The long-term objective is to determine how an enteropathogenic bacterium is able to enter and survive within animal cells. Enteropathogenic Yersinia are able to invade the epithelial cells of their hosts. This phenomenon will be investigated by studying how Yersinia pseudotuberculosis enters cultured cells and by investigating the consequences of invasion-defective mutations on pathogenesis in an animal model. The approach will be to analyze a genetic locus, called inv, that is encoded by this bacterium. To this end, the following goals will be pursued: 1) using a genetic analysis, it will be determined if invasion of cultured cells by Y. pseudotuberculosis requires the product of the inv locus; 2) the role that inv plays in the pathogenesis of disease will be analyzed using a mouse virulence assay; 3) all the products of this locus will be identified by performing DNA sequencing and by isolating nonsense mutations in the major gene product of this locus; 4) antibody will be raised to the major product of this locus, called p108, and this will be used to determine how this protein is localized in the bacterium; 5) the ligand encoded by Y. pseudotuberculosis that binds this bacterium to the animal cell will be identified by isolating mutations that eliminate this structure and by biochemically reconstituting the binding activity. Ingestion of bacteria by epithelial cells is the first step in the infection process of many enteropathogenic organisms. An understanding of how this occurs could allow the development of new chemotherapies that block this step in the infection process. In addition, identification of the components that allow a simple organism to enter an animal cell could result in new techniques to introduce therapeutic agents that would otherwise not be able to enter the host cell.