Doctor of Philosophy (PhD)


Pathobiological Sciences

Document Type



Edwardsiella ictaluri is a gram-negative bacterium of the family Enterobacteriaceae that infects and causes enteric septicemia (ESC) of channel catfish (Ictalurus punctatus), a fatal disease costing the catfish industry millions of dollars in losses each year. Edwardsiella. ictaluri is capable of replicating in catfish head-kidney-derived-macrophages (HKDM), and like many other gram-negative bacteria, E. ictaluri encodes a Type III Secretion System (T3SS) that is required for virulence and intracellular replication. In the case of E. ictaluri, the T3SS translocates effectors from the Edwardsiella containing vacuole (ECV) through the bacterial cell wall and the vacuolar membrane directly to the host cytoplasm. Of the nine known effectors, this work analyzed one effector, EseK to determine the potential for cell-mediated immune (CMI) responses in naïve fish and the fate of non-activated head-kidney-derived macrophages (HKDMs) in response to infection. We determined that the binding partner of EseK is CD74, and an EseK knockout strain provides catfish fingerlings with protection against subsequent WT exposure. RNA sequencing using infected and uninfected HKDMs uncovered strong indications for the M1 phenotype in response to infection. This data also indicates that E. ictaluri evades typical programed-cell death measures, and suppresses the CD40 pathway, which is critical for T cell dependent activation. Finally, in vivo immune responses detected several regulation differences, notably downregulation of CD40L in infected catfish, further implicating the loss of this pathway due to infection. Within the scope of this work, we did not discover any specific differences in immune regulation that could be attributed to EseK, but this research supports the theory that E. ictaluri modifies the macrophage environment in order to persist and replicate in the host, and E. ictaluri evades CMI responses further aiding its survival in the host.



Committee Chair

Thune, Ronald

Available for download on Tuesday, June 29, 2021