Legionnaires' disease

Legionella pneumophila, the etiological agent of Legionnaires' disease was first recognized in 1976, during an outbreak of severe pneumonia at the convention of the American Legion in Philadelphia. Since then the genus Legionella has expanded, and to date more than 50 different species are described. However, L. pneumophila remains the major cause of human disease, as it is responsible for over 90 % of legionellosis cases worldwide. This chapter starts with a description of the genus Legionella and the ecology of these bacteria. We then highlight the epidemiology, clinical features, and the different diagnostic assays and methods available. In the second part, we focus on the pathogenesis, the virulence factors, and the immune response of the host. Special emphasis is placed on the implication of the literally hundreds of different effector proteins secreted by the Dot/Icm type IV secretion system. In the third section we discuss recent knowledge acquired on genomics, transcriptomics, and the metabolic features of Legionella, and, particularly, we present new insight on comparative genomics, evolution, horizontal gene transfer, and the regulation of the life cycle of L. pneumophila.