Extrachromosomal elements of spirochetes.

The spirochetes include some important pathogenic bacteria, Treponema, Borrelia and Leptospira. The pathogeneses of these spirochetes are very diverse. In an attempt to learn more about the virulence factors among the spirochetes, their genetic organization and capacity have been studied. Structural analysis of the genome in Borrelia has shown that the genome is composed of one linear maxi-chromosome with additional linear minichromosomes as well as several supercoiled circular plasmids. Moreover, the molecular analysis of the terminal ends of one of the linear minichromosomes has revealed that this unique replicon has sequence similarities with poxviruses and particularly the viral agent of African swine fever. The presence of nucleic-acid-containing vesicles and its possible role in mediating DNA transfer between borreliae is an additional, very interesting feature of these organisms. Treponema does not contain any linear DNA, chromosomal or extrachromosomal, however molecular characterization of a 2.6-kb plasmid of Treponema denticola has been performed with the aim of establishing cloning vehicles to study the virulence properties of the genus Treponema.