Evolution and distribution of the ospC Gene, a transferable serotype determinant of Borrelia burgdorferi

Abstract

Borrelia burgdorferi, an emerging bacterial pathogen, is maintained in nature by transmission from one vertebrate host to another by ticks. One of the few antigens against which mammals develop protective immunity is the highly polymorphic OspC protein, encoded by the ospC gene on the cp26 plasmid. Intragenic recombination among ospC genes is known, but the extent to which recombination extended beyond the ospC locus itself is undefined. We accessed and supplemented collections of DNA sequences of ospC and other loci from ticks in three U.S. regions (the Northeast, the Midwest, and northern California); a total of 839 ospC sequences were analyzed. Three overlapping but distinct populations of B. burgdorferi corresponded to the geographic regions. In addition, we sequenced 99 ospC flanking sequences from different lineages and compared the complete cp26 sequences of 11 strains as well as the cp26 bbb02 loci of 56 samples. Besides recombinations with traces limited to the ospC gene itself, there was evidence of lateral gene transfers that involved (i) part of the ospC gene and one of the two flanks or (ii) the entire ospC gene and different lengths of both flanks. Lateral gene transfers resulted in different linkages between the ospC gene and loci of the chromosome or other plasmids. By acquisition of the complete part or a large part of a novel ospC gene, an otherwise adapted strain would assume a new serotypic identity, thereby being comparatively fitter in an area with a high prevalence of immunity to existing OspC types.

FIG 1

Unrooted distance phylogram with recombination network for codon-aligned ospC genes of 31 strains of Borrelia burgdorferi and 1 strain of B. bissettii, as implemented by SplitsTree. Nodes with posterior probabilities of ≥0.95 by Bayesian inference are shown. The scale bar indicates the distance.

FIG 2

Population structure of the ospC gene of B. burgdorferi in Ixodes scapularis nymphs in the Northeast (23 sites) and the Midwest (28 sites) regions of the United States for the years 2004 to 2007. Upper panel, overview map of posterior mode of population membership. Middle and lower panels, contour of posterior probabilities (0.1 to 1.0) for two populations defined by 27 ospC sequence types and subtypes identified in these regions. The y coordinates are latitude, and the x coordinates are longitude. The data for the analysis and the resultant posterior probabilities for cluster 1 or 2 for each site are given in Data Set S1 in the supplemental material.

FIG 3

Recombination detection analysis of cp26 plasmids of 11 strains of B. burgdorferi. The ospC coding sequences were removed at the position indicated by the point of the arrowhead. The alignment was subjected to the SciScan algorithm with a window size of 200, a step size of 20, and 100 permutations. The x axis provides the nucleotide positions of the alignment. The bottom panel (“Hits” on the y axis) indicates the sequence region where recombination was detected. The middle panel gives the number of estimated breakpoints per window. The top panel indicates the log₁₀ of the P value for the test statistic for recombination detection.

FIG 4

Patterns of variants of flanking regions for ospC genes of pairs and trios of B. burgdorferi strains. The 13 oligonucleotide characters and their variants are given in Fig. S6 in the supplemental material, and the alignment is given in Data Set S2 in the supplemental material. The characters are indicated by italicized lowercase letters. Five characters (a to e) are from the 5′ flanking region for the ospC gene, two characters (f and g) are from the ospC gene itself, and six characters (h to m) are from the 3′ flanking region. The ospC type or subtype is indicated in the leftmost column. The geographic regions are the Northeast (1), the Midwest (2), northern California (3), and Europe (4). The rrs-rrlA intergenic spacer and MLST genotypes were as defined by Travinsky et al. (25). Groups I, II, and III are described in the text.

FIG 5

Alignment of sequences of the upstream and promoter regions for ospC genes of subtype Ha and Hb strains of B. burgdorferi. The locations of inverted repeats are indicated by arrows. Positions are numbered with respect to the transcriptional start site for ospC.

FIG 6

Nucleotide polymorphisms between the cp26 plasmid sequences, excluding the ospC genes, of strains 72a and 118a of B. burgdorferi. The position in the sequence where the ospC gene sequence was deleted is shown by an arrow.