PROJECT SUMMARY Disease transmission is a multifaceted process mediated by the interactions between the pathogen and host. Salmonella enterica serovars are important human pathogens that cause disease ranging from self-limiting gastroenteritis to persistent systemic infections, such as typhoid fever. Transmission occurs via the fecal-oral route, and epidemiological analyses have revealed heterogeneity in host transmission capabilities. However, relatively little is known about the factors that dictate pathogen shedding and transmission. A number of studies have described mechanisms of Salmonella enterica serovar Typhimurium expansion in mice postantibiotic treatment. However, these mechanisms have not been explored in the absence of antibiotic treatment. Although antibiotic treatment in humans enhances susceptibility to enteric pathogens such as S. Typhimurium and Clostridium difficile, Salmonella colonizes the gut in the absence of antibiotic treatment. Thus, it is important to study the mechanisms of S. Typhimurium emergence and expansion in non-antibiotic treated hosts. We use a mouse model to study molecular mechanisms of S. Typhimurium expansion in the mammalian gut in which ~20-30% of these mice shed ≥108 CFU/g feces and readily transmit to naïve cage mates. We hypothesized that alterations in the diet could influence pathogen shedding. To test this, we fed mice a high cholesterol diet at the time of infection. We have preliminary data demonstrating that high cholesterol diet increases S. Typhimurium shedding levels very rapidly. The long-term goal of this research proposal is to understand how S. Typhimurium usurps and manipulates the gut microbiome and host immune responses during colonization of the mammalian gut. In Aim 1, we will identify host factors that promote cholesterol-dependent expansion of Salmonella in the distal gut. In Aim 2, we will identify and characterize Salmonella factors required for expansion in the distal gut, and determine whether they are specific for expansion in mice fed a high cholesterol diet. These studies are aimed at gaining a better understanding of the molecular mechanisms of host-pathogen interactions during S. Typhimurium infections in the gut, and influence pathogen transmission. The expected results will lead to new methods of transmission control and to the rational design of therapeutics that will benefit public health.