Thesis

Evaluating the dnd gene cluster involvement in the pathogenesis of phosphorothioate Salmonella

Salmonella, consisting of more than 2000 serovars, is the second most common cause of foodborne illness in the US that can survival intracellularly and lead to gastrointestinal illness. Phosphorothioation (PT) is a post-replicative modification that replaces the non-bridging oxygen in the DNA backbone with sulfur molecule to form a phosphorothioate linkage. PT, a result from dnd gene cluster (dndBCDE), instigates DNA degradation (Dnd) phenotype during gel electrophoresis in the present of Tris buffer. This study examined the functional role of the dnd gene cluster present in Salmonella clinical isolates on microbial pathogenesis and the effect it has on the invasion process. Results showed the dnd gene cluster does not confer additional resistance to hydrogen peroxide or to UV irradiation. Scanning electron microscopy showed the presence of the dnd gene cluster had no visual effect on the morphology of the cell. Primers successfully amplified individual dnd genes in PT+ Salmonella, and unexpectedly the dndD gene was also present in PT- isolates, suggesting occurrence of one or more horizontal gene transfer events. Gene expression analysis showed increased gene expression of the dndCD genes when grown in conditions that mimicked in vivo environments simulating invasion of a host cell. The results suggest that dnd genes participate in coping with environmental stress and in invasion of host cells. This study verifies the functional role of the dnd gene cluster and supports a potential role the dnd gene cluster (especially dndCD) may have in the pathogenicity of PT modified Salmonella.

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