Conjugal Transfer of Chromosomal DNA Contributes to Genetic Variation in the Oral Pathogen Porphyromonas gingivalis

Department of Oral Biology and Center for Molecular Microbiology, College of Dentistry, University of Florida, Gainesville, FL 32610-0424, USA

^* To whom correspondence should be addressed. Email: gtribble{at}dental.ufl.edu.

	Abstract

Porphyromonas gingivalis is a major oral pathogen that contributes to the development of periodontal disease. There is a significant degree of genetic variation among strains of P. gingivalis, and the population structure has been predicted to be panmictic, indicating that horizontal DNA transfer and recombination between strains is likely. The molecular events underlying this genetic exchange are not understood, although a putative type IV secretion system is present in the genome sequence of strain W83, implying that DNA conjugation may be responsible for genetic transfer in these bacteria. In this study, we provide in vitro evidence for the horizontal transfer of DNA using plasmid and chromosome-based assays. In the plasmid assays, Bacteroides-derived shuttle vectors were tested for transfer from P. gingivalis strains into E. coli. Of the eight strains tested, five were able to transfer DNA to E. coli by a mechanism most consistent with conjugation. Additionally, strains W83 and 33277 were tested and found positive for transfer of chromosomally-integrated antibiotic resistance markers. Ten chimeras resulting from the chromosomal transfer assay were further analyzed by Southern hybridization, and shown to have exchanged DNA fragments between 1.1 and 5.6 kb, but overall strain identity remained intact. Chimeras showed phenotypic changes in the ability to accrete into biofilms, implying that DNA transfer events are sufficient to generate measurable changes in complex behaviors. This ability to transfer chromosomal DNA between strains may be an adaptation mechanism in the complex environment of the host oral cavity.