Surface protein variation by expression site switching in the relapsing fever agent Borrelia hermsii

Borrelia hermsii, an agent of relapsing fever, undergoes antigenic variatio n of serotype-specifying membrane proteins during mammalian infections. Whe n B. hermsii is cultivated in broth medium, one serotype, 33, eventually pr edominates in the population. Serotype 33 has also been found to be dominan t in ticks but not in mammalian hosts. We investigated the biology and gene tics of two independently derived clonal populations of serotype 33 of B. h ermsii. Both isolates infected immunodeficient mice, but serotype 33 cells were limited in number and were only transiently present in the blood. Prob es for vsp33, which encodes the serotype specifying Vsp33 outer membrane pr otein, revealed that the gene was located on a 53-kb linear plasmid and tha t there was only one locus for the gene in serotype 33. The vsp33 probe and probes for other variable membrane protein genes showed that expression of Vsp33 was determined at the level of transcription and that when the vsp33 expression site was active, an expression site for other variable proteins was silent. The study confirmed that serotype 33 is distinct from other se rotypes of B. hermsii in its biology and demonstrated that B. hermsii can c hange its major surface protein through switching between two expression si tes.