Humans who ingest Listeria monocytogenes (Lm)-contaminated food develop infections that range inseverity from mild, self-limiting gastroenteritis to life-threatening sepsis and meningoencephalitis. Strainlineage, dose, and host susceptibility factors are all likely to influence infection outcomes, but ourunderstanding of the intestinal phase of infection is still severely limited. To overcome this obstacle, werecently developed a murine model of foodborne listeriosis that can be used to study both virulencedeterminants and immune responses in animals that vary in their susceptibility or resistance to infection.Using this model, we found that the vast majority of Lm in the gut were extracellular. This result wasunexpected, because Lm are facultative intracellular pathogens and most research efforts over the pastfew decades have focused on Lm factors that promote invasion, survival and growth in culturedmammalian cells. Mutant Lm that were unable to replicate inside host cells established infection in themurine gut normally, and persisted for a few days, but had a severe defect in disseminating from theintestinal lamina propria (LP) to the mesenteric lymph nodes (MLN). These observations suggest thatintracellular growth is not needed to cause gastroenteritis; however, further systemic spread requiresreplication inside an as-yet-unidentified cell type in the gut. The central hypothesis of this proposal is thatexponential replication in a permissive cell type is needed to evade innate clearance mechanisms in thegut, and to increase the number of Lm above a critical threshold that can promote dissemination by one ofthree distinct mechanisms. In Aim 1, ex vivo flow cytometry and microscopy approaches will be used toidentify all of the cell types that can be productively infected with Lm in the ileum, colon, and the MLN thatdrain these tissues. In Aim 2, we test the hypothesis that Lm can use one of three potentially redundantmechanisms to spread from the gut LP to the MLN: 1) attached to migratory cells that are not efficientlyinvaded; 2) ?stealth transport? in the cytosol of other migratory cells; and 3) extracellular Lm traffickingfree-flowing in lymphatic vessels. Building on the knowledge gained from Aims 1 & 2, Aim 3 seeks toaddress the paradox that mechanisms exist for Lm to spread extracellularly, yet intracellular replication isneeded at some point in the infection cycle for efficient dissemination to the MLN. The replication rate inthe gut will be altered by eliminating the cell types permissive for intracellular growth, and the clearancerate will be altered by depleting neutrophils and inflammatory monocytes. A key strength of this proposal isthe interdisciplinary nature of the approaches that capitalize on recent advances in understandingmononuclear phagocyte differentiation pathways in the steady state and applying those findings to aphysiologically relevant infection model. This study will advance the field by revealing how Lm survive andreplicate in the gut and by identifying the bottlenecks they face in order to cause systemic disease.