Evidence of recombination in Porphyromonas gingivalis and random distribution of putative virulence markers.

The association of Porphyromonas gingivalis to periodontal disease is not clearly understood. Similar proportions of P. gingivalis may be cultivated from both inactive and actively degrading periodontal pockets. Differences in virulence among strains of P. gingivalis exist, but the molecular reason for this remains unknown. We examined the population structure of P. gingivalis to obtain a framework in which to study pathogenicity in relation to evolution. Phylogenetic trees derived from the sequencing of fragments of four housekeeping genes, ahp, thy, rmlB, and infB, in 57 strains were completely different with no correlation between clustering of strains in the four dendrograms. Combining the various alleles of the four gene fragments sequenced resulted in 41 different sequence types. The index of association, I(A), based on a single representative of each sequence type was 0.143 +/- 0.202, indicating a population at linkage equilibrium. Inclusion of all isolates for the calculation of I(A) resulted in a value of 0.206 +/- 0.171. This suggests an epidemic population structure supported by the finding of genetically identical strains in different parts of the world. We observed a random distribution of two virulence-associated mobile genetic elements, the ragB locus and the insertion sequence IS1598, among 132 strains tested. In conclusion, P. gingivalis has a nonclonal population structure characterized by frequent recombination. Our study suggests that particular genotypes, possibly with increased pathogenic potential, may spread successfully in the human population.