Gene sequence-based criteria for identification of new Rickettsia isolates and description of Rickettsia heilongjiangensis sp nov

We propose genetic guidelines for the classification of rickettsial isolates at the genus, group, and species levels by using sequences of the 16S rRNA (rrs) gene and four protein-coding genes, the gltA, ompA, and ompB genes and gene D. To be classified as a member of the genus Rickettsia, an isolate should exhibit degrees of rrs and gltA homology with any of the 20 Rickettsia species studied of greater than or equal to98.1 and greater than or equal to86.5%, respectively. A member of the typhus group should fulfill at least two of the following four criteria: pairwise nucleotide sequence homologies with rrs, gltA, ompB, and gene D of either Rickettsia typhi or Rickettsia prowazekii of greater than or equal to99.4, greater than or equal to96.6, greater than or equal to92.4, and greater than or equal to91.6%, respectively. A member of the spotted fever group should either possess the ompA gene or fulfill at least two of the following four criteria: pairwise nucleotide sequence homologies with rrs, gltA, ompB, and gene D of any member of this group of greater than or equal to98.8, greater than or equal to92.7, greater than or equal to85.8, and greater than or equal to82.2%, respectively. The existence of a distinct "ancestral" group should be questioned. To be classified as a new Rickettsia species, an isolate should not exhibit more than one of the following degrees of nucleotide similarity with the most homologous validated species: greater than or equal to99.8 and greater than or equal to99.9% for the rrs and gltA genes, respectively, and, when amplifiable, greater than or equal to98.8, greater than or equal to99.2, and greater than or equal to99.3% for the ompA and ompB genes and gene D, respectively. By use of our classification scheme, "Rickettsia heilongjiangii" belongs to a new species for which we officially propose the name Rickettsia heilongjiangensis sp. nov.