Analysis of the distribution and molecular heterogeneity of the ospD gene among the Lyme disease spirochetes: evidence for lateral gene exchange

Analysis of the ospD gene has revealed that this gene is not universal among Lyme disease spirochete isolates. The gene was found to be carried by 90, 50, and 24 % of the Borrelia garinii, B. afzelii, and B. burgdorferi isolates tested. Size variability in the ospD-encoding plasmid was also observed. Sequence analysis has demonstrated the presence of various numbers of a 17-bp repeated sequence in the upstream control (promoter) region of the gene. In addition, a region within the coding sequence where various insertions, deletions, and direct repeats occur was identified. ospD gene sequences from 31 different isolates were determined and utilized in pairwise sequence comparisons and construction of a gene tree. These analyses suggest that the ospD gene was the target of several recombinational events and that the gene was recently acquired by Lyme disease spirochetes and laterally transferred between species. Several recent studies have demonstrated that the causative agent of Lyme disease, the Borrelia burgdorferi sensu lato complex (10, 21, 38), comprises at least three distinct Borrelia species (2, 26, 42). A revised nomenclature has been proposed (2), designating these species B. burgdorferi, B. garinii (la), and B. afzelii sp. nov. (11) (previously referred to as group VS461 [2] or group 3 [26]). Several of the outer surface proteins (Osps) associated with these Lyme disease spirochete (LDS) species, including OspA, OspB (7), and OspC (19), have been identified and characterized. Recently a fourth Osp, OspD, has been identified (32). All have been demonstrated to be li-poproteins (9, 32). OspA, OspB, and OspC have been the major focus in efforts to develop a Lyme disease vaccine (17, 18, 33) or diagnostic test (39). As an extension of these efforts, the variability of these proteins among the three LDS specie