Typhoid fever is a communicable, human disease which remains a major public health problem in the world. Recent estimates indicate that approximately 33 million cases occur annually world-wide. However, the virulence determinants and protective immunogenic antigens of Salmonella typhi have yet to be clearly delineated. It is currently unclear whether the host specificity of S. typhi is determined at the level of adherence/invasion, i.e., at the level of cell surface-to-cell surface interactions between host and parasite, or at subsequent levels of interaction with the host, such as survival and growth within phagocytic cells. In this proposal, we will focus on the initial steps in the entry of S. typhi into the mammalian host. We hypothesize that the optimal adherence/invasion of S. typhi into eukaryotic cells depends on the cell surface interactions of S. typhi with the eukaryotic host and that these interactions are mediated, in part, by the expression of specific adherence/invasion genes as well as on the environmental conditions within the distal ileum of the host. Our experiments will help to determine whether or not the initial interactions of S. typhi with host cells are unique or similar to those of S. typhimurium. Direct experimental data to resolve this question are not currently available. Our experiments are an extension of the existing literature and not a duplication. Our LONG TERM GOALS are to identify and characterize unique surface antigens on S. typhi which play a role in virulence and which may ultimately evoke a protective immune response in the host. The SPECIFIC AIMS are: 1. to optimize and assess the effects of diverse environmental growth conditions on the capacity of S. typhi to adhere/invade Henle 407 intestinal epithelial cells. The variables will include osmolarity, pH, oxygen tension, and growth phase; 2. to identify S. typhi unique gene(s) that encode adherence/invasion molecules using TnphoA mutagenesis, and characterize the mutants by: 1) secondary growth characteristics, 2) comparison of their outer membrane profiles with wild type S. typhi, and 3) cloning and sequencing of new genes, and 3. to develop a parallel strategy for the identification of cell surface molecules important in virulence/immunogenicity by generating 2 panels of S. typhi-specific monoclonal antibodies from murine Peyer's patches cells and human peripheral blood. These studies will provide insights into initial steps in the pathogenesis of S. typhi and begin to identify unique virulence determinants of this pathogen.