Intracellular Rickettsiales: Insights into manipulators of eukaryotic cells

The order Rickettsiales comprises obligate intracellular bacteria that are the ancestors of modern eukaryotes. These bacteria infect various vectors and hosts, with some species being pathogenic to man. Rickettsiales have small, degraded genomes and provide a paradigm for increased pathogenicity despite gene loss; significant levels of genetic exchange occur between bacteria that infect the same host and with the eukaryotic hosts themselves. Crosstalk between host and bacteria appears to be mediated by a Type IV secretion system and proteins containing eukaryotic-like repeat motifs. Rickettsiales also manipulate host reproduction and induce host resistance to viruses. Manipulation of its host by Rickettsiales has long been misunderstood because of technical difficulties, but recent advances in understanding bacterial–eukaryotes interactions have been made and are reviewed here.