In other words, given two surface proteins there are four possible combinations. Some bacteria could express only OspA, some only OspC, some might express both, some neither.

"And in fact during the tick feeding process we found all four of these different combinations," da Silva said. Moreover, when the study team looked at another Borrelia surface gene called vlsE, they found a large number of variations generated during tick feeding, compared to only one or two when the tick was not feeding.

"We are excited by the findings because this once again supports the concept that arthropod vectors are not just flying or crawling syringes that go around inoculating bacteria. There's a developing biology going on inside this vector. The bacteria population essentially adapts during the transmission process to maximize the chance of infecting the host," de Silva said According to de Silva, individuals in the bacterial population expressing lots of different sets of surface proteins makes it easier to evade the host's immune response.

"If all the bacteria entering the host have the same set of molecules on the surface, then it's easier to for the immune system to control the infection and it's easier to develop vaccines against it," de Silva explained. "But that fact that the tick introduces so many different flavors probably explains an observation we made several years ago: Lyme spirochetes delivered by ticks are better at evading the host's immune response than cultured spirochetes injected into animals.

The process of bacterial diversity occurs during the blood meal, de Silva pointed out. "So, early on, before the blood meal, the population in the tick gut is still very uniform. Lyme disease ticks feed for 3 to 5 days, and over this period the bacterial population inside the ti
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Contact: Leslie Lang
llang@med.unc.edu
919-843-9687
University of North Carolina School of Medicine
15-Jan-2001