Description of Bartonella bennetti sp. nov., a novel rodent-associated species, with comparative genomics of the Bartonella genus

The genus <i>Bartonella</i> comprises over 40 species, most of which are haemoparasites of mammals. Herein, we describe <i>Bartonella bennetti</i> sp. nov., a novel member of the genus, isolated from field voles (<i>Microtus agrestis</i>) in Kielder Forest, UK. Polyphasic characterization of three strains (C271^T, D105 and J117) of the proposed species indicated that they were closely related to members of phylogenetic lineage 3 (L3) of the genus. The average nucleotide identity (ANI) between C271^T and other L3 species ranged between 88.8 and 90.6%, supporting the proposal of a new species. C271^T shared ANIs approaching 96% with other members of L3 that are yet to be validly published but exhibited marked genomic, ecological and biogeographical differences from them, further justifying the creation of a new taxon. All three <i>B. bennetti</i> sp. nov. strains were found to possess genes encoding three VirB/D4 type IV secretion systems and associated effector proteins and to harbour a chromosomally integrated F-type conjugative plasmid, which forms an Hfr-like configuration not previously observed in the <i>Bartonella</i> genus. This integration could facilitate large-scale chromosomal gene transfer during conjugation, with potential consequences for adaptation, recombination and niche differentiation. The phylogenetic structure of L3, coupled with the ecological partitioning of its members, suggests that covert host specificity, not generalism, is the dominant mode of diversification. The absence of the Trw system, which facilitates host switching in lineage 4 (L4), may constrain ecological breadth in L3, thought to be undergoing a parallel adaptive radiation with L4. The discovery of <i>B. bennetti</i> sp. nov. underscores the importance of combining genomic, ecological and evolutionary evidence when delimiting species boundaries in <i>Bartonella</i> and raises new hypotheses about the role of Hfr-mediated recombination in the evolutionary dynamics of host-adapted pathogens. The type strain of <i>B. bennetti</i> sp. nov. is C271^T (CSUR B1113^T, NCTC 15117^T).