DNA characterization of the spirochete that causes Lyme disease

Abstract

Lyme disease, a tick-borne disease long recognized in Europe but only recently recognized in the United States, was shown in 1982-1983 to be caused by a spirochete, the Lyme disease spirochete. Whether one or more species of the spirochete exists is unknown, as is its taxonomic status. To answer these questions, we determined (i) the DNA base (guanidine-plus-cytosine) content for five strains; (ii) the DNA relatedness of 10 strains from Europe or the United States (isolated from ticks, humans, and a mouse) by DNA hybridization (hydroxyapatite assay at 50 and 65 degrees C); and (iii) the DNA relatedness to other pathogenic spirochetes. The guanine-plus-cytosine content of the Lyme disease spirochete strains was 27.5 to 29.0 mol%, most similar to those of Borrelia hermsii (30.6 mol%) and Treponema hyodysenteriae (25.6 mol%) among the other spirochetes tested. DNA hybridization studies with 32P-labeled DNA from Lyme disease spirochete strain TLO-005, a human blood isolate, revealed divergence (unpaired bases) within related nucleotide sequences of only 0.0 to 1.0% for all nine Lyme disease spirochete strains tested for relatedness to TLO-005. Relatedness values of seven strains to TLO-005 were 58 to 98% (mean, 71%) in 50 degrees C reactions and 50 to 93% (mean, 69%) in 65 degrees C reactions. Two other strains, from which very low yields of DNA were obtained, showed less relatedness (36 to 50 degrees C, 38 to 47% at 65 degrees C). These were nonetheless considered to belong to the same species because of the low amount of divergence in the sequences related to TLO-005 and the absence of decreased relatedness in reactions done at 65 degrees Celsius compared with those done at 50 degrees Celsius. DNA from strain TLO-005 showed relatedness of 1% to DNAs of two leptospires and 16% relatedness to DNA from T. hyodysenteriae. B. hermsii DNA was 30 to 40% related to three Lyme disease spirochete strains in 50 degrees Celsius reactions. Divergence in these reactions was 16.5 to 18.5%, and relatedness in 65 degree Celsius reactions was 8 to 10%. On the bases of phenotypic similarity, guanine-plus-cytosine content, and DNA relatedness to B. hermsii, we conclude the Lyme disease spirochete is single previously undescribed species which belongs in the genus Borrelia.