All members of the Borrelia
genus that have been examined harbour a linear chromosome that is about 900 kbp in length, as well as a plethora of both linear and circular plasmids in the 5–220 kbp size range. Genome sequences for 27 Lyme disease Borrelia
isolates have been determined since the elucidation of the B. burgdorferi
B31 genome sequence in 1997. The chromosomes, which carry the vast majority of the housekeeping genes, appear to be very constant in gene content and organization across all Lyme disease Borrelia
species. The content of the plasmids, which carry most of the genes that encode the differentially expressed surface proteins that interact with the spirochete's arthropod and vertebrate hosts, is much more variable. Lyme disease Borrelia
isolates carry between 7–21 different plasmids, ranging in size from 5–84 kbp. All strains analyzed to date harbor three plasmids, cp26, lp54 and lp17. The plasmids are unusual, as compared to most bacterial plasmids, in that they contain many paralogous sequences, a large number of pseudogenes, and, in some cases, essential genes. In addition, a number of the plasmids have features indicating that they are prophages. Numerous methods have been developed for Lyme disease Borrelia
strain typing. These have proven valuable for clinical and epidemiological studies, as well as phylogenomic and population genetic analyses. Increasingly, these approaches have been displaced by whole genome sequencing techniques. Some correlations between genome content and pathogenicity have been deduced, and comparative whole genome analyses promise future progress in this arena.