Electron microscopy of Treponema cuniculi

Complete genome sequence of Treponema paraluiscuniculi, strain Cuniculi A: the loss of infectivity to humans is associated with genome decay

Treponema paraluiscuniculi is the causative agent of rabbit venereal spirochetosis. It is not infectious to humans, although its genome structure is very closely related to other pathogenic Treponema species including Treponema pallidum subspecies pallidum, the etiological agent of syphilis. In this study, the genome sequence of Treponema paraluiscuniculi, strain Cuniculi A, was determined by a combination of several high-throughput sequencing strategies. Whereas the overall size (1,133,390 bp), arrangement, and gene content of the Cuniculi A genome closely resembled those of the T. pallidum genome, the T. paraluiscuniculi genome contained a markedly higher number of pseudogenes and gene fragments (51). In addition to pseudogenes, 33 divergent genes were also found in the T. paraluiscuniculi genome. A set of 32 (out of 84) affected genes encoded proteins of known or predicted function in the Nichols genome. These proteins included virulence factors, gene regulators and components of DNA repair and recombination. The majority (52 or 61.9%) of the Cuniculi A pseudogenes and divergent genes were of unknown function. Our results indicate that T. paraluiscuniculi has evolved from a T. pallidum-like ancestor and adapted to a specialized host-associated niche (rabbits) during loss of infectivity to humans. The genes that are inactivated or altered in T. paraluiscuniculi are candidates for virulence factors important in the infectivity and pathogenesis of T. pallidum subspecies.

Genome differences between Treponema pallidum subsp. pallidum strain Nichols and T. paraluiscuniculi strain Cuniculi A

The genome of Treponema paraluiscuniculi strain Cuniculi A was compared to the genome of the syphilis spirochete Treponema pallidum subsp. pallidum strain Nichols using DNA microarray hybridization, whole-genome fingerprinting, and DNA sequencing. A DNA microarray of T. pallidum subsp. pallidum Nichols containing all 1,039 predicted open reading frame PCR products was used to identify deletions and major sequence changes in the Cuniculi A genome. Using these approaches, deletions, insertions, and prominent sequence changes were found in 38 gene homologs and six intergenic regions of the Cuniculi A genome when it was compared to the genome of T. pallidum subsp. pallidum Nichols. Most of the observed differences were localized in tpr loci and the vicinity of these loci. In addition, 14 other genes were found to contain frameshift mutations resulting in major changes in protein sequences. Analysis of restriction target sites representing 0.34% of the total genome length and DNA sequencing of three PCR products (0.46% of the total genome length) amplified from Cuniculi A chromosomal regions and comparison to the Nichols genome revealed a sequence similarity of 98.6 to 99.3%. These results are consistent with a close genetic relationship among the T. pallidum strains and subspecies and a strong, but relatively divergent connection between the human and rabbit pathogens.

Tpr homologs in Treponema paraluiscuniculi Cuniculi A strain

Treponema paraluiscuniculi, the etiologic agent of rabbit venereal syphilis, is morphologically indistinguishable from Treponema pallidum subsp. pallidum (T. pallidum), the human syphilis treponeme, and induces similar immune responses and histopathologic changes in the infected host. Because of their high degree of relatedness, comparative studies are likely to identify genetic determinants that contribute to pathogenesis or virulence in human syphilis. The tpr (Treponema pallidum repeat) genes are believed to code for potential virulence factors. In this study, we identified 10 tpr homologs in Treponema paraluiscuniculi Cuniculi A strain and determined their sequence architecture. Half of this group of paralogous genes were predicted to be nonfunctional due to the presence of frameshifts and premature stop codons. Furthermore, the immune response against the T. paraluiscuniculi Tpr homologs in long-term-infected rabbits was studied by enzyme-linked immunosorbent assay and lymphocyte proliferation assay, showing that TprK is the only target of the antibody and T-cell responses during experimental infection and emphasizing the importance of this putative virulence factor in venereal treponematosis.

Susceptibility of rabbits venereally infected with Treponema paraluis-cuniculi to superinfections with Treponema pallidum

Three female rabbits, venereally infected with Treponema paraluis-cuniculi between five and eight months previously, had treponeme-containing genital lesions and positive results to serological tests for syphilis. These rabbits and four normal female rabbits were challenged with Treponema pallidum intradermally on the shaved back with triplicate doses of 10(4), 10(3), 10(2), and 10 treponemes (all at different sites). Significantly fewer syphilitic lesions developed in the rabbits previously infected with T paraluis-cuniculi (4/36) compared with the control rabbits (37/48), and the mean size of the lesions (diameter of induration) was significantly less (4 mm compared with 10 mm). It appears that previous venereal infection of female rabbits with T paraluis-cuniculi induced a level of cross-immunity against infection with T pallidum. Protection was however not complete.