Among the Spirochaetes, the Borrelia burgdorferi sensu lato complex is responsible for Lyme borreliosis. This complex comprises more than 13 Borrelia species. Four of them are clearly pathogenic for humans: B. burgdorferi sensu stricto, B. afzelii, B. garinii and B. spielmanii. They can generate erythema migrans, an initial skin lesion, and can then spread deeply into the host to invade distant tissues, especially the nervous system, the joints or the skin. In humans, Borrelia pathogenicityseems to be linked with taxonomic position, but in vitro studies show the role of plasmids in B. burgdorferi s.l. pathogenesis. The inter- and intraspecies genetic diversity of B. burgdorferi s.l. evidences a clonal evolution of the chromosome, while plasmid genes are quite variable, suggesting their major role in Borrelia adaptability. The plasmid-encoded adhesins and vlse, crasps and osp genes determine invasiveness and host immune evasion of B. burgdorferi s.l., and select the bacterial host spectrum. The geographic distribution of B. burgdorferi s.l. is closely related to its vectors and competent hosts, and its development within these influences its diversity, taxonomy and pathogenesis, primarily via genetic lateral transfer.
Copyright 2009 S. Karger AG, Basel.