Variable tick protein in two genomic groups of the relapsing fever spirochete Borrelia hermsii in western North America

Abstract

Borrelia hermsii is the primary cause of tick-borne relapsing fever in North America. When its tick vector, Ornithodoros hermsi, acquires these spirochetes from the blood of an infected mammal, the bacteria switch their outer surface from one of many bloodstream variable major proteins (Vmps) to a unique protein, Vtp (Vsp33). Vtp may be critical for successful tick transmission of B. hermsii; however, the gene encoding this protein has been described previously in only one isolate. Here we identified and sequenced the vtp gene in 31 isolates of B. hermsii collected over 40 years from localities throughout much of its known geographic distribution. Seven major Vtp types were found. Little or no sequence variation existed within types, but between them significant variation was observed, similar to the pattern of diversity described for the outer surface protein C (OspC) gene in Lyme disease spirochetes. The pattern of sequence relatedness among the Vtp types was incongruent in two branches compared to two genomic groups identified among the isolates by multilocus sequence typing of the 16S rRNA, flaB, gyrB, and glpQ genes. Therefore, both horizontal transfer and recombination within and between the two genomic groups were responsible for some of the variation observed in the vtp gene. O. hermsi ticks were capable of transmitting spirochetes in the newly identified genomic group. Therefore, given the longevity of the tick vector and persistent infection of spirochetes in ticks, these arthropods rather than mammals may be the likely host where the exchange of spirochetal DNA occurs.

FIG. 1.

Plasmid profiles of representative isolates of B. hermsii demonstrating patterns associated with the two genomic groups based on DNA sequence analysis (see subsequent figures). Isolate designations are shown above each lane, and DNA size estimates are shown on the right in kilobases. Arrows on the left show the positions of the circular plasmids (c.p.), chromosome (ch.), and various-sized linear plasmids (l.p.).

FIG. 2.

Phylogram of the concatenated sequences (flaB-glpQ-gyrB) of B. hermsii isolates and B. turicatae 91E135 used for the outgroup. The tree was constructed with CLUSTAL V and the neighbor-joining method with 1,000 bootstrap replicates. Numbers at the nodes are the percentages of bootstraps that supported this pattern. The scale bar for the branch lengths represents the number of substitutions per site.

FIG. 3.

Phylogram of the16S rRNA sequences of one member of each genomic group of B. hermsii, other representative species of relapsing fever spirochetes, and B. burgdorferi used for the outgroup. The tree was constructed and analyzed as described for Fig. 2.

FIG. 4.

(A) Tree of the vtp sequences of the B. hermsii isolates and ospC of B. burgdorferi B31 used for the outgroup. The tree was constructed and analyzed as described for Fig. 2. All isolates in GGII are distinguished from GGI isolates with bold and underlined names. Probable examples of horizontal transfer of vtp sequences between spirochetes in the two genomic groups are shown within dashed-line boxes. (B) Distribution of polymorphic sites in the vtp gene for all isolates in each genomic group. A sliding window of 100 with a step size of 25 bases was used for same length alignments of the vtp genes. The mean nucleotide diversity (π) is on the y axis in relation to the nucleotide position on the x axis.

FIG. 5.

Alignment of deduced amino acid sequences for single isolates in each of the seven Vtp types identified in B. hermsii. The consensus is shaded and shown below the alignment. BYM (type 1), All (type 2), BAK (type 3), FRO (type 4), YOR (type 5), DAH (type 6), SIL (type 7). BYM, ALL, BAK, FRO, and DAH are in GGI; YOR and SIL are in GGII.

FIG. 6.

Mapping the location of the vtp gene with type-specific DNA probes. Plasmids from B. hermsii representing five of the Vtp types are shown in the agarose gel in the left panel. Southern blots containing these DNAs probed with different vtp probes are shown in the other panels, demonstrating their specificity. The isolate source for the DNA is shown above each lane along with its genomic group. The origins of the probes and Vtp types are shown below each panel. Molecular size standards (MSS) are shown on the left in kilobases.