Legionella pneumophila: population genetics, phylogeny and genomics

Legionella pneumophila is a human pathogen that was recognized only about 30 years ago. It is the causative agent of Legionnaires' disease, a severe pneumonia that is transmitted through inhalation of aerosols of contaminated water. Shortly after its discovery, the ability of Legionella to multiply intracellularly in fresh water protozoa was discovered. This long lasting co-evolution between the eukaryotic host and Legionella has led to the selection of a panoply of virulence factors, which allow to exploit important cellular processes during infection. Compelling evidence for the importance of protozoa in the evolution of this bacterium comes from analysis of complete genome sequences. A key feature of the L. pneumophila genomes is the presence of a high number and wide variety of eukaryotic like proteins and protein domains probably acquired through horizontal gene transfer and/or convergent evolution. In the last years several different typing methods aiming in investigating the molecular epidemiology of L. pneumophila have been developed. Furthermore, the access to whole genome sequences of several L. pneumophila strains allowed to apply large scale comparative genomic studies using DNA arrays. A higher genetic diversity among environmental isolates with respect to clinical isolates and the presence of specific clones of L. pneumophila overrepresented in human disease or causing legionellosis world wide, were identified. Further studies analyzing the natural populations of Legionella more in detail will allow to gain a better understanding of the population structure and the ecological diversity of this species. This review describes the latest observations about the structure of L. pneumophila populations, the techniques used to study the molecular epidemiology and evolution of L. pneumophila, the knowledge gained from genome analysis, and discusses future perspectives.