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

The association of Porphyromonas gingivalis to periodontal disease is not c learly understood. Similar proportions of P, gingivalis mag be cultivated f rom both inactive and actively degrading periodontal pockets. Differences i n virulence among strains of P. gingivalis exist, but the molecular reason for this remains unknown. We examined the population structure of P. gingiv alis to obtain a framework in which to study pathogenicity in relation to e volution. Phylogenetic trees derived from the sequencing of fragments of fo ur housekeeping genes, ahp, thy, rmlB, and infB, in 57 strains were complet ely different with no correlation between clustering of strains in the four dendrograms, Combining the various alleles of the four gene fragments sequ enced resulted in 41 different sequence types. The index of association, I, , based on a single representative of each sequence type was 0.143 +/- 0.20 2, indicating a population at linkage equilibrium. Inclusion of all isolate s for the calculation of I, resulted in a value of 0.206 +/- 0.171. This su ggests an epidemic population structure supported by the finding of genetic ally identical strains in different parts of the world. We observed a rando m distribution of two virulence-associated mobile genetic elements, the rag B locus and the insertion sequence IS1598, among 132 strains tested. In con clusion, P, gingivalis has a nonclonal population structure characterized b y frequent recombination, Our study suggests that particular genotypes, pos sibly with increased pathogenic potential, may spread successfully in the h uman population.