Oral anaerobic treponemes are associated with active periodontal disease and may comprise up to 57% of the microbiota in periodontal pockets. Four treponeme strains (designated U2a, U2b, U9b, and U9c) isolated from clinical cases were found to harbor a new 4.2‐kb plasmid when plasmid DNA was extracted and purified employing the Qiagen Plasmid Kit. This plasmid differs from the smaller plasmids (2.0‐, 2.6‐, and 2.7‐kb) reported previously by others inTreponema denticola.The newly discovered 4.2‐kb plasmid was found to be the same in all four treponeme strains by restriction endonuclease analysis. It is a circular plasmid since restriction with Pst I, Pvu II, Sma I, Xma I, Ava I or Bam HI produced a single band of the same size. Bacterial strain U2b was shown to beTreponema socranskiiand U9c to beT. denticola.The plasmid is designated “pTS1”. The presence of the same plasmid in different species of the treponemes isolated from the same patient suggests the possibility of a naturally occurring genetic transfer system within the oral spirochetes or their ability to take up and maintain mobiliz
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