. tarda indicates
its potential to affect fish and human health.