Molecular epidemiology of the aetiological agents of Lyme borreliosis

Ten species are up to now recognized among Borrelia burgdorferi sensu late complex. Among those, only three (Borrelia burgdorferi sensu stricto, B. ga rinii and B. afzelii) are reported to be pathogenic for humans and each res ponsible for a predominant clinical form of Lyme borreliosis. Each species is characterized by its vectors (Ixodidae), its host spectrum, its organotropism (for the pathogenic ones) and its geographical repartiti on. Borrelia are strictly parasitic and essentially clonal bacteria. Our goal w as to explore the diversity of this bacterial complex. We selected, by seve ral molecular markers, atypical isolates and compared them to already known species representative strains by RFLP or sequencing. The results show an unexpected diversity at a level which could be a species one leading to the conclusion that the structure of the Borrelia burgdorferi sensu late compl ex is a high number of small (by their populations) clones among which emer ge some large ones usually corresponding to the pathogenic species. Our dat a also allow to speculate on when, where and how these species evolved and migrated.