It has been estimated that there are ~2 million cases of salmonellosis in the USA per year with an estimated economic impact of 20-200 million dollars. Unlike many infectious diseases, the incidence of Salmonella gastoenteritis in the US and other developed countries has been rising steadily over the past 30 years. People that are immunocompromised or have some other underlying condition (cancer, AIDS, transplant patients, systemic lupus erythematosus, sickle cell hemoglobinopathy) are at increased risk of acquiring and dying from salmonellosis. Salmonella is a food borne pathogen and it must cross the intestinal epithelium to reach its preferred site of multiplication, the reticuloendothelium. How Salmonella does this is not well understood, but both in vivo and in vitro data suggest that these bacteria invade and pass through intestinal epithelial cells. Advances in recent years in the areas of tissue culture technology, cell biology, and molecular genetics have provided the necessary tools to address the question of how these bacteria invade the intestinal epithelium and cause disease. We have isolated thirteen invasion defective mutants of S. enteritidis; these mutants fall into six phenotypic classes. Specifically we propose to address the following questions: 1) How many invasion genes are there and what are their products? The invasion loci identified by TnphoA insertional mutagenesis will be cloned. The genes and their products will be identified and characterized by standard molecular genetic techniques. 2) What is the effect on S. enteritidis virulence? The previously identified invasion mutants will be examined for changes in virulence and pathogenesis in a mouse model. 3) How is the expression of the invasion genes regulated? The invX::phoA fusions will be used to assess the effect of temperature, iron, starvation, and other environmental factors on gene expression. The role of previously identified regulatory systems on expression of the invasion genes will also be examined. Identification of additional regulatory components will be attempted. An anticipated increase in the population of the groups most at risk for salmonellosis - infants, the elderly, cancer patients, and AIDS patients - further underscores a need for more research in the area of Salmonella pathogenesis. With a better understanding of Salmonella pathogenesis it should be possible to devise better methods of treatment and prevention in the future.