BLOOD-STREAM-ASSOCIATED VERSUS TICK-ASSOCIATED VARIANTS OF A RELAPSING FEVER BACTERIUM

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