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Journal of Bacteriology, September 2007, p. 6382-6388, Vol. 189, No. 17
0021-9193/07/$08.00+0 doi:10.1128/JB.00460-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Department of Oral Biology and Center for Molecular Microbiology, College of Dentistry, University of Florida, Gainesville, Florida 32610-0424
Received 27 March 2007/ Accepted 6 June 2007
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 are 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 Escherichia coli. Of the eight strains tested, five were able to transfer DNA into E. coli by a mechanism most consistent with conjugation. Additionally, strains W83 and 33277 tested positive for the transfer of chromosomally integrated antibiotic resistance markers. Ten chimeras resulting from the chromosomal transfer assay were further analyzed by Southern hybridization and were shown to have exchanged DNA fragments of between 1.1 and 5.6 kb, but the 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.
Published ahead of print on 15 June 2007.
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