Bacterial plasmids in the oral and endodontic microflora

Abstract

Bacterial plasmids typically represent covalently closed circular, double‐stranded, supercoiled DNA molecules that replicate independently of chromosomal DNA. Plasmids are frequently not essential to the bacterial host, but can confer traits facilitating survival under atypical conditions e.g. resistance to antibiotics and heavy metals. Plasmids have been identified in clinical and environmental bacteria from all over the world, and range in size from ∼1 to >200 kb. Their copy number in the cell can range from one or two copies per chromosome, to well over 30; the number is usually characteristic for the particular plasmid. Clinical strains of Enterococcus faecalis, a species commonly associated with root canal infections, can carry as many as six co‐resident plasmids with different sizes and copy numbers. In part because of their ability to replicate autonomously, plasmids are used as tools or shuttle vectors in basic molecular biological research. Many plasmids can transfer copies of themselves from one bacterial cell to another by a process called conjugation. In some cases, small peptides called pheromones act as mating signals in this process. Conjugative plasmids that make use of pheromones have been identified previously in E. faecalis and the related plasmids tend to exhibit a narrow host range. Conjugative plasmids in E. faecalis that do not make use of pheromones usually have a much broader host range. Pheromone‐responding plasmids in E. faecalis can carry genes relating to antibiotic resistance as well as virulence traits. In general, plasmids are capable of adaptability and stable maintenance in a wide spectrum of bacterial hosts. The potential role of plasmids in oral and endodontic microbiology is discussed.

Real-time Quantitative Polymerase Chain Reaction and Culture Analyses of Enterococcus faecalis in Root Canals

“…Horizontally transferred genes can encode antibiotic resistance as well as many virulence traits in enterococci (34). However, the clinical significance of remaining nonviable cells or DNA fragments in root canals is not known nor their influence, if any, on the initiation and maintenance of an inflammatory process.…”

Prevalence of Enterococcus faecalis at Multiple Oral Sites in Endodontic Patients Using Culture and PCR

“…Of clinical significance is that lysed cells in biofilms have the potential to act as donors of chromosomal or plasmid DNA, thereby increasing the opportunity for horizontal gene transfer to other bacteria. Horizontally transferred genes can encode antibiotic resistance as well as many virulence traits (27,28). Recent studies using molecular methods have demonstrated that the microflora associated with endodontic infections is more diverse than was previously reported in studies using conventional culture methods (29).…”

Tales of conjugation and sex pheromones

“…35 They were occasionally associated with acute periodontal disease and even with root canal infections (reviewed in ref. 36). We began to obtain a number of these strains and found plasmids in most of them.…”