Doctor of Philosophy (PhD)


Veterinary Medical Sciences - Pathobiological Sciences

Document Type



Edwardsiella ictaluri encodes a type III secretion system (T3SS) required for intracellular replication. Analysis of in vitro gene expression indicates the T3SS translocon proteins express and secrete in acidic pH. Expression of other T3SS genes, however, requires that phosphate be limited in the culture media, in addition to acidic pH. Responses to environmental stimuli are mediated through the T3SS-encoded regulatory proteins EsrA, EsrB, and EsrC. Mutations in these genes result in differing phenotypes. Mutation of EsrA results in moderately reduced expression of T3SS genes, but translocon protein secretion is retained in the mutant. However, the EsrA mutant is attenuated intracellularly and in vivo. Mutation of EsrB results in severely decreased T3SS gene expression and translocon protein secretion, leading to intracellular and in vivo attenuation. EsrB is also required for expression of type VI secretion system (T6SS)-related proteins through the modulation of EsrC expression. EsrC mutation has an effect on T3SS gene expression, but less so than EsrB, and does not abolish T3SS translocon secretion. EsrC mutation, however, does inhibit expression of T6SS-related proteins, indicating one of its functions is to coordinate expression of the T6SS with that of the T3SS. The EsrC mutant is not attenuated intracellularly, but does exhibit attenuated virulence in vivo. Expression of the T3SS also is dependent on two plasmids carried by strains of E. ictaluri virulent for channel catfish. Mutation of both of the plasmids results in severe decreases in T3SS gene expression, resulting in attenuation of E. ictaluri intracellularly and in vivo. The plasmids do not encode proteins with homology to known regulatory proteins, but may integrate into the genome near putative regulatory genes, thereby modulating their expression. The results presented here demonstrate that E. ictaluri responds to conditions mimicking the intracellular environment by expression a T3SS, which is required for intracellular survival. The expression of this T3SS absolutely is dependent on EsrB, and EsrA and EsrC are required for optimal T3SS expression.



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Committee Chair

Ronald Thune