Genetic Diversity in the Oral Pathogen Porphyromonas Gingivalis : Molecular Mechanisms and Biological Consequences

Abstract

Porphyromonas gingivalis is a Gram-negative anaerobic bacterium that colonizes the human oral cavity. It is implicated in the development of periodontitis, a chronic periodontal disease affecting half of the adult population in the USA. To survive in the oral cavity, these bacteria must colonize dental plaque biofilms in competition with other bacterial species. Long-term survival requires P. gingivalis to evade host immune responses, while simultaneously adapting to the changing physiology of the host and to alterations in the plaque biofilm. In reflection of this highly variable niche, P. gingivalis is a genetically diverse species and in this review the authors summarize genetic diversity as it relates to pathogenicity in P. gingivalis. Recent studies revealing a variety of mechanisms by which adaptive changes in genetic content can occur are also reviewed. Understanding the genetic plasticity of P. gingivalis will provide a better framework for understanding the host–microbe interactions associated with periodontal disease.

Genetic exchange and reassignment in Porphyromonas gingivalis

“…While type Ib, II and IV are most frequently associated with periodontitis [14–18], type I and III are less inflammatory and are associated with strains more likely to form localized self-limiting infections [19]. It has been suggested that the fimA type II allele confers a selective advantage in the advanced disease process thus ensuring its distribution throughout the P. gingivalis population [20]. …”

“…In clonal populations of bacteria, genetic diversity primarily arises by point mutations. Point mutations within open reading frames that are limited in size can occur in P. gingivalis at frequencies of 10 –9 , similar to other bacterial species such as Escherichia coli [20]. …”

“…In non-clonal bacterial populations, however, there is a spectrum of phenotypes from commensal to pathogenic phenotype [27] and a number of allele combinations that can be related to disease [20]. Some allele combinations may be more virulent than others and it is reasonable to think that allele combinations of P. gingivalis differ in health and disease.…”

“…Consequently, the genetic variation in P. gingivalis strains is considerable suggesting that exchange of genes occurs to improve the chances of P. gingivalis to survive in vivo . This variability can be extensive and comprise entire genes as well as operons [20]. …”

“…At least 95% of clinical strains from P. gingivalis harbours CRISPR arrays [41]. The presence of transposases and CRISPR-associated sequences in P. gingivalis [11] suggested that some parts of the P. gingivalis genome are mobile so that rearrangement or horizontal gene transfer from one strain to another can take place [20]. In the P. gingivalis W83 genome, all four CRISPR regions were transcribed and one of them was active against dsDNA in vivo [41].…”