Rickettsia raoultii sp. nov., a spotted fever group rickettsia associated with Dermacentor ticks in Europe and Russia

This study describes the characterization and formal naming of Rickettsia raoultii sp. nov., a novel spotted fever group rickettsia identified in Dermacentor ticks collected across Europe and Russia. Researchers cultivated five rickettsial isolates from four different Dermacentor tick species (D. silvarum, D. reticulatus, D. marginatus, and D. nuttalli) collected in France, Russia, and Kazakhstan. Using multigene sequencing of five genes (16S rRNA, gltA, ompA, ompB, and sca4), the isolates were phylogenetically classified within the R. massiliae group but demonstrated sufficient genetic and serological distinctiveness to warrant species status. The organisms share approximately 99% sequence similarity with R. rhipicephali but fall below species-definition thresholds, and exhibit unique serotypes distinguishing them from related rickettsiae. R. raoultii is rifampicin-resistant, has a G+C content of 33.8–33.9 mol%, and grows intracellularly in cultured cells. The type strain, KhabarovskT, was isolated from D. silvarum ticks in the Russian Far East. The bacterium has been detected across a wide geographic range including France, Spain, Croatia, Russia, and Kazakhstan, with evidence of transovarial and transstadial transmission in tick populations.

Key findings

• Rickettsia raoultii sp. nov. is a novel spotted fever group rickettsia isolated from multiple Dermacentor tick species across Europe and Russia
• Genetic and serological analyses demonstrate R. raoultii is phylogenetically distinct from related rickettsiae, particularly R. rhipicephali, despite high sequence similarity
• R. raoultii exhibits rifampicin resistance due to a Phe-to-Leu mutation in the rpoB gene, consistent with the R. massiliae group
• The bacterium has been detected in France, Spain, Croatia, Russia, and Kazakhstan with evidence of maintenance in tick populations through transovarial transmission