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

Initial bacterial adherence and biofilm formation on novel restorative materials used in paediatric dentistry

OBJECTIVE

To evaluate the initial bacterial adherence and biofilm formation on novel restorative materials in paediatric dentistry and compare the results to stainless steel crown and primary enamel.

MATERIALS AND METHODS

Twenty-five samples (Diameter = 4 mm) from five restorative materials (Tetric Power Fill light cured for 3 s or 10 s, Fuji II LC, Equia Forte HT Fil, Cention Forte, Stainless-steel crown) and primary enamel were prepared. Four samples served for recording of surface roughness (Ra) using a contact profilometer, 21 samples were incubated in stimulated human saliva for 2 h (initial bacterial adherence) and 72 h (biofilm formation) and served to determine ion releasing and bacterial growth. After 2 and 72 h, the number of colony-forming units (CFU) per ml was counted and expressed in Log10 CFU/ml. Data were analysed with two-way ANOVA and Tuckey's multiple comparisons test (p < 0.05).

RESULTS

All tested materials showed similar initial bacterial adherence (p > 0.1). Stainless steel crown showed statistically significantly less biofilm formation than all other tested materials (p ≤ 0.02), except for Fuji II LC (p = 0.06). In terms of biofilm formation, the differences between all tested materials were not statistically significant (p ≥ 0.9).

SIGNIFICANCE

Novel restorative materials in paediatric dentistry show similar initial bacterial adherence and biofilm formation. However, compared to other restorative materials, stainless steel crowns demonstrate the lowest level of biofilm formation. Ion-releasing materials may not necessarily show better antimicrobial properties than conventional materials.

Spatiotemporal monitoring of a periodontal multispecies biofilm model: demonstration of prebiotic treatment responses

Biofilms are complex polymicrobial communities which are often associated with human infections such as the oral disease periodontitis. Studying these complex communities under controlled conditions requires in vitro biofilm model systems that mimic the natural environment as close as possible. This study established a multispecies periodontal model in the drip flow biofilm reactor in order to mimic the continuous flow of nutrients at the air-liquid interface in the oral cavity. The design is engineered to enable real-time characterization. A community of five bacteria, Streptococcus gordonii-GFPmut3*, Streptococcus oralis-GFPmut3*, Streptococcus sanguinis-pVMCherry, Fusobacterium nucleatum, and Porphyromonas gingivalis-SNAP26 is visualized using two distinct fluorescent proteins and the SNAP-tag. The biofilm in the reactor develops into a heterogeneous, spatially uniform, dense, and metabolically active biofilm with relative cell abundances similar to those in a healthy individual. Metabolic activity, structural features, and bacterial composition of the biofilm remain stable from 3 to 6 days. As a proof of concept for our periodontal model, the 3 days developed biofilm is exposed to a prebiotic treatment with L-arginine. Multifaceted effects of L-arginine on the oral biofilm were validated by this model setup. L-arginine showed to inhibit growth and incorporation of the pathogenic species and to reduce biofilm thickness and volume. Additionally, L-arginine is metabolized by Streptococcus gordonii-GFPmut3* and Streptococcus sanguinis-pVMCherry, producing high levels of ornithine and ammonium in the biofilm. In conclusion, our drip flow reactor setup is promising in studying spatiotemporal behavior of a multispecies periodontal community.ImportancePeriodontitis is a multifactorial chronic inflammatory disease in the oral cavity associated with the accumulation of microorganisms in a biofilm. Not the presence of the biofilm as such, but changes in the microbiota (i.e., dysbiosis) drive the development of periodontitis, resulting in the destruction of tooth-supporting tissues. In this respect, novel treatment approaches focus on maintaining the health-associated homeostasis of the resident oral microbiota. To get insight in dynamic biofilm responses, our research presents the establishment of a periodontal biofilm model including Streptococcus gordonii, Streptococcus oralis, Streptococcus sanguinis, Fusobacterium nucleatum, and Porphyromonas gingivalis. The added value of the model setup is the combination of simulating continuously changing natural mouth conditions with spatiotemporal biofilm profiling using non-destructive characterization tools. These applications are limited for periodontal biofilm research and would contribute in understanding treatment mechanisms, short- or long-term exposure effects, the adaptation potential of the biofilm and thus treatment strategies.

Polymicrobial Interactions of Oral Microbiota: a Historical Review and Current Perspective

The oral microbiota is enormously diverse, with over 700 microbial species identified across individuals that play a vital role in the health of our mouth and our overall well-being. In addition, as oral diseases such as caries (cavities) and periodontitis (gum disease) are mediated through interspecies microbial interactions, this community serves as an important model system to study the complexity and dynamics of polymicrobial interactions. Here, we review historical and recent progress in our understanding of the oral microbiome, highlighting how oral microbiome research has significantly contributed to our understanding of microbial communities, with broad implications in polymicrobial diseases and across microbial community ecology. Further, we explore innovations and challenges associated with analyzing polymicrobial systems and suggest future directions of study. Finally, we provide a conceptual framework to systematically study microbial interactions within complex communities, not limited to the oral microbiota.

A critical analysis of research methods and experimental models to study intracanal medicaments

In order to ensure predictable decontamination of the root canal system, chemo-mechanical preparation of the root canal space is sometimes supplemented with the use of intracanal medication. As microbial control of the root canal space is fundamental to the resolution of apical periodontitis, root canal disinfection strategies haven been researched intensively. The use of intracanal medication as a supplementary step to the chemo-mechanical preparation of the root canal space is one of them. Because of the costs and limitations of clinical research it is relevant and common practice to first evaluate alternative or new root canal disinfection modalities in laboratory studies. This involves the simulation of a root canal infection in a laboratory model, on which different disinfection strategies can be tested. When modelling the infected root canal, different levels of infection can be discriminated: suspended bacteria, microbial biofilms and infected dentine. This review describes the experimental models associated with these infection levels and critically appraises their value and methodological details. Suggestions for relevant research methods and experimental models are given, as well as some good practices for laboratory-based microbiological studies.

Effect of TrisEDTA and Chlorhexidine 0.12% on an In Vitro-Defined Biofilm Representing the Subgingival Plaque Biofilm of the Dog

This study was designed to investigate the effects of chlorhexidine 0.12%, TrisEDTA (tromethamine ethylenediamintetraacetic acid), and a combination of chlorhexidine 0.12% and TrisEDTA on an in vitro plaque biofilm model comprised of three bacterial species commonly found in canine subgingival plaque. Porphyromonas gulae, Actinomyces canis, and Neisseria canis were grown in a biofilm on polished hydroxyapatite coated titanium alloy pucks for 72 h prior to exposure to one of four test solutions: TrisEDTA, chlorhexidine 0.12%, a combination of TrisEDTA and chlorhexidine 0.12%, or sterile deionized water as a control. Following exposure to the test solution, a sample was collected of the biofilm either immediately or following 24 h of additional incubation in a broth medium. Lower numbers of CFU/mL of Porphyromonas gulae resulted when the biofilm was treated with a solution of chlorhexidine 0.12% and TrisEDTA compared to with chlorhexidine 0.12% alone, TrisEDTA alone, or the control and so this solution can be said to be synergistic against Porphyromonas gulae in this controlled in vitro model. Greater reductions in the numbers of CFU/mL of Actinomyces canis and Neisseria canis resulted from treatment with chlorhexidine 0.12% alone than if treated with the combination of TrisEDTA and chlorhexidine 0.12%. When treated biofilm samples were allowed 24 h of additional growth in fresh media, greater variance resulted and this variance highlights the complex dynamics involved in bacterial growth within a biofilm.

Mimicking biofilm formation and development: Recent progress in in vitro and in vivo biofilm models

Biofilm formation in living organisms is associated to tissue and implant infections, and it has also been linked to the contribution of antibiotic resistance. Thus, understanding biofilm development and being able to mimic such processes is vital for the successful development of antibiofilm treatments and therapies. Several decades of research have contributed to building the foundation for developing in vitro and in vivo biofilm models. However, no such thing as an "all fit" in vitro or in vivo biofilm models is currently available. In this review, in addition to presenting an updated overview of biofilm formation, we critically revise recent approaches for the improvement of in vitro and in vivo biofilm models.

Modified SHI medium supports growth of a disease-state subgingival polymicrobial community in vitro

Developing a laboratory model of oral polymicrobial communities is essential for in vitro studies of the transition from healthy to diseased oral plaque. SHI medium is an enriched growth medium capable of supporting in vitro biofilms with similar diversity to healthy supragingival inocula; however, this medium does not maintain the diversity of gram‐negative bacteria more associated with subgingival plaque. Here, we systematically modified SHI medium components to investigate the impacts of varying nutrients and develop a medium capable of supporting a specific disease‐state subgingival community. A diseased subgingival plaque sample was inoculated in SHI medium with increasing concentrations of sucrose (0%, 0.1%, 0.5%), fetal bovine serum (FBS) (0%, 10%, 20%, 30%, 50%), and mucin (0.1, 2.5, 8.0 g/L) and grown for 48 hrs, then the 16S rRNA profiles of the resulting biofilms were examined. In total, these conditions were able to capture 89 of the 119 species and 43 of the 51 genera found in the subgingival inoculum. Interestingly, biofilms grown in high sucrose media, although dominated by acidogenic Firmicutes with a low final pH, contained several uncultured taxa from the genus Treponema, information that may aid culturing these periodontitis‐associated fastidious organisms. Biofilms grown in a modified medium (here named subSHI‐v1 medium) with 0.1% sucrose and 10% FBS had a high diversity closest to the inoculum and maintained greater proportions of many gram‐negative species of interest from the subgingival periodontal pocket (including members of the genera Prevotella and Treponema, and the Candidate Phyla Radiation phylum Saccharibacteria), and therefore best represented the disease community.

Endodontic-Like Oral Biofilms as Models for Multispecies Interactions in Endodontic Diseases

Oral bacteria possess the ability to form biofilms on solid surfaces. After the penetration of oral bacteria into the pulp, the contact between biofilms and pulp tissue may result in pulpitis, pulp necrosis and/or periapical lesion. Depending on the environmental conditions and the availability of nutrients in the pulp chamber and root canals, mainly Gram-negative anaerobic microorganisms predominate and form the intracanal endodontic biofilm. The objective of the present study was to investigate the role of different substrates on biofilm formation as well as the separate and collective incorporation of six endodontic pathogens, namely Enterococcus faecalis, Staphylococcus aureus, Prevotella nigrescens, Selenomonas sputigena, Parvimonas micra and Treponema denticola into a nine-species "basic biofilm". This biofilm was formed in vitro as a standard subgingival biofilm, comprising Actinomyces oris, Veillonella dispar, Fusobacterium nucleatum, Streptococcus anginosus, Streptococcus oralis, Prevotella intermedia, Campylobacter rectus, Porphyromonas gingivalis, and Tannerella forsythia. The resulting endodontic-like biofilms were grown 64 h under the same conditions on hydroxyapatite and dentin discs. After harvesting the endodontic-like biofilms, the bacterial growth was determined using quantitative real-time PCR, were labeled using fluorescence in situ hybridization (FISH) and analyzed by confocal laser scanning microscopy (CLSM). The addition of six endodontic pathogens to the "basic biofilm" induced a decrease in the cell number of the "basic" species. Interestingly, C. rectus counts increased in biofilms containing E. faecalis, S. aureus, P. nigrescens and S. sputigena, respectively, both on hydroxyapatite and on dentin discs, whereas P. intermedia counts increased only on dentin discs by addition of E. faecalis. The growth of E. faecalis on hydroxyapatite discs and of E. faecalis and S. aureus on dentin discs were significantly higher in the biofilm containing all species than in the "basic biofilm". Contrarily, the counts of P. nigrescens, S. sputigena and P. micra on hydroxyapatite discs as well as counts of P. micra and T. denticola on dentin discs decreased in the all-species biofilm. Overall, all bacterial species associated with endodontic infections were successfully incorporated into the standard multispecies biofilm model both on hydroxyapatite and dentin discs. Thus, future investigations on endodontic infections can rely on this newly established endodontic-like multispecies biofilm model.

Combined DNase and Proteinase Treatment Interferes with Composition and Structural Integrity of Multispecies Oral Biofilms

Modification of oral biofilms adhering to dental hard tissues could lead to new treatment approaches in cariology and periodontology. In this study the impact of DNase I and/or proteinase K on the formation of a simulated supragingival biofilm was investigated in vitro. Six-species biofilms were grown anaerobically in the presence of DNase I and proteinase K. After 64 h biofilms were either harvested and quantified by culture analysis or proceeded to staining followed by confocal laser scanning microscopy. Microbial cells were stained using DNA-dyes or fluorescent in situ hybridization. Exopolysaccharides, eDNA and exoproteins were stained with Calcofluor, anti-DNA-antibody, and Sypro^TM Ruby, respectively. Overall, results showed that neither DNase I nor proteinase K had an impact on total colony-forming units (CFUs) compared to the control without enzymes. However, DNase I significantly suppressed the growth of Actinomyces oris, Fusobacterium nucleatum, Streptococcus mutans, Streptococcus oralis and Candida albicans. Proteinase K treatment induced significant increase in S. mutans and S. oralis CFUs (p < 0.001), whereas C. albicans and V. dispar showed lower CFUs compared to the control. Interestingly, confocal images visualized the biofilm degradation caused by DNase I and proteinase K. Thus, enzymatic treatment should be combined with conventional antimicrobial agents aiming at both bactericidal effectiveness and biofilm dispersal.

Polymicrobial oral biofilm models: simplifying the complex

Over the past century, numerous studies have used oral biofilm models to investigate growth kinetics, biofilm formation, structure and composition, antimicrobial susceptibility and host-pathogen interactions. In vivo animal models provide useful models of some oral diseases; however, these are expensive and carry vast ethical implications. Oral biofilms grown or maintained in vitro offer a useful platform for certain studies and have the advantages of being inexpensive to establish and easy to reproduce and manipulate. In addition, a wide range of variables can be monitored and adjusted to mimic the dynamic environmental changes at different sites in the oral cavity, such as pH, temperature, salivary and gingival crevicular fluid flow rates, or microbial composition. This review provides a detailed insight for early-career oral science researchers into how the biofilm models used in oral research have progressed and improved over the years, their advantages and disadvantages, and how such systems have contributed to our current understanding of oral disease pathogenesis and aetiology.

Fusobacterium Species and Subspecies Differentially Affect the Composition and Architecture of Supra- and Subgingival Biofilms Models

Fusobacteria are common obligately anaerobic Gram-negative bacteria of the oral cavity that may act as a bridge between early and late colonizing bacteria in dental plaque and have a role in oral and extra-oral infections. Fusobacterium nucleatum has a crucial role in oral biofilm structure and ecology, as revealed in experimental and clinical biofilm models. The aim of this study was to investigate the impact of various Fusobacterium species on in vitro biofilm formation and structure in three different oral biofilm models namely a supragingival, a supragingival “feeding”, and a subgingival biofilm model. The standard six-species supragingival and “feeding” biofilm models employed contained Actinomyces oris, Candida albicans, Streptococcus mutans, Streptococcus oralis, Veillonella dispar, and Fusobacterium sp. The subgingival biofilm model contained 10 species (A. oris, Campylobacter rectus, F. nucleatum ssp. nucleatum, Porphyromonas gingivalis, Prevotella intermedia, Streptococcus anginosus, S. oralis, Tannerella forsythia, Treponema denticola, and V. dispar). Six different Fusobacterium species or subspecies, respectively, were tested namely F. nucleatum ssp. fusiforme, F. nucleatum ssp. nucleatum, F. nucleatum ssp. polymorphum, F. nucleatum ssp. vincentii, F. naviforme, and F. periodonticum). Biofilms were grown anaerobically on hydroxyapatite disks in 24-well culture dishes. After 64 h, biofilms were either harvested and quantified by culture analysis or proceeded to fluorescent in situ hybridization (FISH) and confocal laser scanning microscopy (CLSM). All Fusobacterium species tested established well in the biofilms, with CFUs ranging from 1.4E+04 (F. nucleatum ssp. fusiforme) to 5.6E+06 (F. nucleatum ssp. nucleatum). The presence of specific Fusobacterium sp./ssp. induced a significant decrease in C. albicans levels in the supragingival model and in V. dispar levels in the “feeding” supragingival model. In the subgingival model, the counts of A. oris, S. oralis, P. intermedia, P. gingivalis, and C. rectus significantly decreased in the presence of specific Fusobacterium sp./ssp. Collectively, this study showed variations in the growing capacities of different fusobacteria within biofilms, affecting the growth of surrounding species and potentially the biofilm architecture. Hence, clinical or experimental studies need to differentiate between Fusobacterium sp./ssp., as their biological properties may well vary.

Microcosm biofilms cultured from different oral niches in periodontitis patients

Objective: Periodontal diseases are triggered by dysbiotic microbial biofilms. Therefore, it is essential to develop appropriate biofilm models. Aim of the present study was to culture microcosm biofilms inoculated from different niches in periodontitis patients and compare their microbial composition to those inoculated from subgingival plaque. Methods: Saliva, subgingival plaque, tongue and tonsils were sampled in five periodontitis patients to serve as inocula for culturing biofilms in vitro in an active attachment model. Biofilms were grown for 14 or 28 d and analyzed for their microbial composition by 16S rDNA sequencing. Results: As classified by HOMD, all biofilms were dominated by periodontitis-associated taxa, irrespective which niche had been used for inoculation. There was a low similarity between 14 d biofilms and their respective inocula (Bray-Curtis similarity 0.26), while biofilms cultured for 14 and 28 d shared high similarity (0.69). Principal components analysis showed much stronger clustering per patient than per niche indicating that the choice of patients may be more crucial than choice of the respective niches in these patients. Conclusion: Saliva, tongue scrapings or tonsil swabs may represent sufficient alternative inocula for growing microcosm biofilms resembling periodontitis-associated microbial communities in cases when sampling subgingival plaque is not possible.

Development and characterization of an oral multispecies biofilm implant flow chamber model

Peri-implant infections are the most common cause of implant failure in modern dental implantology. These are caused by the formation of biofilms on the implant surface and consist of oral commensal and pathogenic bacteria, which harm adjacent soft and hard tissues and may ultimately lead to implant loss. In order to improve the clinical situation, there has to be a better understanding of biofilm formation on abiotic surfaces. Therefore, we successfully developed a system to cultivate an oral multispecies biofilm model in a flow chamber system, optimized for the evaluation of biofilm formation on solid materials by direct microscopic investigation. The model contains four relevant oral bacterial species: Streptococcus oralis, Actinomyces naeslundii, Veillonella dispar and Porphyromonas gingivalis in ratios similar to the native situation. The reliability of the developed “Hanoverian Oral Multispecies Biofilm Implant Flow Chamber” (HOBIC) model was verified. Biofilm volume and live/dead distribution within biofilms were determined by fluorescence staining and confocal laser scanning microcopy (CLSM). The individual species distribution was analyzed using quantitative real time PCR with propidium monoazide pretreatment (PMA-qRT-PCR) and by urea-NaCl fluorescence in situ hybridization (urea-NaCl-FISH). This in vitro model may be used to analyze biofilm formation on dental implants in more detail and to develop future implant systems with improved material properties.

Phenalen-1-One-Mediated Antimicrobial Photodynamic Therapy and Chlorhexidine Applied to a Novel Caries Biofilm Model

Antimicrobial photodynamic therapy (aPDT) may be useful as a supportive antimicrobial measure for caries-active subjects. In this study, the antimicrobial efficacy of aPDT with a phenalen-1-one photosensitizer was evaluated in a novel in vitro biofilm model comprising Actinomyces naeslundii, Actinomyces odontolyticus, and Streptococcus mutans and was compared to chlorhexidine. The proposed biofilm model allows high-throughput screening for antimicrobial efficacy while exhibiting a differentiated response to different antimicrobial approaches. While chlorhexidine 0.2% showed a reduction of ≈4 log10 for all species, aPDT led to a more pronounced reduction of S. mutans (2.8 log10) than of Actinomyces spp. (1.2 or 1.3 log10). A similar effect was also observed in monospecies biofilms. Therefore, aPDT may be more effective against S. mutans than against Actinomyces spp. when in biofilms, and this antimicrobial approach merits further investigations.

Streptococcus oralis maintains homeostasis in oral biofilms by antagonizing the cariogenic pathogen Streptococcus mutans

Bacteria residing in oral biofilms live in a state of dynamic equilibrium with one another. The intricate synergistic or antagonistic interactions between them are crucial for determining this balance. Using the six-species Zürich "supragingival" biofilm model, this study aimed to investigate interactions regarding growth and localization of the constituent species. As control, an inoculum containing all six strains was used, whereas in each of the further five inocula one of the bacterial species was alternately absent, and in the last, both streptococci were absent. Biofilms were grown anaerobically on hydroxyapatite disks, and after 64 h they were harvested and quantified by culture analyses. For visualization, fluorescence in situ hybridization and confocal laser scanning microscopy were used. Compared with the control, no statistically significant difference of total colony-forming units was observed in the absence of any of the biofilm species, except for Fusobacterium nucleatum, whose absence caused a significant decrease in total bacterial numbers. Absence of Streptococcus oralis resulted in a significant decrease in Actinomyces oris, and increase in Streptococcus mutans (P < .001). Absence of A. oris, Veillonella dispar or S. mutans did not cause any changes. The structure of the biofilm with regards to the localization of the species did not result in observable changes. In summary, the most striking observation of the present study was that absence of S. oralis resulted in limited growth of commensal A. oris and overgrowth of S. mutans. These data establish highlight S. oralis as commensal keeper of homeostasis in the biofilm by antagonizing S. mutans, so preventing a caries-favoring dysbiotic state.

Osteogenic nanostructured titanium surfaces with antibacterial properties under conditions that mimic the dynamic situation in the oral cavity

The study aim was to assess the impact of different surface nanofeatures on otherwise smooth titanium surfaces on bacterial adhesion as well as on their osteogenic potential.

The making of a miscreant: tobacco smoke and the creation of pathogen-rich biofilms

We have previously reported that oral biofilms in clinically healthy smokers are pathogen-rich, and that this enrichment occurs within 24 h of biofilm formation. The present investigation aimed to identify a mechanism by which smoking creates this altered community structure. By combining in vitro microbial–mucosal interface models of commensal (consisting of Streptococcus oralis, Streptococcus sanguis, Streptococcus mitis, Actinomyces naeslundii, Neisseria mucosa and Veillonella parvula) and pathogen-rich (comprising S.oralis, S.sanguis, S.mitis, A.naeslundii, N.mucosa and V.parvula, Fusobacterium nucleatum, Porphyromonas gingivalis, Filifactor alocis, Dialister pneumosintes, Selenonomas sputigena, Selenominas noxia, Catonella morbi, Parvimonas micra and Tannerella forsythia) communities with metatranscriptomics, targeted proteomics and fluorescent microscopy, we demonstrate that smoke exposure significantly downregulates essential metabolic functions within commensal biofilms, while significantly increasing expression of virulence genes, notably lipopolysaccharide (LPS), flagella and capsule synthesis. By contrast, in pathogen-rich biofilms several metabolic pathways were over-expressed in response to smoke exposure. Under smoke-rich conditions, epithelial cells mounted an early and amplified pro-inflammatory and oxidative stress response to these virulence-enhanced commensal biofilms, and a muted early response to pathogen-rich biofilms. Commensal biofilms also demonstrated early and widespread cell death. Similar results were observed when smoke-free epithelial cells were challenged with smoke-conditioned biofilms, but not vice versa. In conclusion, our data suggest that smoke-induced transcriptional shifts in commensal biofilms triggers a florid pro-inflammatory response, leading to early commensal death, which may preclude niche saturation by these beneficial organisms. The cytokine-rich, pro-oxidant, anaerobic environment sustains inflammophilic bacteria, and, in the absence of commensal antagonism, may promote the creation of pathogen-rich biofilms in smokers.

Tobacco smoke inhibits the metabolism of beneficial bacteria in biofilms, while activating specific genes in pathogenic bacteria. This suggests a mechanism to explain how smoking quickly leads to the formation of damaging biofilms in the mouth and respiratory tract. Purnima Kumar and colleagues at Ohio State University, USA studied the effect of tobacco smoke on cultured biofilms used to model those that form on mucous membranes. They detected specific and varied changes in the activity of genes, proteins and metabolism that allowed pathogenic bacteria to displace beneficial “commensal” bacteria. The research suggests the transition toward pathogen-rich biofilms may contribute to the health effects of smoking by causing increased inflammation of mucous membranes and the production of damaging oxidant chemicals. Further research should investigate the chemical constituents of smoke responsible for these effects.

Behavior of two Tannerella forsythia strains and their cell surface mutants in multispecies oral biofilms

As a member of subgingival multispecies biofilms, Tannerella forsythia is commonly associated with periodontitis. The bacterium has a characteristic cell surface (S‐) layer modified with a unique O‐glycan. Both the S‐layer and the O‐glycan were analyzed in this study for their role in biofilm formation by employing an in vitro multispecies biofilm model mimicking the situation in the oral cavity. Different T. forsythia strains and mutants with characterized defects in cell surface composition were incorporated into the model, together with nine species of select oral bacteria. The influence of the T. forsythia S‐layer and attached glycan on the bacterial composition of the biofilms was analyzed quantitatively using colony‐forming unit counts and quantitative real‐time polymerase chain reaction, as well as qualitatively by fluorescence in situ hybridization and confocal laser scanning microscopy. This revealed that changes in the T. forsythia cell surface did not affect the quantitative composition of the multispecies consortium, with the exception of Campylobacter rectus cell numbers. The localization of T. forsythia within the bacterial agglomeration varied depending on changes in the S‐layer glycan, and this also affected its aggregation with Porphyromonas gingivalis. This suggests a selective role for the glycosylated T. forsythia S‐layer in the positioning of this species within the biofilm, its co‐localization with P. gingivalis, and the prevalence of C. rectus. These findings might translate into a potential role of T. forsythia cell surface structures in the virulence of this species when interacting with host tissues and the immune system, from within or beyond the biofilm.

An oral multispecies biofilm model for high content screening applications

Peri-implantitis caused by multispecies biofilms is a major complication in dental implant treatment. The bacterial infection surrounding dental implants can lead to bone loss and, in turn, to implant failure. A promising strategy to prevent these common complications is the development of implant surfaces that inhibit biofilm development. A reproducible and easy-to-use biofilm model as a test system for large scale screening of new implant surfaces with putative antibacterial potency is therefore of major importance. In the present study, we developed a highly reproducible in vitro four-species biofilm model consisting of the highly relevant oral bacterial species Streptococcus oralis, Actinomyces naeslundii, Veillonella dispar and Porphyromonas gingivalis. The application of live/dead staining, quantitative real time PCR (qRT-PCR), scanning electron microscopy (SEM) and urea-NaCl fluorescence in situ hybridization (urea-NaCl-FISH) revealed that the four-species biofilm community is robust in terms of biovolume, live/dead distribution and individual species distribution over time. The biofilm community is dominated by S. oralis, followed by V. dispar, A. naeslundii and P. gingivalis. The percentage distribution in this model closely reflects the situation in early native plaques and is therefore well suited as an in vitro model test system. Furthermore, despite its nearly native composition, the multispecies model does not depend on nutrient additives, such as native human saliva or serum, and is an inexpensive, easy to handle and highly reproducible alternative to the available model systems. The 96-well plate format enables high content screening for optimized implant surfaces impeding biofilm formation or the testing of multiple antimicrobial treatment strategies to fight multispecies biofilm infections, both exemplary proven in the manuscript.

Red and Green Fluorescence from Oral Biofilms

Red and green autofluorescence have been observed from dental plaque after excitation by blue light. It has been suggested that this red fluorescence is related to caries and the cariogenic potential of dental plaque. Recently, it was suggested that red fluorescence may be related to gingivitis. Little is known about green fluorescence from biofilms. Therefore, we assessed the dynamics of red and green fluorescence in real-time during biofilm formation. In addition, the fluorescence patterns of biofilm formed from saliva of eight different donors are described under simulated gingivitis and caries conditions. Biofilm formation was analysed for 12 hours under flow conditions in a microfluidic BioFlux flow system with high performance microscopy using a camera to allow live cell imaging. For fluorescence images dedicated excitation and emission filters were used. Both green and red fluorescence were linearly related with the total biomass of the biofilms. All biofilms displayed to some extent green and red fluorescence, with higher red and green fluorescence intensities from biofilms grown in the presence of serum (gingivitis simulation) as compared to the sucrose grown biofilms (cariogenic simulation). Remarkably, cocci with long chain lengths, presumably streptococci, were observed in the biofilms. Green and red fluorescence were not found homogeneously distributed within the biofilms: highly fluorescent spots (both green and red) were visible throughout the biomass. An increase in red fluorescence from the in vitro biofilms appeared to be related to the clinical inflammatory response of the respective saliva donors, which was previously assessed during an in vivo period of performing no-oral hygiene. The BioFlux model proved to be a reliable model to assess biofilm fluorescence. With this model, a prediction can be made whether a patient will be prone to the development of gingivitis or caries.

Experimental Models of Oral Biofilms Developed on Inert Substrates: A Review of the Literature

The oral ecosystem is a very complex environment where more than 700 different bacterial species can be found. Most of them are organized in biofilm on dental and mucosal surfaces. Studying this community is important because a rupture in stability can lead to the preeminence of pathogenic microorganisms, causing dental decay, gingivitis, or periodontitis. The multitude of species complicates biofilm analysis so its reproduction, collection, and counting are very delicate. The development of experimental models of dental biofilms was therefore essential and multiple in vitro designs have emerged, each of them especially adapted to observing biofilm formation of specific bacteria within specific environments. The aim of this review is to analyze oral biofilm models.

Incorporation of staphylococci into titanium-grown biofilms: an in vitro "submucosal" biofilm model for peri-implantitis

Objectives

Staphylococcus spp. are postulated to play a role in peri‐implantitis. This study aimed to develop a “submucosal” in vitro biofilm model, by integrating two staphylococci into its composition.

Materials and methods

The standard “subgingival” biofilm contained Actinomyces oris, Fusobacterium nucleatum, Streptococcus oralis, Veillonella dispar, Campylobacter rectus, Prevotella intermedia, Streptococcus anginosus, Porphyromonas gingivalis, Tannerella forsythia and Treponema denticola, and was further supplemented with Staphyoccous aureus and/or Staphylococcus epidermidis. Biofilms were grown anaerobically on hydroxyapatite or titanium discs and harvested after 64 h for real‐time polymerase chain reaction, to determine their composition. Confocal laser scanning microscopy and fluorescence in situ hybridization were used for identifying the two staphylococci within the biofilm.

Results

Both staphylococci established within the biofilms when added separately. However, when added together, only S. aureus grew in high numbers, whereas S. epidermidis was reduced almost to the detection limit. Compared to the standard subgingival biofilm, addition of the two staphylococci had no impact on the qualitative or quantitative composition of the biofilm. When grown individually in the biofilm, S. epidermidis and S. aureus formed small distinctive clusters and it was confirmed that S. epidermidis was not able to grow in presence of S. aureus.

Conclusions

Staphyoccous aureus and S. epidermidis can be individually integrated into an oral biofilm grown on titanium, hence establishing a “submucosal” biofilm model for peri‐implantitis. This model also revealed that S. aureus outcompetes S. epidermidis when grown together in the biofilm, which may explain the more frequent association of the former with peri‐implantitis.

Quantitative proteomics reveal distinct protein regulations caused by Aggregatibacter actinomycetemcomitans within subgingival biofilms

Periodontitis is an infectious disease that causes the inflammatory destruction of the tooth-supporting (periodontal) tissues, caused by polymicrobial biofilm communities growing on the tooth surface. Aggressive periodontitis is strongly associated with the presence of Aggregatibacter actinomycetemcomitans in the subgingival biofilms. Nevertheless, whether and how A. actinomycetemcomitans orchestrates molecular changes within the biofilm is unclear. The aim of this work was to decipher the interactions between A. actinomycetemcomitans and other bacterial species in a multi-species biofilm using proteomic analysis. An in vitro 10-species "subgingival" biofilm model, or its derivative that included additionally A. actinomycetemcomitans, were anaerobically cultivated on hydroxyapatite discs for 64 h. When present, A. actinomycetemcomitans formed dense intra-species clumps within the biofilm mass, and did not affect the numbers of the other species in the biofilm. Liquid chromatography-tandem mass spectrometry was used to identify the proteomic content of the biofilm lysate. A total of 3225 and 3352 proteins were identified in the biofilm, in presence or absence of A. actinomycetemcomitans, respectively. Label-free quantitative proteomics revealed that 483 out of the 728 quantified bacterial proteins (excluding those of A. actinomycetemcomitans) were accordingly regulated. Interestingly, all quantified proteins from Prevotella intermedia were up-regulated, and most quantified proteins from Campylobacter rectus, Streptococcus anginosus, and Porphyromonas gingivalis were down-regulated in presence of A. actinomycetemcomitans. Enrichment of Gene Ontology pathway analysis showed that the regulated groups of proteins were responsible primarily for changes in the metabolic rate, the ferric iron-binding, and the 5S RNA binding capacities, on the universal biofilm level. While the presence of A. actinomycetemcomitans did not affect the numeric composition or absolute protein numbers of the other biofilm species, it caused qualitative changes in their overall protein expression profile. These molecular shifts within the biofilm warrant further investigation on their potential impact on its virulence properties, and association with periodontal pathogenesis.

Establishment of an oral infection model resembling the periodontal pocket in a perfusion bioreactor system

Periodontal infection involves a complex interplay between oral biofilms, gingival tissues and cells of the immune system in a dynamic microenvironment. A humanized in vitro model that reduces the need for experimental animal models, while recapitulating key biological events in a periodontal pocket, would constitute a technical advancement in the study of periodontal disease. The aim of this study was to use a dynamic perfusion bioreactor in order to develop a gingival epithelial-fibroblast-monocyte organotypic co-culture on collagen sponges. An 11 species subgingival biofilm was used to challenge the generated tissue in the bioreactor for a period of 24 h. The histological and scanning electron microscopy analysis displayed an epithelial-like layer on the surface of the collagen sponge, supported by the underlying ingrowth of gingival fibroblasts, while monocytic cells were also found within the sponge mass. Bacterial quantification of the biofilm showed that in the presence of the organotypic tissue, the growth of selected biofilm species, especially Campylobacter rectus, Actinomyces oris, Streptococcus anginosus, Veillonella dispar, and Porphyromonas gingivalis, was suppressed, indicating a potential antimicrobial effect by the tissue. Multiplex immunoassay analysis of cytokine secretion showed that interleukin (IL)-1 β, IL-2, IL-4, and tumor necrosis factor (TNF)-α levels in cell culture supernatants were significantly up-regulated in presence of the biofilm, indicating a positive inflammatory response of the organotypic tissue to the biofilm challenge. In conclusion, this novel host-biofilm interaction organotypic model might resemble the periodontal pocket and have an important impact on the study of periodontal infections, by minimizing the need for the use of experimental animal models.

Role of Porphyromonas gingivalis gingipains in multi-species biofilm formation

Background

Periodontal diseases are polymicrobial diseases that cause the inflammatory destruction of the tooth-supporting (periodontal) tissues. Their initiation is attributed to the formation of subgingival biofilms that stimulate a cascade of chronic inflammatory reactions by the affected tissue. The Gram-negative anaerobes Porphyromonas gingivalis, Tannerella forsythia and Treponema denticola are commonly found as part of the microbiota of subgingival biofilms, and they are associated with the occurrence and severity of the disease. P. gingivalis expresses several virulence factors that may support its survival, regulate its communication with other species in the biofilm, or modulate the inflammatory response of the colonized host tissue. The most prominent of these virulence factors are the gingipains, which are a set of cysteine proteinases (either Arg-specific or Lys-specific). The role of gingipains in the biofilm-forming capacity of P. gingivalis is barely investigated. Hence, this in vitro study employed a biofilm model consisting of 10 “subgingival” bacterial species, incorporating either a wild-type P. gingivalis strain or its derivative Lys-gingipain and Arg-gingipan isogenic mutants, in order to evaluate quantitative and qualitative changes in biofilm composition.

Results

Following 64 h of biofilm growth, the levels of all 10 species were quantified by fluorescence in situ hybridization or immunofluorescence. The wild-type and the two gingipain-deficient P. gingivalis strains exhibited similar growth in their corresponding biofilms. Among the remaining nine species, only the numbers of T. forsythia were significantly reduced, and only when the Lys-gingipain mutant was present in the biofilm. When evaluating the structure of the biofilm by confocal laser scanning microscopy, the most prominent observation was a shift in the spatial arrangement of T. denticola, in the presence of P. gingivalis Arg-gingipain mutant.

Conclusions

The gingipains of P. gingivalis may qualitatively and quantitatively affect composition of polymicrobial biofilms. The present experimental model reveals interdependency between the gingipains of P. gingivalis and T. forsythia or T. denticola.

Integration of non-oral bacteria into in vitro oral biofilms

Biofilms are polymicrobial communities that grow on surfaces in nature. Oral bacteria can spontaneously form biofilms on the surface of teeth, which may compromise the health of the teeth, or their surrounding (periodontal) tissues. While the oral bacteria exhibit high tropism for their specialized ecological niche, it is not clear if bacteria that are not part of the normal oral microbiota can efficiently colonize and grow within oral biofilms. By using an in vitro "supragingival" biofilm model of 6 oral species, this study aimed to investigate if 3 individual bacterial species that are not part of the normal oral microbiota (Eschericia coli, Staphylococcus aureus, Enterococcus faecails) and one not previously tested oral species (Aggregatibacter actinomycetemcomitans) can be incorporated into this established supragingival biofilm model. Staphylococcus aureus and A. actinomycetemcomitans were able to grow efficiently in the biofilm, without disrupting the growth of the remaining species. They localized in sparse small aggregates within the biofilm mass. Enterococcus faecalis and E. coli were both able to populate the biofilm at high numbers, and suppressed the growth of A. oris and S. mutants. Enterococcus faecalis was arranged in a chain-like conformation, whereas E. coli was densely and evenly spread throughout the biofilm mass. In conclusion, it is possible for selected species that are not part of the normal oral microbiota to be introduced into an oral biofilm, under the given experimental micro-environmental conditions. Moreover, the equilibrated incorporation of A. actinomycetemcomitans and S. aureus in this oral biofilm model could be a useful tool in the study of aggressive periodontitis and peri-implantitis, in which these organisms are involved, respectively.

Colonisation of gingival epithelia by subgingival biofilms in vitro: role of "red complex" bacteria

OBJECTIVES

Biofilm formation on tooth surface results in colonisation and invasion of the juxtaposed gingival tissue, eliciting strong inflammatory responses that lead to periodontal disease. This in vitro study investigated the colonisation of human gingival multi-layered epithelium by multi-species subgingival biofilms, and evaluated the relative effects of the "red complex" species (Porphyromonas gingivalis, Tannerella forsythia and Treponema denticola).

METHODS

The grown biofilm consisted of Fusobacterium nucleatum, Campylobacter rectus, Veillonella dispar, P. gingivalis, Prevotella intermedia, T. forsythia, T. denticola, Actinomyces oris, Streptococcus anginosus and Streptococcus oralis, or its variant lacking the "red complex". After 48h in co-culture with the gingival epithelia, the bacterial species in the biofilm were quantified, whereas their localisation on the cell surface was investigated by combining confocal-laser scanning microscopy (CLSM) and fluorescence in situ hybridisation (FISH), as well as by scanning electron microscopy (SEM).

RESULTS

Exclusion of the "red complex" quantitatively affected S. oralis, but not other species. The "red-complex" species were all able to colonise the gingival epithelial cells. A co-localisation trend was observed between P. gingivalis and T. denticola, as determined by FISH. However, in the absence of all three "red complex" bacteria from the biofilm, an immense colonisation of streptococci (potentially S. oralis) was observed on the gingival epithelia, as confirmed by both CLSM and SEM.

CONCLUSIONS

While the "red complex" species synergise in colonizing gingival epithelia, their absence from the biofilm enhances streptococcal colonisation. This antagonism with streptococci reveals that the "red complex" may regulate biofilm virulence, with potential implications in periodontal pathogenesis.

Impact of early colonizers on in vitro subgingival biofilm formation

The aim of this study was to investigate the impact of early colonizing species on the structure and the composition of the bacterial community developing in a subgingival 10-species biofilm model system. The model included Streptococcus oralis, Streptococcus anginosus, Actinomycesoris, Fusobacterium nucleatum subsp. nucleatum, Veillonella dispar, Campylobacter rectus, Prevotella intermedia, Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola. Based on literature, we considered Streptococcus oralis, Streptococcus anginosus, and Actinomyces oris as early colonizers and examined their role in the biofilms by either a delayed addition to the consortium, or by not inoculating at all the biofilms with these species. We quantitatively evaluated the resulting biofilms by real-time quantitative PCR and further compared the structures using confocal laser scanning microscopy following fluorescence in situ hybridisation. The absence of the early colonizers did not hinder biofilm formation. The biofilms reached the same total counts and developed to normal thickness. However, quantitative shifts in the abundances of individual species were observed. In the absence of streptococci, the overall biofilm structure appeared looser and more dispersed. Moreover, besides a significant increase of P. intermedia and a decrease of P. gingivalis , P. intermedia appeared to form filamented long chains that resembled streptococci. A. oris, although growing to significantly higher abundance in absence of streptococci, did not have a visible impact on the biofilms. Hence, in the absence of the early colonizers, there is a pronounced effect on P. intermedia and P. gingivalis that may cause distinct shifts in the structure of the biofilm. Streptococci possibly facilitate the establishment of P. gingivalis into subgingival biofilms, while in their absence P. intermedia became more dominant and forms elongated chains.