Rickettsia hoogstraalii sp. nov., isolated from hard- and soft-bodied ticks

A novel spotted fever group Rickettsia was found in Haemaphysalis sulcata ticks collected from sheep and goats in Croatia in 2006. At the same time, a genetically identical organism was co-isolated with the embryonic cell line CCE3 obtained from the soft tick Carios capensis in Georgia, USA. In this study, further phenotypic and genotypic characteristics of the novel rickettsial strain present in H. sulcata ticks were investigated. Based on the cultivation of bacteria in mosquito and Vero cell cultures, the presence of rickettsiae in tick tissues and cell cultures [confirmed by transmission electron microscopy (TEM)] and the amplification and sequencing of five rickettsial genes, it was demonstrated that the novel Rickettsia strain fulfils the criteria to be classified as a novel species. The name Rickettsia hoogstraalii sp. nov. is proposed for the new strain. Rickettsia hoogstraalii sp. nov., an obligately intracellular bacterium, was grown in Vero cells and arthropod CCE3, ISE6 and C6/36 cell lines. The morphology of the cells of the novel species was typical of SFG rickettsiae. The small coccobacillary appearance of the bacteria was apparent with light microscopy. A Gram-negative bacterial cell wall and a cytoplasmic membrane separated by a narrow periplasmic space were visible by TEM. To date, Rickettsia hoogstraalii sp. nov. has been isolated from two species of ticks, H. sulcata and C. capensis. The novel species appears to be geographically widely distributed, having been detected in Croatia, Spain and Georgia, USA. Although no information is available regarding the possible pathogenicity of the novel species for vertebrate hosts, R. hoogstraalii sp. nov. has a cytopathic effect in Vero, CCE3 and ISE6 cells. Sequence analyses of the 16S rRNA, 17 kDa, gltA, ompA and ompB genes indicated that even though R. hoogstraalii sp. nov. was closely related to Rickettsia felis, it represents a separate species within the spotted fever group. The type strain of R. hoogstraalii sp. nov. is strain CroaticaT (=DSM 22243T=UTMB 00003T).