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Bartonella in a Phylogenetic Context

The genus Bartonella belongs to the Rhizobiales in the a-subdivision of the Proteo-bacteria, which consist of bacteria that live in close association with eukaryotes and are widely distributed in many different environments. The Rhizobiales encompass facultative intracellular pathogens like Bartonella and Brucella as well as plant-associated soil bacteria such as Agrobacterium tumefaciens and Sinorhizo-bium meliloti. Species of the genus Brucella may cause abortions in pregnant animals, but differ from Bartonella in that they are not vector-borne. The Rickettsiales consist of obligate intracellular bacteria, such as Rickettsia and Wolbachia. Like Bartonella, Rickettsia species are vector-borne bacteria that circulate among mammalian hosts and arthropod vectors. Interestingly, both genera contain louse-borne human pathogens, Rickettsia prowazekii, the typhus pathogen, and B. quin-

tana, the trench fever agent. These are, however, not phylogenetically related but have developed similar lifestyles independently of each other.

Phylogenetic reconstructions have predicted that the common ancestor of the a-proteobacteria contained 3000-5000 genes [1]. There are two different evolutionary routes from this ancestor to the modern species: intracellular bacteria like Bartonella, Rickettsia, and Wolbachia have evolved by gene loss, whereas the soil-growing bacteria associated with plants have evolved by genome expansion [1].

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