Porphyromonas gingivalis strain variability and periodontitis

Abstract

To determine if there is variability in virulence among strains of Porphyromonas gingivalis in human periodontitis, their distribution in a group of subjects with clear indicators of periodontitis and in a healthy, age-matched control group was examined. The presence of heteroduplex types of P. gingivalis in the two groups was determined with a PCR-based assay. This assay relied on detection of polymorphisms in the ribosomal internal spacer region (ISR). ISR fragments generated by PCR with P. gingivalis-specific primers were hybridized to fragments from reference strains, and the formation of heteroduplexes from the hybridization of nonidentical sequences was observed by polyacrylamide gel electrophoresis. Characteristic fingerprints from comparison with a panel of reference strains allowed the identification of heteroduplex types in clinical samples. One hundred thirty adults with periodontitis and 181 controls were sampled. With this approach, 11 heteroduplex types of P. gingivalis were detected in the population. Sufficient numbers were available for statistical analysis of six of these types. Heteroduplex type hW83 was found to be very strongly associated with periodontitis (P = 0.0000), and two additional types, h49417 and hHG1691, were also significantly associated with disease. The remaining types, h23A4, h381, and hA7A1, were detected more frequently in subjects with periodontitis than in healthy subjects, but the difference was not significant. These data indicate that virulence in human periodontitis varies among strains of P. gingivalis, and they identify an apparently highly virulent subgroup.

FIG. 1

Heteroduplex gel of a sample containing P. gingivalis heteroduplex type h23A4. ISR DNA amplified from a sample was hybridized to ISR DNA from a series of reference strains of P. gingivalis. Heteroduplex bands were seen for duplexes formed with all strains except 23A4. The migration pattern of heteroduplex bands generated with the sample was identical to that observed for 23A4 duplexed to the same strains (data not shown).

FIG. 2

Prevalence of heteroduplex types of P. gingivalis in the healthy group and the group with periodontitis. Since multiple strains were present in many subjects, the total percentage is more than 100% for each group.

FIG. 3

Presence of multiple heteroduplex types of P. gingivalis in periodontally healthy subjects and subjects with periodontitis. The expected distribution of number of heteroduplex types is also shown. This was calculated for each group based on the binomial probability function and the number of heteroduplex types observed within the group. Both the healthy group and the group with periodontitis showed significant differences between the observed distribution of numbers of strains per individual and the expected distribution. In the healthy group fewer subjects harbored a single strain than expected (P = 0.0003). In the periodontitis group more subjects harbored a single strain than expected (P = 0.026).

FIG. 4

Odds (log scale) of finding heteroduplex types of P. gingivalis in subjects with periodontitis compared to healthy subjects as determined by a nominal logistic regression. The 95% confidence intervals are shown as horizontal bars. The odds are greater than 1 for hW83, h49417, and HG1691 at an α level of 0.05.