Yersinia pestis is the agent of plague, an acute, often fatal bacterial infection that is transmitted by flea bite or aerosol. Y. pestis is considered to be a facultative intracellular pathogen. Previous data indicate that Y. pestis survives and replicates within macrophages during the first few hours of infection, while extracellular growth is predominant at later time points. The mechanism by which Y. pestis survives and replicates in macrophages is not known. The long-term objective of the proposed studies is to determine the molecular and cellular basis for intracellular survival of Y. pestis in macrophages. To achieve this goal we will (1) Characterize macrophage defenses that are important for limiting intracellular survival and replication of Y. pestis; and (2) Identify genes in Y. pestis that are important for combating intracellular defenses of macrophages. Understanding the intracellular survival mechanism of Y. pestis may aid the development of new ways to prevent or treat plague infections in the human population.