The tick-borne bacteria responsible for Lyme disease, Borrelia burgdorfi is a wily invader. It causes an array of vague symptoms, or sometimes none at all, making diagnosis difficult. Yet if left untreated, the disease can lead to serious inflammation of the nervous system, heart and joints. The newly approved Lyme vaccine protects 75 to 80 percent of people exposed to infected ticks, but recent research suggests that the effects may not last over the long term, and some doubts have been raised over its safety. So with an eye toward developing better vaccines, researchers at the University of North Carolina and the Centers for Disease Control and Prevention decided to further probe the biology of B. burgdorfi transmission. Their findings, published yesterday in the Proceedings of the National Academy of Sciences, reveal that the microbe is even sneakier than scientists thought.

"Previous studies showed there were certain surface proteins that were expressed in the tick gut and others that were expressed in the host, and that a switch occurred en route from the tick to the host," team member Aravinda M. de Silva notes. "This paper shows us that things are a lot more complicated." On closer examination of the surface protein expression, the researchers made a key observation: during feeding, the tick actually transmits a highly variable population of B. burgdorfi into the host. In contrast, when the tick isn't feeding, the population is fairly homogeneous. What this means, says de Silva, is that "the bacteria essentially adapts during the transmission process to maximize the chance of infecting the host." Intriguingly, the fact that the tick spits so many different "flavors" of B. burgdorfi into the host seems to explain an observation made several years ago, de Silva muses: lyme bacteria delivered by ticks evade the host's immune response more successfully than do cultured bacteria injected into animals.

The team suggests that future efforts to develop better vaccines could focus either on antigens produced within the tick before the bacteria population diversifies or on surface proteins common to all of the otherwise variable bacteria.

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