The protozoa are a diverse group of eukaryotic microorganisms that employ a variety of life strategies. Many protozoa are free-living while others are commensal organisms that thrive within the gut of vertebrates. In addition, a relatively small group of parasitic protozoa represent serious health threats to humans and animals. Certain free-living, commensal, and parasitic protozoa are bactivorous and rely on bacteria as a food source. However, some intracellular bacterial pathogens survive engulfment and thrive within these protozoa. This dissertation describes the investigation of Salmonella hypervirulence following bacterial engulfment and characterizes physiologic interactions between various protozoa and bacteria.

Initial studies entailed characterization of in vitro and in vivo Salmonella hypervirulence following engulfment by free-living and commensal bactivorous protozoa. This work revealed that Salmonella enterica serotype Typhimurium phagetype DT104 survive protozoan engulfment and use the intracellular environment as a venue for plasmid transfer and activation of virulence genes. In subsequent studies, receptors from ciliated protozoa were investigated with a strategy involving molecular cloning, RNA interference, and heterologous expression in yeast. These studies revealed the involvement of protozoan G protein-coupled receptors and a cyclic nucleotide binding protein in bacterial engulfment. The yeast expression system was then used to deorphanize a G protein-coupled receptor from Entamoeba histolytica, demonstrating that bacterial lipopolysaccharide is an agonist of this receptor. Engulfment assays demonstrated that E. histolytica trophozoites preferred to engulf bacteria with intact LPS and this process was sensitive to inhibition of G protein signaling. In summary, a variety of protozoa facilitate salmonellosis, G protein-coupled receptors play a role in bacterial engulfment by protozoa, and these receptors represent novel drug targets worthy of further exploration.