Bartonella species are fastidious Gram-negative bacteria that are highly adapted to a mammalian reservoir host and within which the bacteria usually cause a long-lasting intraerythrocytic bacteremia. These facts are of particular importance to veterinarians and physicians, as an increasing number of animal reservoir hosts have been identified for various Bartonella species. Among numerous other examples, Bartonella henselae has co-evolved with cats, Bartonella vinsonii subsp. berkhoffii has co-evolved with dogs and wild canines, and Bartonella bovis has co-evolved with cattle. Importantly, the list of reservoir-adapted Bartonella species, including a large number of rodent species that might serve as "pocket pets", continues to grow exponentially, as new Bartonella spp. are discovered. Prior to 1990, there were only two named Bartonella species, whereas there are now at least 32 named and numerous unnamed or candidatus species, based upon deposited GenBank sequences or preliminary reports, respectively.In the natural reservoir host, chronic bacteremia with a Bartonella species can frequently be detected by blood culture or PCR in outwardly healthy individuals. In contrast, the diagnostic detection of a Bartonella spp. in a non-reservoir adapted host can be extremely difficult. Most, although not all diseases caused by Bartonella spp., occur in accidental hosts and these organisms are being increasingly implicated as a cause of zoonotic infections. Until recently, mechanisms that facilitate persistentBartonella bacteremia in mammals were not well understood. Recent reports have identified an intra-endothelial and intra-erythrocytic localization for these bacteria, which represents a unique strategy for bacterial persistence. Non-hemolytic intracellular colonization of erythrocytes and endothelial cells would preserve the organisms for efficient vector transmission, protect Bartonella from the host immune response, and potentially contribute to decreased antimicrobial efficacy. Other in vitro studies indicate that Bartonella spp. can infect dendritic cells, microglial cells, pericytes, monocytes and CD34+ bone marrow progenitor cells.
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