Legionella pneumophila causes the severe and sometimes lethal pneumonia termed Legionnaire's disease. Legionnaire's disease has been shown to arise in individuals that undergo prolonged exposure to aerosols from domestic water supplies that contain Legionella. The first evidence that air-handling devices were the source of infection stems from a confirmed Legionella epidemic in 1976. There have recently been a large number of infections that have arisen aboard cruise ships as well. Approximately 15% of all domestic water supplies are thought to contain Legionella. It has been found that Legionella replicates almost exclusively intracellularly during infection and in the environment. Thus, Legionella must enter a eukaryotic cell in order to survive and the genes involved in entry should be critical for production of disease. The investigator's recent data indicate that Legionella grown in an environmental host, the amoebae Acanthamoeba castellani, are approximately two to four logs more invasive for host cells than bacteria grown in the standard laboratory medium. Electron microscopic examination of entry has shown that a novel mechanism, termed "coiling phagocytosis", is responsible for bacterial uptake. This mechanism appears to result in enhanced survival of Legionella after entry into activated macrophages. Taken together, these studies indicate that a key factor in the production of virulent Legionella in water supplies is growth within the appropriate environmental host, without which the investigator hypothesizes that human infections will most likely not occur. The specific aims of the current proposal are to: 1) isolate and characterize the genes involved in invasion and 2) determine the involvement of these genes in virulence of Legionella. Through an examination of the mechanisms of entry and the factors that regulate it, the investigator hopes to further the understanding of how Legionella causes disease as well as provide insight into novel methods for disease prevention.