The polymicrobial pathogenicity of Porphyromonas gingivalis.

Accumulating microbiome data and mechanistic studies in vitro and in vivo have refined our understanding of the oral microbiota as a functionally integrated polymicrobial community. Emergent properties of these communities are driven to a large extent by interspecies communication which can be based on physical association, secreted small molecules or nutritional exchange. Porphyromonas gingivalis is a consensus periodontal pathogen; however, virulence is only expressed in the context of a polymicrobial community. Multivalent fimbriae mediate attachment to other oral species which can initiate a distinct transcriptional program in both constituents of the binding pair. P. gingivalis also responds to small molecules and nutritional cues produced by partner organisms. Physiological interdependence forms the basis of complex networks of cooperating organisms which begin to resemble an organismal entity exhibiting a spectrum of pathogenic potential.

Interspecies metabolite transfer fuels the methionine metabolism of Fusobacterium nucleatum to stimulate volatile methyl mercaptan production.

The major oral odor compound methyl mercaptan (CH3SH) is strongly associated with halitosis and periodontitis. CH3SH production stems from the metabolism of polymicrobial communities in periodontal pockets and on the tongue dorsum. However, understanding of CH3SH-producing oral bacteria and their interactions is limited. This study aimed to investigate CH3SH production by major oral bacteria and the impact of interspecies interactions on its generation. Using a newly constructed large-volume anaerobic noncontact coculture system, Fusobacterium nucleatum was found to be a potent producer of CH3SH, with that production stimulated by metabolic interactions with Streptococcus gordonii, an early dental plaque colonizer. Furthermore, analysis of extracellular amino acids using an S. gordonii arginine-ornithine antiporter (ArcD) mutant demonstrated that ornithine excreted from S. gordonii is a key contributor to increased CH3SH production by F. nucleatum. Further study with 13C, 15N-methionine, as well as gene expression analysis, revealed that ornithine secreted by S. gordonii increased the demand for methionine through accelerated polyamine synthesis by F. nucleatum, leading to elevated methionine pathway activity and CH3SH production. Collectively, these findings suggest that interaction between S. gordonii and F. nucleatum plays a key role in CH3SH production, providing a new insight into the mechanism of CH3SH generation in oral microbial communities. A better understanding of the underlying interactions among oral bacteria involved in CH3SH generation can lead to the development of more appropriate prophylactic approaches to treat halitosis and periodontitis. An intervention approach like selectively disrupting this interspecies network could also offer a powerful therapeutic strategy.IMPORTANCEHalitosis can have a significant impact on the social life of affected individuals. Among oral odor compounds, CH3SH has a low olfactory threshold and halitosis is a result of its production. Recently, there has been a growing interest in the collective properties of oral polymicrobial communities, regarded as important for the development of oral diseases, which are shaped by physical and metabolic interactions among community participants. However, it has yet to be investigated whether interspecies interactions have an impact on the production of volatile compounds, leading to the development of halitosis. The present findings provide mechanistic insights indicating that ornithine, a metabolite excreted by Streptococcus gordonii, promotes polyamine synthesis by Fusobacterium nucleatum, resulting in a compensatory increase in demand for methionine, which results in elevated methionine pathway activity and CH3SH production. Elucidation of the mechanisms related to CH3SH production is expected to lead to the development of new strategies for managing halitosis.

Organotypic model of the gingiva for studying bacterial and viral pathogens implicated in periodontitis.

Background: Three-dimensional (3D) tissue models bridge the gap between conventional two-dimensional cell cultures and animal models. The aim of this study was to develop an organotypic 3D gingival (OTG) model to provide a tool to investigate bacterial and viral pathogens in periodontitis. Methods: The OTG model composed of gingival fibroblasts (GFs) and telomerase-immortalized gingival keratinocytes (TIGKs) was constructed and applied to study infections by Porphyromonas gingivalis and herpes simplex virus 1 (HSV-1). Immunohistochemical staining, confocal microscopy, qPCR, titration techniques, and colony-forming unit counts were applied to interrogate epithelial markers expression, monitor P. gingivalis and HSV-1 presence, and evaluate the immune response along with the efficiency of antimicrobial drugs. Results: The OTG model resembled the morphology of the human gingiva. During infection, both pathogens penetrated deep into the tissue and persisted for a few days with P. gingivalis also forming a biofilm on the cell surface. The infection triggered the expression of inflammatory mediators in cells and both pathogens were efficiently eliminated by specific antimicrobials. Conclusions: Presented OTG model constitutes a simple and convenient tool to study the interaction between bacterial and viral pathogens within the gingival tissue, including penetration, persistence and biofilm formation. It is also suitable to examine the efficiency of antimicrobial drugs.