Molecular characterization and pathogenicity evaluation of Edwardsiella sp. from freshwater ornamental fish

Abstract: Edwardsiellatarda is an opportunistic pathogenic bacterium that causes edwardsiellosis in cultured and wild fish.It is one of the most important bacterial diseases in fish worldwide that cause severe economic losses. This study was carried out to determine the occurrence and characterize E. tarda from ornamental freshwater fishes in Alappuzha, Ernakulam and Thrissur district, Kerala. A total of 30 fish samples were collected from different fish farms and Aquarium shop during the month of August 2018 to February 2019. Clinical and post mortem examination of sampled fish was done followed by bacteriological examination of the collected internal organs (intestine, kidney and liver). Identification of the bacterium was done using conventional biochemical tests and sequencing of 16S rRNA.Three isolates showed 100% similarity to E. tarda. The isolates were screened for the presence of selected virulence genes by polymerase chain reaction (PCR), Five virulence genes were detected in the isolate that enhance bacterial survival and pathogenesis in the host including; gyrB, mukF, gadB, esrBand fimA.Although the study confirmed only one E. tarda isolate ETAB2 positive for five virulence genes. Also these isolates ETAB1, ETAB2 and ETBM was screened for the presence of other phenotypic virulent property like Congo red binding, Biofilm formation and hemolytic assay. One of the E. tarda isolates ETAB2 was bound to Congo red and positive for thick biofilm formation, whereas none of them were found to be hemolytic. Although the study confirmed only one E. tarda isolate, the isolate tested positive for several virulence genes, multiple antidrug resistant, thick biofilm formation and Congo red binding, hence it indicating their potential to cause disease in fish and since the bacterium is of public health importance, awareness should be created amongst aquarium fish handlers to take precaution to avoid disease outbreak.