Bartonella bacteria in nature: Where does population variability end and a species start?

► Bartonella are emergent pathogens with a unique hemotropic parasitic strategy. ► Bartonella genetic diversity is great, particularly among rodent and bat species. ► We propose a species complex approach that will help resolve species definitions. The application of new molecular approaches has permitted the differentiation of numerous strains belonging to the genus Bartonella and identification of new Bartonella species. However, the molecular typing of these organisms should be coupled with studies aimed at defining the biological properties of the newly described species. The long-history of co-adaptation between bartonella1Bartonella species are capitalized and italicized in accordance with the guidance for bacterial nomenclature. The common name ‘bartonella’ is neither capitalized nor italicized and here is commonly used as a non-specific adjective when we talk about a bartonella bacterium or about a bartonella infection. Bartonellae is the plural non-specific and neither capitalized nor italicized.1 bacteria and their mammalian hosts and possibly arthropod vectors provides a unique opportunity for applying this information for the sub-genus taxonomy. There can be a varying level of association between the bacteria and their hosts, ranging from animal species to animal genus to animal community. The commonality is that any level of association provides a certain degree of isolation for a given bartonella population that can mimic ‘biological isolation’. Such an association defines a specific ecological niche and determines some specific characteristics, including sequence types that can be used as markers for demarcation of bacterial species. Usage of a combination of genetic markers and ecological information can delineate a number of species complexes that might combine several genospecies, named strains, and unique genotypes. The identification of such species complexes can be presented as (1) separate phylogenetic lineages distantly related to other species (e.g. Bartonella bacilliformis); (2) clusters of genetically similar strains associated with a specific mammalian group (e.g. Bartonella elizabethae species complex); and (3) clusters of genetically similar strains that combine a number of ecotypes (e.g. Bartonella vinsonii species complex).