The Lyme disease spirochete, Borrelia burgdorferi is a bacterial species of the spirochete class of the genus Borrelia, which has a double-membrane envelope. Borrelia burgdorferi is predominant in North America, but also exists in Europe, and is the agent of Lyme disease. Borrelia burgdorferi alters the proteins expressed on its outer surface, depending on the state of each host.