Bloodstream- versus tick-associated variants of a relapsing fever bacterium.

The relapsing fever spirochete, Borrelia hermsii, alternates infections between a mammal and a tick vector. Whether the spirochete changes phenotypically in the different hosts was examined by allowing the tick vector Ornithodoros hermsi to feed on mice infected with serotype 7 or serotype 8 of B. hermsii. Upon infection of ticks, the spirochetal serotype-specific variable major proteins (Vmps) 7 and 8 became undetectable and were replaced by Vmp33. This switch from a bloodstream- to tick-associated phenotype could be induced in culture by a decrease in temperature. After tick-bite transmission back to mice, the process was reversed and the spirochetes resumed expression of the same Vmp present in the previous infectious blood meal.