Borrelia hermsii and Borrelia burgdorferi, two closely related spirochetes, are the etiological agents of tick-borne relapsing fever and
Lyme disease, respectively. Previous studies have shown the loss of infectivity of B. burgdorferi is associated with in vitro cultivation. This diminished infectivity of B. burgdorferi has occurred as early as three in vitro passages, and the loss of plasmids have been observed with these less virulent to noninfective cultures. The effects of long-term in vitro cultivation on B. hermsii have not been investigated. However, understanding the degree of genomic degradation during in vitro cultivation is important for investigating pathogenic mechanisms of spirochetes. In this study, we analyzed the effects of continuous in vitro cultivation on the genomic composition and infectivity of B. hermsii and B. turicatae.We report that all seven isolates of B. hermsii and the one isolate of B. turicatae examined retained infectivity in mice after 1 year of continuous in vitro cultivation. Furthermore, there were few apparent differences in the plasmid profiles after long-term cultivation. Two isolates of B. hermsii remained infective after high passage despite losing a portion of the 200-kb linear plasmid containing the fhbA gene encoding the factor H binding protein. Also, sequence analysis of multiple B. hermsii isolates demonstrated two types of fhbA with complete congruence with the two genomic groups of B. hermsii spirochetes. Therefore, these results suggest that relapsing fever spirochetes are genetically stable during in vitro cultivation, and the fhbA-containing segment of DNA that is lost during cultivation is not required for infection.