Characterization of specific virulence traits of the channel catfish pathogen, Edwardsiella ictaluri

Edwardsiella ictaluri, the causative agent of enteric septicemia of catfish (ESC), is one of the most important pathogens to the commercial catfish industry. However, insufficient information is known about the virulence factors of this pathogen for effective treatment and control of ESC. The goal of this study was to examine whether laboratory adaptation of E. ictaluri results in an attenuated phenotype and downregulation of virulence factor expression. Using this model, potential virulence factors of E. ictaluri were identified.; In this study, phenotypic comparisons were made between wild type virulent strains and laboratory adapted attenuated strains of E. ictaluri to determine whether the presence of these characteristics correlates with virulence. Attenuation through serial subcultivation resulted in shorter generation times, loss of expression of the outer membrane protein Eip55, changes in the core oligosaccharides, and a significant decrease in hemolysin activity in E. ictaluri strain R4383.; Using the findings obtained by phenotypic comparisons, isogenic mutant strains of both eip55 and hemolysin were constructed to elucidate the roles of these factors in virulence. The hemolysin of E. ictaluri was identified as a member of the two-component hemolysin family, but expression of the hemolysin was not essential for full virulence in the channel catfish. The eip55 mutant strain of E. ictaluri that was constructed was unstable, which resulted in a reversion to the wild type genotype. Because of this instability, the role of this protein in virulence of E. ictaluri was not established. Further studies concerning the role of this protein in virulence and survival are warranted.