Advancing our Understanding of the Edwardsiella

Diseases caused by Edwardsiella spp. are responsible for significant losses in wild and cultured fishes around the world. Historically, Edwardsiella tarda has been considered the most phenotypically and genotypically heterogeneous member of the genus. Investigations into intraspecific variability of E. tarda demonstrated isolates previously classified as E. tarda actually represent three genetically distinct yet phenotypically ambiguous groups, leading to the adoption of E. piscicida and E. anguillarum as discrete taxa. Current genomic investigations have demonstrated significant differences between these organisms. To this end, real-time quantitative PCR assays were developed to quickly and accurately detect the pathogens in pond water, fish tissue and broth culture. Additionally, whole genome sequencing was performed for representative isolates of each Edwardsiella spp. Furthermore, forty-seven Edwardsiella isolates, representing all five taxa, from different hosts and a wide temporal and geographic range were analyzed using commercial microbial identification kits, repetitive sequence-mediated polymerase chain reaction, fatty acid methyl ester analysis, antimicrobial resistance profiles, in addition to 16S, gyrB, sodB and plasmid sequencing. This analysis demonstrated key differences in gene sequences and plasmid profiles among these important bacterial pathogens and further supported contemporary taxonomic classifications. Additionally, a real-time multiplex PCR was developed to accurately discriminate between all Edwardsiella spp. affecting fish; namely E. ictaluri, E. piscicida, E. tarda and E. anguillarum. Moreover, recent publications suggest E. piscicida is more commonly associated with disease outbreaks in Mississippi catfish aquaculture than E. tarda or E. anguillarum. To this end, several different challenge models were evaluated. Comparative virulence was assessed, along with histopathological lesions and posterior kidney clearance rates in channel catfish (Ictalurus punctatus). Diagnostic case submissions suggest E. piscicida is more commonly associated with disease outbreaks in blue catfish (Ictalurus furcatus) x channel catfish hybrids compared to channel catfish. This led to investigations into the relative pathogenicity of E. piscicida in hybrid and channel catfish, which demonstrated a significantly lower median lethal dose (LD50) for E. piscicida in hybrid catfish; an important finding given the increased production of hybrid catfish in U.S. farm-raised catfish in the southeastern United States