ABSTRACT
Intracellular parasites offer a unique challenge to biologists, particularly the obligately intracellular bacteria. The study of bacterial phylogeny has not always been fruitful. The natural classification of species according to their evolutionary diversion and ascent has traditionally relied on grouping together organisms with similar morphological features. Zuckerkand and Pauling suggested a novel approach to the study of the evolutionary history of species. They proposed that the genes of organisms were archeological relics of their ancestors. The order Rickettsiales includes several genera of parasitic bacteria which demonstrate extreme dependence on eukaryotic host cells. Most of the species are obligate intracellular pathogens, although there are examples of epicellular bacteria, and some forms may be commensal. A taxonomic dissection of the order Rickettsiales reveals three families: Rickettsiaceae, Bartonellaceae, and Anaplasmataceae. The tribe Rickettsiae contains three genera, Rickettsia, Rochalimaea, and Coxiella.
