Aetiology
Borrelia burgdorferi sl is an obligate parasite whose life cycle is maintained by hard ticks in the genus Ixodes and a large spectrum of mammalian, avian and reptilian hosts (see (3) for a review)(13)- The spirochaete does not survive for long periods outside a vector or a host-
Borrelia burgdorferi sl has a unique genome consisting of one linear chromosome and the largest number of plasmids of any characterized bacterium, comprising approximately 40% of the genome of the B.
burgdorfers type strain. Only 8% of the genes on the plasmids have homologues outside Borrelia, and these similarities are not known to be bacterial virulence genes, suggesting that the plasmid genes may encode specific functions unique to this genus and that B. burgdorferi interacts with its hosts in basically different ways than other known bacterial pathogens(14)- At least one of the plasmids has been demonstrated to be essential under all growth conditions tested, and loss of others correlates with loss or reduction of infectivity in laboratory animals. Correspondingly, many of the genes necessary for evading the immune system and causing disease are probably plasmid-encoded. The plasmids seem to be in a state of rapid evolution, and their number, size and gene order vary substantially among strains and between species. Importantly, horizontal gene transfer between strains of Borrelia does occur(14).Each genospecies of B. burgdorferi sl may to a certain extent be characterized by its vectors, host spectrum, organ affinity and geographical distribution. Different genospe- cies are associated with distinct clinical manifestations of Lyme borreliosis in humans. Accurate typing of genospe- cies relies heavily on molecular methods (see Wang, 1999 or Bergstrom et al., 2002 for a review)(15)(16). Serotyping of European and North American strains, using monoclonal antibodies against two outer surface proteins, OspA and OspC, has allowed identification of several serotypes(17). Other methods of classification require cultivation of the spirochaetes, as ribotyping, pulsed-field gel electrophoresis