Characterization of a 4.2-kb plasmid isolated from periodontopathic spirochetes
Oral anaerobic spirochetes are strongly implicated in the initiation and progression of periodontal disease.
Dr. Suzanne Caudry
Periodontist
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Background & clinical context
Oral anaerobic spirochetes are strongly implicated in the initiation and progression of periodontal disease. In active disease, they can comprise up to 57% of the total subgingival microbiota. Among these, Treponema denticola is the most extensively studied species and is considered a key member of the "red complex" of periodontal pathogens.
Plasmids are of significant interest in microbiology because they can carry genes that enhance virulence and, critically, can be transferred between bacteria, potentially spreading those traits across species. Smaller plasmids (2.0–2.7 kb) had previously been identified in T. denticola, but their full significance was not well understood.
This study reports the discovery and characterization of a previously undescribed, larger 4.2-kb plasmid found in clinical spirochete isolates; now designated pTS1.
Key findings
Discovery of the plasmid
Subgingival plaque samples were collected from periodontal pockets of patients attending the McGill University Dental Clinic. Nine spirochete strains were purified from these samples.
When plasmid DNA was extracted and analyzed by agarose gel electrophoresis, four of the nine strains — designated U2a, U2b, U9b, and U9c — were found to carry plasmid DNA. The plasmid appeared as two bands: a faster-migrating supercoiled form and a slower-migrating relaxed circular form, both characteristic of a circular plasmid.
Structural characterization
The plasmid was subjected to extensive restriction endonuclease analysis to map its structure and confirm its identity across all four isolates. Single-enzyme digestion with six different enzymes each produced a single 4.2-kb band, confirming the plasmid is circular.
Double digests with multiple enzyme combinations generated consistent, reproducible fragment patterns across all four strains, confirming the plasmid is identical or very closely related in all four isolates.
Analysis | Finding |
|---|---|
Plasmid size | 4.2 kb — larger than previously described spirochete plasmids (2.0–2.7 kb) |
Topology | Circular — confirmed by single-enzyme linearization producing one band |
Identity across strains | Restriction profiles from all four isolates (U2a, U2b, U9b, U9c) were identical or very closely related |
Restriction sites absent | No sites found for EcoR I, Kpn I, Sal I, Bgl I, Xba I, Not I, Nru I, and 11 additional enzymes |
Designation | pTS1 — first plasmid described in T. socranskii |
Species identification of the host strains
Two of the four plasmid-harboring strains were identified at the species level. Strain U2b was identified as Treponema socranskii based on its G+C mol% content of 51%, its slower growth rate (consistent with T. socranskii reference strains), and 99% 16S rRNA sequence homology with T. socranskii. Strain U9c was identified as T. denticola based on a G+C mol% of 39% and growth kinetics matching the reference strain. Strains U2a and U2b are therefore considered T. socranskii, while U9b and U9c are T. denticola.
The most significant finding |
|---|
Strains U2b (T. socranskii) and U9c (T. denticola) were isolated from the same patient, yet both carried the identical 4.2-kb plasmid. The presence of the same plasmid in two different spirochete species from a single host strongly suggests that horizontal genetic transfer occurs naturally between oral treponeme species, or that these bacteria can acquire and stably maintain mobilizable plasmids from their environment. This has direct implications for how virulence traits may spread within the subgingival microbiome. |
Why plasmids in periodontal bacteria matter
Plasmids are vehicles for genetic mobility. In other bacterial species, plasmids are well established as carriers of antibiotic resistance genes, toxin genes, and other virulence factors.
The discovery that oral spirochetes harbour plasmids raises important questions about whether these elements contribute to their pathogenicity and whether virulence traits can be shared between species within the same periodontal pocket.
The pTS1 plasmid's similarity to a class of mobilizable plasmids from Gram-positive bacteria (previously noted for the smaller pTD1 plasmid in T. denticola) further suggests these spirochetes may participate in broader inter-species genetic exchange, a finding with potential implications for both disease progression and the evolution of antimicrobial resistance in the oral cavity.
Context and limitations
This is a characterization study — it describes the plasmid's structure and confirms its presence across strains, but does not yet identify what genes the plasmid carries or what function it serves.
Whether pTS1 encodes any virulence-related genes, confers antibiotic resistance, or influences the pathogenicity of its host species remains to be determined. The study also screened only nine strains from a single clinical setting, so the prevalence of this plasmid across broader populations of periodontal patients is unknown.
