Intracellular parasitism, the driving force of the evolution of virulence of Legionella pneumophila and the genus Legionella

The genus Legionella comprises 65 species that replicate as intracellular parasites of amoebae but that can also cause a severe pneumonia, called Legionnaires’ disease. Adaptation to the host environment and exploitation of host cell functions are critical to the success of these intracellular pathogens. Genomics analyses of L. pneumophila identified a large number of genes coding proteins with eukaryoticlike properties as witness of the tight co-evolution between Legionella and their protozoan hosts. Functional analyses showed that these proteins are secreted effectors that act in the host like their eukaryotic homologues – thus molecular mimicry is a major virulence strategy of Legionella. Recently, functional and comparative genomics used to deconstruct the entire genus Legionella revealed the surprising parallel evolutionary trajectories that have led to the capacity of Legionella to replicate in protozoa and in human cells.