The mammalian intestine is home to a dense community of bacteria that are integral to host health. Alterations in this bacterial population ca cause intestinal distress and even in some cases inflammatory disease. Aside from bacteria, viruses are also abundant in the intestinal tract. Both eukaryotic and prokaryotic viruses make up the intestinal virome, however, viruses that infect bacteria, known as bacteriophages (phages), dominate. The majority of intestinal phages exist as lysogenic prophages that are stably integrated into bacterial chromosomes. Prophages typically undergo lytic induction to produce infectious phage particles when bacterial cells are stressed or when they are stimulated by nutrients. Preliminary studies have revealed that the commensal bacterium Enterococcus faecalis V583 induces the prophages when growing in the mouse intestine. In vitro, prophage induction depends on elevated amino acid concentrations; however, the exact mechanisms underlying nutrient-mediated induction of prophages are unknown. Further, in vivo studies indicate that the release of lytic phage particles allows E. faecalis to compete with genetically related enterococci vying for similar intestinal resources. Together, these initial findings indicae that phages can impact the dynamics of bacterial colonization in the mammalian intestinal ecosystem and have established E. faecalis prophages as model intestinal phages for the study of phage-host interactions. The long-term goal of this research is to understand how intestinal bacteriophages impact human physiology and health. The specific research proposed under this K01 award mechanism will advance this goal by defining the mechanisms that elicit phage production in the intestine, by determining how bacteriophage impact host immunity, and by exploring how phage predation impacts bacterial interactions with the host. These studies will provide novel insights into how intestinal phages influence host health and physiology, and will set the stage for future exploration of phage as therapeutic tools for manipulating the microbiota to improve health.