Fatty acids synthesized by oral treponemes in chemically defined media

Culturing the Human Oral Microbiota, Updating Methodologies and Cultivation Techniques

Recent years have been marked by a paradigm shift in the study of the human microbiota, with a re-emergence of culture-dependent approaches. Numerous studies have been devoted to the human microbiota, while studies on the oral microbiota still remain limited. Indeed, various techniques described in the literature may enable an exhaustive study of the microbial composition of a complex ecosystem. In this article, we report different methodologies and culture media described in the literature that can be applied to study the oral microbiota by culture. We report on specific methodologies for targeted culture and specific culture techniques and selection methodologies for cultivating members of the three kingdoms of life commonly found in the human oral cavity, namely, eukaryota, bacteria and archaea. This bibliographic review aims to bring together the various techniques described in the literature, enabling a comprehensive study of the oral microbiota in order to demonstrate its involvement in oral health and diseases.

A review of in vitro attempts to develop the axenic culture of Treponema pallidum and genomics-based suggestions to achieve this elusive goal

To date, the axenic culture of Treponema pallidum remains a challenge in the field of microbiology despite countless attempts. Here, we conducted a comprehensive bibliographic analysis using several databases and search engines, namely Pubmed, Google scholar, Google, Web of Science and Scopus. Numerous unsuccessful empiric studies have been conducted and evaluated using as criteria dark-field microscopic observation of motile spiral shaped cells in the culture and virulence of the culture through rabbit infectivity. All of these studies failed to induce rabbit infectivity, even when deemed positive after microscopic observation leading to the misnomer of avirulent T. pallidum. In fact, this criterion was improperly chosen because not all spiral shaped cells are T. pallidum. However, these studies led to the formulation of culture media particularly favourable to the growth of several species of Treponema, including Oral Microbiology and Immunology, Zürich medium (OMIZ), Oral Treponeme Enrichment Broth (OTEB) and T-Raoult, thus allowing the increase in the number of cultivable strains of Treponema. The predicted metabolic capacities of T. pallidum show limited metabolism, also exhibited by other non-cultured and pathogenic Treponema species, in contrast to cultured Treponema species. The advent of next generation sequencing represents a turning point in this field, as the knowledge inferred from the genome can finally lead to the axenic culture of T. pallidum.

Novel Pathogens in Periodontal Microbiology

Periodontitis is a polymicrobial disease caused by complex interactions between distinct pathogens in a biofilm resulting in the destruction of periodontal tissues. It seems evident that unknown microorganisms might be involved in onset or progression of periodontitis. For many decades, research in the field of oral microbiology failed to identify certain subgingival microbiota due to technical limitations but, over a period of 12 years using molecular approaches and sequencing techniques, it became feasible to reveal the existence of new periodontal pathogens. Therefore, it is evident that in addition to conventional periodontal pathogens, other microbes might be involved in onset and progression of periodontitis. The novel pathogens enlisted under periodontal phylogeny include Cryptobacterium curtum, Dialister pneumosintes, Filifactor alocis, Mitsuokella dentalis, Slackia exigua, Selenomonas sputigena, Solobacterium moorei, Treponema lecithinolyticum, and Synergistes. The polymicrobial etiology of periodontitis has been elucidated by comprehensive techniques, and studies throwing light on the possible virulence mechanisms possessed by these novel periodontal pathogens are enlisted.

Differentiation of oral bacteria in in vitro cultures and human saliva by secondary electrospray ionization - mass spectrometry

The detection of bacterial-specific volatile metabolites may be a valuable tool to predict infection. Here we applied a real-time mass spectrometric technique to investigate differences in volatile metabolic profiles of oral bacteria that cause periodontitis. We coupled a secondary electrospray ionization (SESI) source to a commercial high-resolution mass spectrometer to interrogate the headspace from bacterial cultures and human saliva. We identified 120 potential markers characteristic for periodontal pathogens Aggregatibacter actinomycetemcomitans (n = 13), Porphyromonas gingivalis (n = 70), Tanerella forsythia (n = 30) and Treponema denticola (n = 7) in in vitro cultures. In a second proof-of-principle phase, we found 18 (P. gingivalis, T. forsythia and T. denticola) of the 120 in vitro compounds in the saliva from a periodontitis patient with confirmed infection with P. gingivalis, T. forsythia and T. denticola with enhanced ion intensity compared to two healthy controls. In conclusion, this method has the ability to identify individual metabolites of microbial pathogens in a complex medium such as saliva.

Saliva as the Sole Nutritional Source in the Development of Multispecies Communities in Dental Plaque

Dental plaque is a polymicrobial biofilm that forms on the surfaces of teeth and, if inadequately controlled, can lead to dental caries or periodontitis. Nutrient availability is the fundamental limiting factor for the formation of dental plaque, and for its ability to generate acid and erode dental enamel. Nutrient availability is also critical for bacteria to grow in subgingival biofilms and to initiate periodontitis. Over the early stages of dental plaque formation, micro-organisms acquire nutrients by breaking down complex salivary substrates such as mucins and other glycoproteins. Once dental plaque matures, dietary carbohydrates become more important for supragingival dental plaque, and gingival crevicular fluid forms the major nutrient source for subgingival microorganisms. Many species of oral bacteria do not grow in laboratory monocultures when saliva is the sole nutrient source, and it is now clear that intermicrobial interactions are critical for the development of dental plaque. This chapter aims to provide an overview of the key metabolic requirements of some well-characterized oral bacteria, and the nutrient webs that promote the growth of multispecies communities and underpin the pathogenicity of dental plaque for both dental caries and periodontitis.

Porphyromonas gingivalis and Treponema denticola exhibit metabolic symbioses

Porphyromonas gingivalis and Treponema denticola are strongly associated with chronic periodontitis. These bacteria have been co-localized in subgingival plaque and demonstrated to exhibit symbiosis in growth in vitro and synergistic virulence upon co-infection in animal models of disease. Here we show that during continuous co-culture a P. gingivalis:T. denticola cell ratio of 6∶1 was maintained with a respective increase of 54% and 30% in cell numbers when compared with mono-culture. Co-culture caused significant changes in global gene expression in both species with altered expression of 184 T. denticola and 134 P. gingivalis genes. P. gingivalis genes encoding a predicted thiamine biosynthesis pathway were up-regulated whilst genes involved in fatty acid biosynthesis were down-regulated. T. denticola genes encoding virulence factors including dentilisin and glycine catabolic pathways were significantly up-regulated during co-culture. Metabolic labeling using ¹³C-glycine showed that T. denticola rapidly metabolized this amino acid resulting in the production of acetate and lactate. P. gingivalis may be an important source of free glycine for T. denticola as mono-cultures of P. gingivalis and T. denticola were found to produce and consume free glycine, respectively; free glycine production by P. gingivalis was stimulated by T. denticola conditioned medium and glycine supplementation of T. denticola medium increased final cell density 1.7-fold. Collectively these data show P. gingivalis and T. denticola respond metabolically to the presence of each other with T. denticola displaying responses that help explain enhanced virulence of co-infections.

Unlike the traditional view that most diseases are caused by infection with a single bacterial species, some chronic diseases including periodontitis result from the perturbation of the natural microbiota and the proliferation of a number of opportunistic pathogens. Both Porphyromonas gingivalis and Treponema denticola have been associated with the progression and severity of chronic periodontitis and have been shown to display synergistic virulence in animal models. However, the underlying mechanisms to these observations are unclear. Here we demonstrate that these two bacteria grow synergistically in continuous co-culture and modify their gene expression. The expression of T. denticola genes encoding known virulence factors and enzymes involved in the uptake and metabolism of the amino acid glycine was up-regulated in co-culture. T. denticola stimulated the proteolytic P. gingivalis to produce free glycine, which T. denticola used as a major carbon source. Our study shows P. gingivalis and T. denticola co-operate metabolically and this helps to explain their synergistic virulence in animal models and their intimate association in vivo.

Validation of a quantitative real-time PCR assay and comparison with fluorescence microscopy and selective agar plate counting for species-specific quantification of an in vitro subgingival biofilm model

BACKGROUND AND OBJECTIVE

Subgingival biofilms are the prime etiological factor of periodontal disease. Owing to their complex polymicrobial nature, quantification of individual bacterial species within the biofilm for research and diagnostic purposes can be methodologically challenging. The aims of this study were to establish a quantitative real-time PCR (qPCR) assay to quantify the bacteria used in our 10-species in vitro 'subgingival' biofilm model and to compare the quantitative outcome with fluorescence microscopy and colony-forming unit (CFU) counts on selective agar plates.

MATERIAL AND METHODS

The 10 species included in the in vitro biofilm were Streptococcus oralis, Streptococcus anginosus, Veillonella dispar, Fusobacterium nucleatum, Treponema denticola, Tannerella forsythia, Actinomyces oris, Campylobacter rectus, Porphyromonas gingivalis and Prevotella intermedia. The numbers of each species were quantified at two time points using qPCR, microscopy counting following fluorescence in-situ hybridization (FISH) or immunofluorescence staining, and counting of CFUs after growth on selective agar plates.

RESULTS

All 10 species were successfully quantified using qPCR and FISH or immunofluorescence, and the eight species culturable on selective agar plates were also quantified by counting the numbers of CFUs after growth on selective agar. In early biofilm cultures, all methods showed a significant correlation, although the absolute numbers differed between methods. In late biofilm cultures, measurements obtained using qPCR and FISH or immunofluorescence, but not by CFU counts, maintained significant correlation. CFU counts yielded lower values than did measurements made using the other two methods.

CONCLUSION

Quantitative PCR and epifluorescence microscopy can be easily combined with each other to determine species-specific bacterial numbers within biofilms. However, conventional bacterial cultures cannot be as efficiently combined using these molecular detection methods. This may be crucial in designing and selecting appropriate clinical diagnostic methods for subgingival biofilm samples.

Advancement of the 10-species subgingival Zurich biofilm model by examining different nutritional conditions and defining the structure of the in vitro biofilms

Background

Periodontitis is caused by a highly complex consortium of bacteria that establishes as biofilms in subgingival pockets. It is a disease that occurs worldwide and its consequences are a major health concern. Investigations in situ are not possible and the bacterial community varies greatly between patients and even within different loci. Due to the high complexity of the consortium and the availability of samples, a clear definition of the pathogenic bacteria and their mechanisms of pathogenicity are still not available. In the current study we addressed the need of a defined model system by advancing our previously described subgingival biofilm model towards a bacterial composition that reflects the one observed in diseased sites of patients and analysed the structure of these biofilms.

Results

We further developed the growth media by systematic variation of key components resulting in improved stability and the firm establishment of spirochetes in the 10-species subgingival Zurich biofilm model. A high concentration of heat-inactivated human serum allowed the best proliferation of the used species. Therefore we further investigated these biofilms by analysing their structure by confocal laser scanning microscopy following fluorescence in situ hybridisation. The species showed mutual interactions as expected from other studies. The abundances of all organisms present in this model were determined by microscopic counting following species-specific identification by both fluorescence in situ hybridisation and immunofluorescence. The newly integrated treponemes were the most abundant organisms.

Conclusions

The use of 50% of heat-inactivated human serum used in the improved growth medium resulted in significantly thicker and more stable biofilms, and the quantitative representation of the used species represents the in vivo community of periodontitis patients much closer than in biofilms grown in the two media with less or no human serum. The appearance of T. denticola, P. gingivalis, and T. forsythia in the top layer of the biofilms, and the high abundance of T. denticola, reflects well the microbial situation observed at diseased sites. The improved model biofilms will allow further investigations of interactions between individual species and of the effects of atmospheric or nutritional changes, as well as interactions with tissue cells.

Treponema denticola interactions with host proteins

Oral Treponema species, most notably T. denticola, are implicated in the destructive effects of human periodontal disease. Progress in the molecular analysis of interactions between T. denticola and host proteins is reviewed here, with particular emphasis on the characterization of surface-expressed and secreted proteins of T. denticola involved in interactions with host cells, extracellular matrix components, and components of the innate immune system.

Fatty acid profiles in smokers with chronic periodontitis

We hypothesized that tobacco smoke induces alterations to the 3-OH fatty acids present in lipid A in a manner consistent with a microflora of reduced inflammatory potential. Whole saliva samples and full-mouth clinical periodontal recordings were obtained from persons with (22 smokers; 15 non-smokers) and without (14 smokers; 15 non-smokers) chronic periodontitis. Clear differences in the contributions of multiple saturated 3-OH fatty acid species were noted in the group with disease compared with healthy individuals. Increases in the long-chain fatty acids associated with anaerobic bacterial periodontopathogens, particularly 3-OH-C(i17.0) (146.7%, relative to controls), were apparent. Significant reductions in the 3-OH fatty acids associated with the consensus (high potency) enteric LPS structure (3-OH-C(12.0) and 3-OH-C(14.0); 33.3% and 15.8% reduction, respectively) were noted in smokers compared with non-smokers with chronic periodontitis. Thus, smoking is associated with specific structural alterations to the lipid-A-derived 3-OH fatty acid profile in saliva that are consistent with an oral microflora of reduced inflammatory potential. These findings provide much-needed mechanistic insight into the established clinical conundrum of increased infection with periodontal pathogens but reduced clinical inflammation in smokers.

In vitro modeling of host-parasite interactions: the 'subgingival' biofilm challenge of primary human epithelial cells

Background

Microbial biofilms are known to cause an increasing number of chronic inflammatory and infectious conditions. A classical example is chronic periodontal disease, a condition initiated by the subgingival dental plaque biofilm on gingival epithelial tissues. We describe here a new model that permits the examination of interactions between the bacterial biofilm and host cells in general. We use primary human gingival epithelial cells (HGEC) and an in vitro grown biofilm, comprising nine frequently studied and representative subgingival plaque bacteria.

Results

We describe the growth of a mature 'subgingival' in vitro biofilm, its composition during development, its ability to adapt to aerobic conditions and how we expose in vitro a HGEC monolayer to this biofilm. Challenging the host derived HGEC with the biofilm invoked apoptosis in the epithelial cells, triggered release of pro-inflammatory cytokines and in parallel induced rapid degradation of the cytokines by biofilm-generated enzymes.

Conclusion

We developed an experimental in vitro model to study processes taking place in the gingival crevice during the initiation of inflammation. The new model takes into account that the microbial challenge derives from a biofilm community and not from planktonically cultured bacterial strains. It will facilitate easily the introduction of additional host cells such as neutrophils for future biofilm:host cell challenge studies. Our methodology may generate particular interest, as it should be widely applicable to other biofilm-related chronic inflammatory diseases.

Uncultivated Tannerella BU045 and BU063 are slim segmented filamentous rods of high prevalence but low abundance in inflammatory disease-associated dental plaques

Uncultivated clones BU045 and BU063 and Tannerella forsythia, a 'consensus periodontal pathogen', are the closest known relatives within the genus Tannerella. They have been described to inhabit different ecological niches of the human oral cavity. In this study, fluorescent in situ hybridization (FISH) and immunofluorescence were combined to investigate the prevalence and abundance of BU045 and BU063 in comparison to T. forsythia in plaques from gingivitis, necrotizing ulcerative gingivitis (NUG) and chronic periodontitis. Phylotype-specific FISH probes identified BU045 and BU063 as elongated thin rods with a segmented structure. Two structurally similar and previously unknown, rare phylotypes (127+ and 997+) were also identified due to partial 16S rRNA sequence identity with T. forsythia. In gingivitis, NUG and periodontitis patients, BU045, BU063, 127+, 997+ and T. forsythia were detected with prevalences of 50/83/71/14 and 81%, 100/100/86/17 and 53%, and 100/100/12/0 and 100%, respectively. Supragingivally, colonization density of all five organisms was generally low, rarely exceeding 0.1% of the total biota. In periodontal pocket samples, however, cell numbers of T. forsythia, but not of the uncultivable phylotypes, were greatly elevated. Our data demonstrate that Tannerella phylotypes BU045, BU063, 127+ and 997+ consist of long slim rods with segments, which, with respect to FISH stainability, often behaved as independent units. The phylotypes are frequent but low-level colonizers of various periodontal disease-associated plaques. Their apparent inability to proliferate to high density seems to exclude any relevance for the pathogenesis of periodontal diseases.