Detection of identical ribotypes of Porphyromonas gingivalis in patients residing in the United States, Sudan, Romania and Norway

Lessons learned and unlearned in periodontal microbiology

Periodontal diseases are initiated by bacterial species living in polymicrobial biofilms at or below the gingival margin and progress largely as a result of the inflammation elicited by specific subgingival species. In the past few decades, efforts to understand the periodontal microbiota have led to an exponential increase in information about biofilms associated with periodontal health and disease. In fact, the oral microbiota is one of the best-characterized microbiomes that colonize the human body. Despite this increased knowledge, one has to ask if our fundamental concepts of the etiology and pathogenesis of periodontal diseases have really changed. In this article we will review how our comprehension of the structure and function of the subgingival microbiota has evolved over the years in search of lessons learned and unlearned in periodontal microbiology. More specifically, this review focuses on: (i) how the data obtained through molecular techniques have impacted our knowledge of the etiology of periodontal infections; (ii) the potential role of viruses in the etiopathogenesis of periodontal diseases; (iii) how concepts of microbial ecology have expanded our understanding of host-microbe interactions that might lead to periodontal diseases; (iv) the role of inflammation in the pathogenesis of periodontal diseases; and (v) the impact of these evolving concepts on therapeutic and preventive strategies to periodontal infections. We will conclude by reviewing how novel systems-biology approaches promise to unravel new details of the pathogenesis of periodontal diseases and hopefully lead to a better understanding of their mechanisms.

Porphyromonas gingivalis: a clonal pathogen?: Diversities in housekeeping genes and the major fimbriae gene

The introduction of multilocus sequence typing (MLST) in infectious disease research has allowed standardized typing of bacterial clones. Through multiple markers around the genome, it is possible to determine the sequence type (ST) of bacterial isolates to establish the population structure of a species. For the periodontal pathogen, Porphyromonas gingivalis, the MLST scheme has been established at www.pubmlst.org/pgingivalis, and data from the database indicate a high degree of genetic diversity and a weakly clonal population structure comparable with Neisseria menigitidis. The major fimbriae (FimA) have been held responsible for the adhesive properties of P. gingivalis and represent an important virulence factor. The fimA genotyping method (PCR based) indicate that fimA genotype II, IV and Ib are associated with diseased sites in periodontitis and tissue specimens from cardiovascular disease. fimA genotyping of the isolates in the MLST database supports the association of genotypes II and IV with periodontitis. As a result of multiple positive PCR reactions in the fimA genotyping, sequencing of the fimA gene revealed only minor nucleotide variation between isolates of the same and different genotypes, suggesting that the method should be redesigned or re-evaluated. Results from several investigations indicate a higher intraindividual heterogeneity of P. gingivalis than found earlier. Detection of multiple STs from one site in several patients with "refractory" periodontitis, showed allelic variation in two housekeeping genes indicating recombination between different clones within the periodontal pocket.

Porphyromonas gingivalis strain diversity

Porphyromonas gingivalis is implicated in the etiology of chronic periodontitis. Genotyping studies suggest that genetic variability exists among P. gingivalis strains; however, the extent of variability remains unclear and regions of variability remain largely unidentified. To assess P. gingivalis strain diversity, we previously used heteroduplex analysis of the ribosomal operon intergenic spacer region (ISR) to type strains in clinical samples and identified 22 heteroduplex types. Additionally, we used ISR sequence analysis to determine the relatedness of P. gingivalis strains to one another and demonstrated a link between ISR sequence phylogeny and the disease-associated phenotype of the strains. In the current study, heteroduplex analysis of the ISR was used to determine the worldwide genetic variability and distribution of P. gingivalis, and microarray-based comparative genomic hybridization (CGH) analysis was used to more comprehensively examine the variability of major heteroduplex type strains by using the entire genome. Heteroduplex analysis of clinical samples from geographically diverse populations identified 6 predominant geographically widespread heteroduplex types (prevalence, > or = 5%) and 14 rare heteroduplex types (prevalence, <2%) which are found in one or a few locations. CGH analysis of the genomes of seven clinically prevalent heteroduplex type strains identified 133 genes from strain W83 that were divergent in at least one of the other strains. The relatedness of the strains to one another determined on the basis of genome content (microarray) analysis was highly similar to their relatedness determined on the basis of ISR sequence analysis, and a striking correlation between the genome contents and disease-associated phenotypes of the strains was observed.

Genetic diversity of Porphyromonas gingivalis isolates recovered from single "refractory" periodontitis sites

Multilocus sequence typing and fimA genotyping were performed on Porphyromonas gingivalis isolates from 15 subjects with "refractory" periodontitis. Several sequence types were detected for most individual pockets. The variation indicated recombination at the recA and pepO genes. The prevalence of fimA genotypes II and IV confirmed their association with periodontitis.

Genotypic characterization of Porphyromonas gingivalis isolated from Swedish patients with periodontitis and from periodontal abscesses

INTRODUCTION

A significant genetic polymorphism has been shown for Porphyromonas gingivalis isolates from different geographical areas. It is, however, possible that genetic similarities can be found among isolates obtained from a more specific population. The aim of the present study was to evaluate genetic heterogeneity among P. gingivalis isolates obtained from Swedish subjects with chronic periodontitis and from periodontal abscess lesions.

METHODS

A total of 78 P. gingivalis strains, including 55 fresh clinical isolates obtained from 52 Swedish periodontitis subjects, eight isolates from eight Swedish periodontal abscess subjects and 15 reference strains, were subjected to amplified fragment length polymorphism (AFLP) and random amplified polymorphic DNA (RAPD) genotyping assays.

RESULTS

A total of 62 AFLP genotypes and 70 RAPD genotypes were identified among the 78 P. gingivalis strains. Forty-six strains were clustered at 70% similarity level into 15 clusters. Six identical RAPD genotypes were identified among the strains. The AFLP/RAPD profiles were compared for identical genotypes. A total of 56 AFLP/RAPD genotypes were found. Four pairs of identical AFLP/RAPD genotypes were found for two strains obtained from two different periodontal pockets each of four subjects. Interestingly, two strains showed an RAPD/AFLP genotype, which was identical to the type strain W83.

CONCLUSION

The present study demonstrated that Swedish P. gingivalis isolates exhibit a wide variety of genotypes with only a weak clustering pattern. No predominant genotype at the whole chromosomal DNA level was present among Swedish P. gingivalis strains.

Multilocus sequence typing of Porphyromonas gingivalis strains from different geographic origins

Porphyromonas gingivalis is an important periodontal pathogen that can be isolated from both active and inactive periodontal lesions. Apparently, differences in virulence between P. gingivalis strains exist, but the mechanisms underlying these differences are not yet fully understood. To obtain more information about pathogenicity and virulence of P. gingivalis, it is relevant to assess the genetic population structure of the species and to examine the occurrence of putative virulence factors against the genetic background. Presently, multilocus sequence typing (MLST) is the best method for analyzing bacterial population structures. Forty P. gingivalis strains from worldwide sources were analyzed by MLST. Internal 310- to 420-bp DNA fragments of the eight ubiquitous chromosomal genes, ftsQ, hagB, gdpxJ, pepO, mcmA, recA, pga, and nah, were amplified by PCR and then sequenced. The number of alleles at individual loci ranged from 2 to 19, and a total of 33 allelic profiles, or sequence types (STs), were identified. Nucleotide variation between alleles was located at one or a few sites. Identical or similar STs were found in isolates from different geographic regions. Our results showed signs of a clonal population structure with a level of recombination not as high as that previously suggested for the species. We also found that P. gingivalis isolates from individual patients were genetically heterogeneous.

Distribution of Porphyromonas gingivalis biotypes defined by alleles of the kgp (Lys-gingipain) gene

Paired subgingival plaque samples representing the most-diseased and least-diseased sites were collected from 34 adult patients with diagnosed chronic periodontitis. The percentage of Porphyromonas gingivalis relative to the total anaerobic and gram-negative bacterial load at each site was determined by real-time PCR. Based on variations in the noncatalytic C terminus of the Lys-gingipain (Kgp), it was reasoned that DNA sequence variation in the 3'-coding region of the kgp gene might determine functional biotypes. Perusal of the available sequence information in GenBank indicated three such forms of the kgp gene corresponding to P. gingivalis strains HG66, 381, and W83. Analysis of patient samples revealed the presence of a fourth genotype (W83v) that showed duplication of a sequence recognized by the W83 reverse primer. The four biotypes, HG66, 381, W83, and W83v, were present in the study group in the ratio 8:11:6:5, respectively. Each subject was colonized by one predominant biotype, and only three patients were colonized by a trace amount of a second biotype.

IgG antibody levels to Porphyromonas gingivalis and clinical measures in children

BACKGROUND

Periodontopathic clinical markers are poorly understood in the pediatric population. Several studies have proposed Porphyromonas gingivalis (P. gingivalis) and an antibody response to the microorganism as factors in periodontal tissue destruction in children. The objective of this study was to examine the prevalence of P. gingivalis in dental plaque and of serum immunoglobulin G (IgG) antibody levels to P. gingivalis, and their relationship to periodontal clinical measures in children.

METHODS

Thirty-one subjects, aged 20 to 163 months, participated in this study. Clinical measures examined included gingivitis, plaque, alveolar bone height, age, gender, ethnicity, medical status, caries, and IgG antibody levels to P. gingivalis. Five ml of blood was collected for serum analysis, and IgG antibody levels to P. gingivalis were determined by using enzyme-linked immunosorbent assay. Plaque samples were examined for the presence of P. gingivalis by DNA-DNA checkerboard. Data were analyzed on a person-level basis for relationships to serum IgG antibody levels to P. gingivalis and on a site-specific level for relationships to the presence of P. gingivalis in plaque.

RESULTS

A majority (77%) of the subjects were systemically healthy, non-white (74%), and did not have detectable P. gingivalis in their plaque. Fifty-two percent of the subjects had positive serum IgG antibody levels to P. gingivalis. Based on univariate linear regression, factors related to IgG antibody levels to P. gingivalis (P<0.05) included age, average gingival index (GI), average probing depth, and number of teeth with alveolar bone crest to cemento-enamel junction (ABC-CEJ) distances >2 mm. When all clinical measures were considered together, only age remained statistically significantly related to serum IgG antibody levels to P. gingivalis.

CONCLUSIONS

Age is one of the most important factors in the development of the immune response to putative microorganisms such as P. gingivalis in children. The role of IgG as a time-sensitive measure of periodontal health in children needs to be investigated further.

Characterization of two outer membrane protein antigens of Porphyromonas gingivalis that are protective in a murine lesion model

Porphyromonas gingivalis is a key periodontal pathogen that has been implicated in the aetiology of chronic adult periodontitis. The aim of this study was to characterize two potential vaccine candidates (PG32 and PG33) identified from a previous genomic sequence analysis. Gene knockout studies suggested that these proteins play an important role in bacterial growth and are transcriptionally linked. Analysis of 14 laboratory and clinical isolates of P. gingivalis found that in all strains, both genes were present with a high level of conservation and that the two proteins were also expressed in vitro. Truncated recombinant PG32 and PG33 proteins were produced in Escherichia coli in an attempt to increase the solubility of the proteins while retaining their native conformation. While most of the truncated proteins remained insoluble, two truncated proteins showed good solubility and high levels of protection in the P. gingivalis murine lesion model and may be considered as potential vaccine candidates for further testing in models of human periodontal disease.

The genetic relatedness of Porphyromonas gingivalis clinical and laboratory strains assessed by analysis of insertion sequence (IS) element distribution

OBJECTIVES

Porphyromonas gingivalis is frequently found in periodontitis lesions. This organism contains a large number of insertion sequence (IS) elements. We sought to determine the distribution of seven IS elements from strain W83 among nine P. gingivalis laboratory strains and nine clinical isolates and to use these findings to determine strain relationships.

METHODS

Southern blots of BamHI digested genomic DNA digests were probed with insertion sequence elements ISPg1-7.

RESULTS

The restriction fragment length polymorphism (RFLP) patterns revealed that five of the nine laboratory strains, including strain W83, were nearly identical for all seven IS elements. Two of nine clinical isolates were similar to the five laboratory strains. Two of the four remaining laboratory strains had similar or identical RFLP patterns. The remaining two laboratory strains had limited similarity to clinical strains. Four of the clinical isolates had identical RFLP patterns for all seven IS elements. The three remaining clinical isolates were unique in their RFLP patterns. Several strains lacked from one to four of the IS elements. Similar strain relationships were suggested regardless of the IS element examined.

CONCLUSIONS

Transposition and recombination between IS elements are not sufficiently pervasive to obscure strain relationships, though this does not preclude the possibility that such events play an important role in allowing P. gingivalis to adapt to new environments. Given the level of genetic diversity observed, it may be especially important to examine genetically diverse strains when drawing conclusions based on the W83 P. gingivalis genomic database.

Characterization and expression of a novel Porphyromonas gingivalis outer membrane protein, Omp28

We report the characterization of a Porphyromonas gingivalis gene, designated omp28, encoding a protein that we have previously purified and characterized as a 28-kDa outer membrane protein. The deduced amino acid sequence of the omp28 open reading frame displayed an outer membrane leader sequence and lipoprotein attachment site but did not exhibit any significant overall sequence identity with protein sequences in the databases. A small stretch of amino acids (19 residues) exhibits 50% sequence identity with a segment of a fimbrial protein from Dichelobacter nodosus involved in adhesion, suggesting that Omp28 may be a surface adhesin/receptor of P. gingivalis. Using the pET-24 vector we expressed recombinant Omp28 (rOmp28) in Escherichia coli. Western blot analyses of purified rOmp28 with rabbit antisera to a P. gingivalis outer membrane preparation, protective rat anti-whole P. gingivalis antisera and pooled human sera from chronic periodontitis patients showed that the recombinant was recognized by all antisera. Further, anti-rOmp28 antisera exhibited strong reactivity with a panel of four laboratory strains and 10 clinical isolates of P. gingivalis from the United States, Sudan, Romania and Norway. These results suggest that Omp28 is expressed by a wide distribution of P. gingivalis strains.

Subgingival microbiota of Brazilian subjects with untreated chronic periodontitis

BACKGROUND

Different periodontopathogenic microbiota have been associated with periodontal diseases in several populations. The present investigation determined the subgingival microbiota of untreated chronic periodontitis Brazilians using the checkerboard DNA-DNA hybridization technique.

METHODS

Twenty-five periodontitis patients (mean age, 41 +/- 2; mean probing depth [PD], 3.3 +/- 0.2; mean attachment level [AL], 3.6 +/- 0.2) with no history of previous periodontal therapy and a control group of 14 healthy subjects (mean age, 34 +/- 0.6; mean PD, 1.8 +/- 0.2; mean AL, 1.7 +/- 0.1) were selected. Measurements of PD, AL, bleeding on probing, plaque accumulation, and suppuration were recorded at 6 sites/tooth. Subgingival plaque samples were obtained from 4 sites in each tooth/subject in both groups. The presence and levels of 41 subgingival species were determined in 4,032 plaque samples using whole genomic DNA probes and the checkerboard method.

RESULTS

Periodontal pathogens, as well as some unusual species (E. faecalis, E. coli and Bartonella sp.), were detected significantly more often and/or in higher levels in the periodontitis group (P < 0.05). Most species were more frequently detected in interproximal sites. B. forsythus, P. gingivalis, E. nodatum, and F. nucleatum ss vincentii showed a significant positive correlation with mean PD and AL (P < 0.05).

CONCLUSIONS

The subgingival microbiota of Brazilians with untreated chronic periodontitis were complex, including high proportions of periodontopathogens commonly found in other populations, as well as some unusual species.

Prevalence and risk indicators for destructive periodontal diseases in 3 urban American minority populations

BACKGROUND, AIMS

Destructive periodontal diseases have been reported disproportionately more prevalent and severe in African-Americans relative to other American populations. Differences in subgingival microbiota and host immune response have also been reported for African-Americans, implying that risk factors for disease progression may also differ for these populations. Since it is not clear whether these differences are truly genetic or due to confounding variables such as social economic status, we examined a series of clinical, environmental, demographic, and microbiologic features associated with periodontal disease status in a group of 185 urban minority subjects resident within the greater New York metropolitan area.

METHODS

The study population consisted of 56 Asian-American, 71 African-American and 58 Hispanic subjects. Clinical data recorded included pocket depth, attachment level, gingival erythema, bleeding upon probing, suppuration, and the presence of supragingival plaque. Environmental and demographic data recorded included smoking history, years resident in the United States, whether the subject reported a private dentist and occupational status. Subgingival plaque was sampled from the mesial aspect of all teeth exclusive of third molars and the levels of 40 subgingival species enumerated using checkerboard DNA-DNA hybridization.

RESULTS

The African-American group had more missing teeth, deeper periodontal pocket depth and more attachment loss than the Asian-American or Hispanic groups. However, the African-American group were less likely to report having a private dentist, had a greater proportion of smokers and a greater proportion of unskilled individuals. The profile of subgingival species differed among the three ethnic/racial groups with A. actinomycetemcomitans, N. mucosa, S. noxia and T. socranskii significantly elevated in the Asian-American group and P. micros significantly elevated in the African-American group. When subset by occupational status, numbers of missing teeth, pocket depth, attachment level and prior disease activity were all found increased in the unskilled relative to the professional group. Local factors including the mean % of sites with plaque, marginal gingival erythema, bleeding upon probing and suppuration were also elevated in the unskilled group. The microbial profile differed among the 3 occupational groups with the unskilled group having elevated numbers of species associated with destructive periodontal diseases.

CONCLUSIONS

Although greater destructive periodontal disease prevalence and severity were found in the African-American group, these results suggest that environmental and demographic variables, such as occupational status, may have a greater influence on risk indicators associated with disease prevalence and progression in these populations.

Fimbriae of Porphyromonas gingivalis induce opsonic antibodies that significantly enhance phagocytosis and killing by human polymorphonuclear leukocytes

Porphyromonas gingivalis has been strongly implicated in the pathogenesis of human periodontitis. Fimbriae mediate adherence and colonization of the oral cavity by this organism and may, therefore, have potential for use as antigen in an anti-P. gingivalis vaccine. The purpose of our study was to determine whether P. gingivalis fimbriae have opsonic target sites and whether they are accessible on the cell surfaces and cross-reactive among P. gingivalis fimbrial types and serotypes. Rabbits were immunized with a vaccine. The antiserum reacted with a 43-kDa fimbrillin monomer and a 43-kDa component in whole-cell sonicates of P. gingivalis 33277, but it showed only very weak reactivity in the 43-kDa region of Western blots of a whole-cell sonicate of strain DPG3, a mutant that does not express functional fimbriae. The antibody enhanced chemiluminescence approximately six-fold relative to preimmune serum values and significantly enhanced phagocytosis and killing of P. gingivalis 33277 by human polymorphonuclear leukocytes. Peak opsonic activity was observed at week 6 followed by a plateau that remained until week 16. The fimbria-deficient mutant DPG3 did not bind antifimbrial antibody and was not opsonized, whereas strain 381, the parent of the mutant, was opsonized. The specific antibody bound to and opsonized P. gingivalis strains 33277 and 381 (fimbria type I) but not W50, A7A-1-28, 9-14K-1 or FAY-19M-1 (fimbrial types II-V). Specific antibody bound to strain 2561 (fimbrial type I) but, as assessed by chemiluminescence, did not opsonize it. While fimbriae have opsonic target sites that are accessible on P. gingivalis cell surfaces, the relevant opsonic target sites do not appear to be shared across serotypes or fimbrial types. Thus, a vaccine containing, as antigen, fimbrial protein from a single P. gingivalis strain would likely be ineffective against infections by P. gingivalis strains expressing other fimbrial types.

Functional characteristics of antibodies induced by Arg-gingipain (HRgpA) and Lys-gingipain (Kgp) from Porphyromonas gingivalis

Arginine-specific gingipain (HRgpA) and lysine-specific gingipain (Kgp), enzymes produced by Porphyromonas gingivalis, may be candidates for an anti-P. gingivalis vaccine. The purpose of our study was to determine whether HRgpA and Kgp have opsonic target sites and whether these sites are available and accessible on intact P. gingivalis cells. Rabbits were used to generate polyclonal antibodies to both proteins. Animals were immunized and immunoglobulin G (IgG) fractions were isolated from preimmune and immune sera. Functional characteristics of the antibodies were assessed by determining antibody titers by enzyme-linked immunosorbent assay (ELISA), generating Western immunoblots, and measuring antibody enhancement of P. gingivalis opsonization, phagocytosis and killing by polymorphonuclear leukocytes (PMN) of intact cells of strains of P. gingivalis representative of the four serotypes. Strains studied included 33277 (serotype A), A7A1-28 (serotype B), W50 (serotype C) and 381 (serotype D). Both HRgpA and Kgp induced high titers of IgG antibody. Anti-HRgpA and anti-Kgp bound to both HRgpA and Kgp demonstrating a large proportion of shared antigenic epitopes. The two antibodies bound equally well to all four P. gingivalis serotypes with titers ranging from 77 to 205 ELISA units when compared to preimmune IgG set at 1 ELISA unit. The immunoblot patterns of binding of the two antibodies to HRgpA and Kgp and to sonicates of the four P. gingivalis serotypes were virtually identical. Both antibodies detected components in HRgpA at 27, 35 and 45 kDa and in Kgp at 27, 32, 35, 40 and 55 kDa. The antibodies also detected components at or near these same positions in addition to multiple high molecular mass components in the cell sonicates of P. gingivalis. Both proteins induced antibodies that significantly enhanced opsonization as assessed by chemiluminescence, with values ranging from 130 mV to 375 mV for anti-HRgpA IgG and from 240 mV to 475 mV for anti-Kgp IgG. Both antibodies significantly enhanced PMN-mediated bacterial killing of the four P. gingivalis serotypes, although the percentage of killing varied among the serotypes (24-81% for anti-HRgpA and 37-89% for anti-Kgp). Thus, both HRgpA and Kgp express opsonic target sites and induce high titers of antibodies that opsonize and enhance killing of all four serotypes of P. gingivalis. These two proteins appear to be potential candidate antigens for an anti-P. gingivalis vaccine.

Immunoglobulin G response of periodontitis patients to Porphyromonas gingivalis capsular carbohydrate and lipopolysaccharide antigens

Porphyromonas gingivalis clonal types that participate in periodontal infections express serologically distinct surface antigens. This investigation sought to determine whether serum antibodies titers against the serotype-specific capsular carbohydrate K antigen and lipopolysaccharide antigens of P. gingivalis might reveal which serotypes are most likely to be responsible for subgingival infections in subjects with adult periodontitis. Immunoglobulin G (IgG) titers to purified K antigen and lipopolysaccharide from different P. gingivalis strains were measured by ELISA for 28 healthy controls and 51 patients with periodontal pockets known to be infected with genetically and serologically distinct P. gingivalis clonal types. Titers to purified K antigen from strains W50, HG184, A7A1-28, 49417, HG1690 and HG1691, representing serotypes K1-K6, respectively, and lipopolysaccharide from strains 381, HG1691 and W50, representing serotypes O1-O3, respectively, were measured for all subjects. Chi-square likelihood ratios, Mann-Whitney tests and receiver-operating characteristic sensitivity-specificity plots were used to compare the accuracy with which titer results for different target antigens classified subjects with or without disease. Results from assays targeting K2, K3, K4, K5, O1 and O2 generally gave poor diagnostic accuracy, whether evaluated separately or as summed titer pairs corresponding to the K/O combinations actually expressed by the target antigen parent strains. Exceptions were O3 (from W50) and K5+O2 (both from HG1690), which gave moderate accuracy in classifying subjects. In contrast, highly significant diagnostic accuracy was achieved using individual K1 (W50) and K6 (HG1691) titer data and K1+O3 (W50) and K6+O2 (HG1691) titer sum values. These observations suggest that P. gingivalis clonal types expressing K/O serotypes matching those of W50 (K1/O3) and HG1691 (K6/O2) are more likely than others to participate in periodontal infections in adult periodontitis patients and thus are more likely than others to express relevant virulence factors.

Identification of vaccine candidate antigens from a genomic analysis of Porphyromonas gingivalis

Porphyromonas gingivalis is a key periodontal pathogen which has been implicated in the etiology of chronic adult periodontitis. Our aim was to develop a protein based vaccine for the prevention and or treatment of this disease. We used a whole genome sequencing approach to identify potential vaccine candidates. From a genomic sequence, we selected 120 genes using a series of bioinformatics methods. The selected genes were cloned for expression in Escherichia coli and screened with P. gingivalis antisera before purification and testing in an animal model. Two of these recombinant proteins (PG32 and PG33) demonstrated significant protection in the animal model, while a number were reactive with various antisera. This process allows the rapid identification of vaccine candidates from genomic data.

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.

Characterization of Porphyromonas gingivalis insertion sequence-like element ISPg5

Porphyromonas gingivalis, a black-pigmented, gram-negative anaerobe, is found in periodontitis lesions, and its presence in subgingival plaque significantly increases the risk for periodontitis. In contrast to many bacterial pathogens, P. gingivalis strains display considerable variability, which is likely due to genetic exchange and intragenomic changes. To explore the latter possibility, we have studied the occurrence of insertion sequence (IS)-like elements in P. gingivalis W83 by utilizing a convenient and rapid method of capturing IS-like sequences and through analysis of the genome sequence of P. gingivalis strain W83. We adapted the method of Matsutani et al. (S. Matsutani, H. Ohtsubo, Y. Maeda, and E. Ohtsubo, J. Mol. Biol. 196:445-455, 1987) to isolate and clone rapidly annealing DNA sequences characteristic of repetitive regions within a genome. We show that in P. gingivalis strain W83, such sequences include (i) nucleotide sequence with homology to tRNA genes, (ii) a previously described IS element, and (iii) a novel IS-like element. Analysis of the P. gingivalis genome sequence for the distribution of the least used tetranucleotide, CTAG, identified regions in many of the initial 218 contigs which contained CTAG clusters. Examination of these CTAG clusters led to the discovery of 11 copies of the same novel IS-like element identified by the repeated sequence capture method of Matsutani et al. This new 1,512-bp IS-like element, designated ISPg5, has features of the IS3 family of IS elements. When a recombinant plasmid containing much of ISPg5 was used in Southern analysis of several P. gingivalis strains, including clinical isolates, diversity among strains was apparent. This suggests that ISPg5 and other IS elements may contribute to strain diversity and can be used for strain fingerprinting.

Antigenic cross-reactivity among Porphyromonas gingivalis serotypes

The goal of our research program is to develop a Porphyromonas gingivalis vaccine. Vaccine development requires identification of antigenic components shared by the many clonal types of P. gingivalis. The purpose of the present study was to evaluate the extent and nature of antigenic cross-reactivity among serotypes of P. gingivalis and to identify shared antigenic components. Strains selected to represent serotypes A-D were 33277, A7A1-28 W50 and 381, respectively. Using intact cells, antibodies were raised in rabbits. Titers were assessed by enzyme-linked immunosorbent assay (ELISA) using intact cells as antigen, Western blots were prepared and biologic activity was measured as opsonization (chemiluminescence expressed as mV) and enhancement of phagocytosis and killing by polymorphonuclear leukocytes. Extensive cross-reactivity that varied greatly among serotypes was observed by ELISA. The Western blots showed an even greater extent of cross-reactivity, with shared protein components at approximately 140, 130, 37, 32 and 28 kDa and a shared variable molecular mass smear considered to be lipopolysaccharide and other carbohydrate. Additional protein components at 110, 85, 35 and 20 kDa appeared to be shared by some but not all serotypes. In the functional assays, strains 33277 and 381 were equally well opsonized by anti-33277 and anti-381 (500-650 mV) but opsonized to a much lesser extent by anti-A7A1-28 and anti-W50 (roughly 125 mV and 350 mV respectively). A7A1-28 and W50 were opsonized by all four immune sera almost equally but to a much lower extent (roughly 400 mV and 250 mV respectively). Enhancement of phagocytosis and killing in the presence of active complement mirrored opsonization with the exception that 381 was reasonably well opsonized by anti-A7A1-28 (400 mV) and anti-W50 (350 mV), but poorly killed. The protein components at 140, 130, 37 and 28 kDa shared by all of the four serotypes appear to have potential as vaccine candidate antigens.

Intrafamilial Transmission of Black-pigmented, Putative Periodontal Pathogens

Porphyromonas gingivalis and Prevotella intermedia are black-pigmented, putative periodontopathogenic bacteria considered to cause some forms of periodontal disease. Porphyromonas gingivalis and P. intermedia can be transmitted between humans and produce periodontal disease in susceptible hosts. In this article, studies using molecular typing methods for determining the transmission of black-pigmented, putative periodontopathogens between family members are reviewed. As individuals living close to each other are more prone to transmit bacteria, the studies on transmission of periodontopathogens have been performed on family members. It has been shown that black-pigmented bacteria are not only transferred between spouses but also between parents and child. Since only a limited number of studies have been done, longitudinal and controlled studies should be carried out to elucidate further the transmittance potential of these bacteria.

Milestones in periodontal research and the remaining critical issues

A significant recent development in periodontal research has been the convergence of basic and clinical research resulting in a logarithmic increase in the rate of progress. Scientific consensus has been reached in many areas. In most populations, moderate to severe periodontitis affects a relatively small segment of adults who are at high risk. The microbial etiology is accepted and the identity of the major pathogenic bacterial species is known. The mechanisms through which resistant individuals successfully fend off the microbial challenge are known, and the immuno-inflammatory pathways activated by bacteria that underlie destruction of the alveolar bone and the connective tissues of the periodontium are reasonably well understood. The evidence shows that these pathways are held in common by all forms of periodontitis. Therapeutic modulation of these pathways is now possible, and new treatments based on such modulation are now becoming available. Although bacteria are essential for disease to occur, they are insufficient; a susceptible host is also necessary. Host susceptibility, disease progression and response to treatment are determined predominantly by heredity and environmental and acquired risk factors. Some of these can be changed while others are immutable. Concepts and procedures for treatment are generally scientifically based and appropriately applied. Preventive measures are largely successful and widely practiced in industrialized countries. Clearly, control of these ancient chronic diseases is now within our reach. In spite of the tremendous progress, many unresolved issues remain. The purpose of this paper is to summarize some of the major accomplishments of periodontal research, and identify and discuss some of the more important critical issues that still need to be addressed.

Oral ecology and person-to-person transmission of Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis

The ecological characteristics of the oral cavity are dissimilar for A. actinomycetemcomitans and for P. gingivalis, as judged by differences in their colonization preferences and patterns, associations with periodontal disease parameters, relationships with the subgingival microbiota and the type of periodontitis and their clonal persistence in the oral cavity. These features also suggest that as a periodontal pathogen, A. actinomycetemcomitans is different from P. gingivalis. Probably in most infected individuals, low levels of A. actinomycetemcomitans can persist for years in equilibrium with the host and the resident oral microbiota. However, it is well established that A. actinomycetemcomitans can cause disease in some individuals or in some circumstances when the regulatory mechanisms are unable to maintain homeostasis in the ecosystem. Elevated A. actinomycetemcomitans proportions of the biota can be regarded as a sign of ecological imbalance, leading to increased risk of periodontal destruction. There is also evidence showing elevated pathogenic potential of certain A. actinomycetemcomitans clones. Although A. actinomycetemcomitans seems to be relatively rarely transmitted between cohabiting adults, transmission can occur to periodontally healthy children of A. actinomycetemcomitans-positive parents. Parents and children may share factors that promote successful oral colonization of A. actinomycetemcomitans, or the window of opportunity is in childhood. Therefore, to prevent parent-child transmission of A. actinomycetemcomitans, bacterium-positive parents of young children are optimal targets for enhanced information and treatment. In selected populations, screening for specific clones of A. actinomycetemcomitans has been employed in prevention of peridontitis. Future research aiming at finding the reasons which cause the changes in the oral homeostasis to allow the growth of A. actinomycetemcomitans may give insight into novel prevention strategies for A. actinomycetemcomitans-associated periodontitis. Compared with A. actinomycetemcomitans, P. gingivalis shows a different pattern of coexistence with the host. In periodontal health or in children, P. gingivalis is absent or only rarely detected. When present, P. gingivalis is commonly recovered in high numbers from dentitions exhibiting inflamed periodontitis and poor oral hygiene. Contrary to A. actinomycetemcomitans, the data on the vertical transmission of P. gingivalis are limited. The major infection route of P. gingivalis seems to be between adults, indicating that P. gingivalis commonly colonizes in an established oral microbiota. These characteristics suggest that the degree of tolerance between P. gingivalis and the host is inferior to that between A. actinomycetemcomitans and the host. It appears that the association of P. gingivalis with disease is a rule rather than an accidental incident. On these grounds, it seems that the host-P. gingivalis relationship approaches antibiosis. Since P. gingivalis infection is related to a typical periodontal eco-pathology, the susceptibility to person-to-person transmission of this pathogen may be controlled by periodontal treatment and emphasizing the significance of high standard oral hygiene.

Antigenic variation in Porphyromonas gingivalis ribotypes recognized by serum immunoglobulin G of adult periodontitis patients

We obtained clinical isolates of Porphyromonas gingivalis of known ribotype from patients diagnosed with adult periodontitis and used Western blot methodology to evaluate profiles of antigens recognized by IgG in heterologous and homologous patient sera. Our aims were to identify isolates belonging to different serogroups, to learn if serogroup membership is related to ribotype to assess variation in IgG responses of patients to antigens is homologous and heterologous ribotypes, and to determine the frequency of shared and variable antigens in different biochemical classes recognized across different serogroups and ribotypes. Blots of separation patterns of 28 isolates were developed in sera from patients and bound IgG was quantified by digital image densitometry. The membership of isolates in different serogroups was determined by correlation and hierarchical cluster analysis of isolate whole-cell IgG binding profiles. Two major isolate clusters, each with two subclusters, were found. Isolates within the same ribotype clustered together in some cases but not others. Homologous isolates ranked high in IgG binding levels relative to those from different patients irrespective of ribotype. Patient subgroups with IgG responses dominant for different ribotypes and serogroups were revealed by correlation analysis. The IgG binding profiles observed for individual protein and proteinase-resistant antigens across both homologous and heterologous isolates were very dissimilar. Furthermore, the frequency of antigens both shared across all ribotypes and recognized by IgG in patient sera was unexpectedly low. Only two protein antigens (Mr 44 kDa and 27 kDa) were strongly recognized across all ribotypes by different sera. We conclude that the IgG response of patients infected with a particular P. gingivalis serotype or ribotype is directed mainly against antigens that are not shared by other potentially infective clonal types.

Clonal analysis by ribotyping of Fusobacterium nucleatum isolates obtained from healthy young adults with optimal plaque control

Fusobacterium nucleatum is a Gram-negative anaerobic rod implicated in the pathogenesis of periodontal disease. However, this organism has also been frequently identified in high numbers in healthy adults. These observations suggest that the species may comprise different clonal types, some of which may participate in disease. The purpose of the present investigation was to use restriction endonuclease analysis (REA) and ribotyping to characterize F. nucleatum clonal types isolated from healthy young adults with optimal plaque control and investigate the stability of some of these clonal types. A group comprising 11 dental students and 11 dental outpatients with optimal plaque control was sampled. Clonal stability was investigated by sampling the dental student group at baseline and at 16 months. One hundred and thirty-two clinical isolates of F. nucleatum were successfully recovered from 15/22 individuals. For the positive subjects, 29 different clonal types were identified by REA and ribotyping, each subject and site being colonized by 1-4 clonal types. For the dental students, 9 and 15 different clonal types were identified at baseline and 16 months, respectively. None of the students harboured identical clonal types at both sampling times. Our results show that ribotyping is a useful technique for monitoring the distributions of F. nucleatum clonal types and indicate that healthy individuals with optimal plaque control can be colonized by more than one F. nucleatum clonal type and that these clonal types appear to be unstable.