Doctor of Philosophy (PhD)


Veterinary Medical Sciences - Pathobiological Sciences

Document Type



Edwardsiella ictaluri is a gram negative bacterium that is the causative agent of enteric septicemia of catfish. In 2011, this bacterium was identified as the causative agent of massive death in zebrafish populations in U.S. In this project, we found that isolates of E. ictaluri from zebrafish comprise a unique strain that differs from the type strain of E. ictaluri phenotypically as well as genetically. Also, strains of E. ictaluri from zebrafish are non-infectious in channel catfish Ictalurus punctatus by immersion. Here we sequenced the zebrafish strains of E. ictaluri LADL11-100 and LADL11-194 and compared the potential virulence genes in these strains with their homologous genes from the catfish strain LADL93-146. One of the major differences between the catfish strain and the zebrafish strain was found in the LPS related genes, more specifically, the O-antigen biosysnthesis cluster. The catfish strain and the zebrafish strain each contained unique genes in their O-antigen biosynthesis cluster. Three of the genes were shared by both strains but with relatively low identities. In contrast, the entire O-antigen biosynthesis cluster of the zebrafish strain of E. ictaluri was nearly identical to that of E. piscicida C07-087 with higher than 90% similarity. The differences in the O antigen were further confirmed by observing the different banding patterns of the purified LPS samples from the catfish and the zebrafish strain of E. ictaluri. Comparative genomic DNA analysis revealed that the major part of the type III secretion system is present and consistent among the zebrafish strains and the catfish strain. One variable portion is the effector EseI and its potential chaperone EscD are missing from the zebrafish strains. Also, there are more than twenty single nucleotide polymorphisms (SNPs) in one of the type III secretion system regulators esrA, and two genes eseC and eseD, which encode translocon proteins. These variations in the type III secretion system may contribute to the differences in virulence between the zebrafish and catfish strains. The type IV secretion system harbored the most variations between the catfish strain and the zebrafish strain compared to the other secretion systems. The sequences of the putative proteins in this type IV secretion system of both the zebrafish and the catfish strain were aligned and very low similarities were observed for most of the proteins in this system. In fact, two of the putative proteins in the zebrafish strain of E. ictaluri, had different conserved domains than their related proteins in the catfish strain, indicating possible different functions of these proteins. Other potential virulence related systems, the type VI secretion system and the urease system, are conserved between the catfish and zebrafish strains of E. ictaluri with only few SNPs. In addition, to protect against outbreaks of edwardsiellosis in zebrafish populations, the wild type zebrafish strain of E. ictaluri was mutated with the goal of generating attenuated strains that could serve as live attenuated vaccines. Both of our mutants, the ureG and esrC mutant, were proven to be fully attenuated by immersion in zebrafish. Further study is needed to test their efficacy as live attenuated vaccines.



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Committee Chair

Hawke, John P.