Entry into host cells by Legionella

Many respiratory diseases are caused by extracellular bacterial pathogens; however, two very important lung infections are due to intracellular pathogens, Legionnaires' disease and tuberculosis. Legionnaires' disease remains problematic due to our inability to predict where sporadic epidemics will occur and the speed at which the bacterium debilitates its victims. The development of better methods for prevention would greatly alleviate public concern and the economic impacts of eradication efforts where infections occur. Legionella, the causative agent of Legionnaires' disease, has been shown to replicate within eukaryotic cells both during disease and in the environment. During disease these bacteria are found primarily within macrophages, though they have the ability to enter and survive within a number of different mammalian cell types. In the environment Legionella replicate within free-living protozoa. Thus, the ability to enter into host cells successfully and efficiently is critical to the ability of Legionella to survive. The process by which Legionella gains access to the intracellular environment involves a number of steps; including, finding an appropriate host cell, adherence, signal transduction, entry and initial survival. Unless Legionella accomplishes each of these steps properly, few viable bacteria will be observed intracellularly and reduced intracellular replication may occur. However, the importance of each of these individual steps in the pathogenesis of Legionella is unclear. Herein we discuss the potential mechanisms of entry by Legionella into host cells, a critical early event in the production of Legionnaires' disease.