sp. nov.: Relapsing Fever Spirochetes Transmitted By the Tick Designated Previously as Genomic Group II.

The taxonomic status of the relapsing fever spirochete in western North America was established in 1942 and based solely on its specific association with the soft tick vector . Multilocus sequence typing (MLST) of the , , , , and intergenic spacer of isolates collected over many years from various geographic locations and biological sources identified two distinct clades designated previously as Genomic Group I (GGI) and Genomic Group II (GGII). To better assess the taxonomic relationship of these two genomic groups to each other and other species of , DNA sequences of the entire linear chromosome were determined. Genomic DNA samples were prepared from 11 spirochete isolates grown in Barbour-Stoenner-Kelly-H medium. From these preparations, DNA sequences of the entire linear chromosome of two isolates of belonging to each genomic group and seven additional species were determined. Chromosomal sequences of four isolates of contained 919,212 to 922,307 base pairs. DNA sequence identities between the two genomic groups of were 95.86-95.99%, which were more divergent than chromosomal sequences comparing and (97.13%), and (97.07%), and and (97.09%). The 3' end of the chromosome of the two GGII isolates also contained a unique intact gene absent from all other species examined. Previous MLST and the chromosomal sequences presented herein support the division of the species complex into two species, sensu stricto ( = GGI) and sp. nov. ( = GGII). We name this unique relapsing fever spirochete in honor of our late friend and colleague Dr. Nathan Nieto for his outstanding contributions to our understanding of tick-borne relapsing fever.