Interdomain horizontal gene transfer shaped the genomes of legionella pneumophila and legionella longbeachae

Abstract

Legionella are the causative agents of legionellosis or Legionnaires' disease-a severe pneumonia that can be fatal. Legionella are ubiquitous in natural and artificial freshwater environments worldwide and can also be present in moist soil and composted material, where these bacteria are infecting aquatic and soil amoebae to replicate therein. It is thought that the interaction of Legionella with freshwater amoebae and other protozoa has allowed this bacterium to acquire the factors necessary to replicate within eukaryotic cells. Indeed, the long coevolution with protozoa is reflected in its genome, as a large number of so-called eukaryotic-like genes have been identified. Many of these are secreted virulence factors used by Legionella to subvert host functions. Recent evolutionary studies indicated that these genes have been acquired by horizontal gene transfer (HGT) from aquatic protozoa, or other organisms living in amoebae, but some may have evolved by convergent evolution. In addition to the eukaryotic-like proteins, many other genes of Legionella seem to have been acquired by HGT or are involved in HGT events. In this chapter, we discuss the evolution of the eukaryotic-like proteins of Legionella and HGT among different domains of life in the context of Legionella and aquatic protozoa. Furthermore, a comparison of Legionella with the genomes of other amoeba-associated bacteria is presented, indicating that these bacteria share many common features like frequent HGT from protozoa to bacteria. Legionella is a privileged model to study the evolution of virulence and to understand the emergence of human pathogens.