The establishment of reproducible, complex communities of oral bacteria in the chemostat using defined inocula

Evaluating models and assessment techniques for understanding oral biofilm complexity

Oral biofilms are three-dimensional (3D) complex entities initiating dental diseases and have been evaluated extensively in the scientific literature using several biofilm models and assessment techniques. The list of biofilm models and assessment techniques may overwhelm a novice biofilm researcher. This narrative review aims to summarize the existing literature on biofilm models and assessment techniques, providing additional information on selecting an appropriate model and corresponding assessment techniques, which may be useful as a guide to the beginner biofilm investigator and as a refresher to experienced researchers. The review addresses previously established 2D models, outlining their advantages and limitations based on the growth environment, availability of nutrients, and the number of bacterial species, while also exploring novel 3D biofilm models. The growth of biofilms on clinically relevant 3D models, particularly melt electrowritten fibrous scaffolds, is discussed with a specific focus that has not been previously reported. Relevant studies on validated oral microcosm models that have recently gaining prominence are summarized. The review analyses the advantages and limitations of biofilm assessment methods, including colony forming unit culture, crystal violet, 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide inner salt assays, confocal microscopy, fluorescence in situ hybridization, scanning electron microscopy, quantitative polymerase chain reaction, and next-generation sequencing. The use of more complex models with advanced assessment methodologies, subject to the availability of equipment/facilities, may help in developing clinically relevant biofilms and answering appropriate research questions.

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.

Rapid infectious disease identification by next-generation DNA sequencing

Currently, there is a critical need to rapidly identify infectious organisms in clinical samples. Next-Generation Sequencing (NGS) could surmount the deficiencies of culture-based methods; however, there are no standardized, automated programs to process NGS data. To address this deficiency, we developed the Rapid Infectious Disease Identification (RIDI™) system. The system requires minimal guidance, which reduces operator errors. The system is compatible with the three major NGS platforms. It automatically interfaces with the sequencing system, detects their data format, configures the analysis type, applies appropriate quality control, and analyzes the results. Sequence information is characterized using both the NCBI database and RIDI™ specific databases. RIDI™ was designed to identify high probability sequence matches and more divergent matches that could represent different or novel species. We challenged the system using defined American Type Culture Collection (ATCC) reference standards of 27 species, both individually and in varying combinations. The system was able to rapidly detect known organisms in <12h with multi-sample throughput. The system accurately identifies 99.5% of the DNA sequence reads at the genus-level and 75.3% at the species-level in reference standards. It has a limit of detection of 146cells/ml in simulated clinical samples, and is also able to identify the components of polymicrobial samples with 16.9% discrepancy at the genus-level and 31.2% at the species-level. Thus, the system's effectiveness may exceed current methods, especially in situations where culture methods could produce false negatives or where rapid results would influence patient outcomes.

Effect of periodontal pathogens on the metatranscriptome of a healthy multispecies biofilm model

Oral bacterial biofilms are highly complex microbial communities with up to 700 different bacterial taxa. We report here the use of metatranscriptomic analysis to study patterns of community gene expression in a multispecies biofilm model composed of species found in healthy oral biofilms (Actinomyces naeslundii, Lactobacillus casei, Streptococcus mitis, Veillonella parvula, and Fusobacterium nucleatum) and the same biofilm plus the periodontopathogens Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans. The presence of the periodontopathogens altered patterns in gene expression, and data indicate that transcription of protein-encoding genes and small noncoding RNAs is stimulated. In the healthy biofilm hypothetical proteins, transporters and transcriptional regulators were upregulated while chaperones and cell division proteins were downregulated. However, when the pathogens were present, chaperones were highly upregulated, probably due to increased levels of stress. We also observed a significant upregulation of ABC transport systems and putative transposases. Changes in Clusters of Orthologous Groups functional categories as well as gene set enrichment analysis based on Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways showed that in the absence of pathogens, only sets of proteins related to transport and secondary metabolism were upregulated, while in the presence of pathogens, proteins related to growth and division as well as a large portion of transcription factors were upregulated. Finally, we identified several small noncoding RNAs whose predicted targets were genes differentially expressed in the open reading frame libraries. These results show the importance of pathogens controlling gene expression of a healthy oral community and the usefulness of metatranscriptomic techniques to study gene expression profiles in complex microbial community models.

Chapter 4: In vitro biofilm models: an overview

Observing naturally occurring biofilms in situ or ex situ has revealed the wide distribution of sessile microbial communities. The ubiquity, variety and complexity of biofilms is now widely accepted by microbiologists. While they are associated with many beneficial functions such as nutrient cycling, bioremediation and colonization resistance, adverse effects including recalcitrance, their involvement in industrial fouling, contamination and infection have made biofilms a priority research topic. We know that most biofilms, other than within certain infections and laboratory flasks, are composed of multiple species and that there is arguably no unifying biofilm architecture. Biofilms do however share certain properties including the presence of gradients of nutrients, gasses and metabolic products, relatively increased cell density, deposition of extracellular polymeric substances and marked recalcitrance towards antimicrobial treatments. Much of our understanding of biofilm physiology and micro-ecology originates from experiments using in vitro biofilm models. Broadly speaking, such models may be used to replicate environmental conditions within the laboratory or to focus on selected variables such a growth rate or fluid flow, etc. This chapter provides an overview of some commonly used biofilm models including microtitre plate systems, flow cells, the constant depth film fermenter, annular reactors and the perfused biofilm fermenter. While perfused biofilm fermenters, in particular, enable growth rate to be controlled within thin, relatively homogenous, quasi steady-state biofilms through modulation of flow rate nutrient availability, other models provide representative modelling of in situ conditions where steady states may be uncommon.

An in vitro evaluation of hydrolytic enzymes as dental plaque control agents

The plaque-control potential of commercially available amylase, lipase and protease was evaluated by observing their effects on coaggregation and on bacterial viability within various plaque microcosms. A quantitative coaggregation assay indicated that protease significantly inhibited the extent of coaggregation of Actinomyces naeslundii and Streptococcus oralis (P <0.05) and of Porphyromonas gingivalis and S. oralis. Amylase significantly (P <0.05) increased the coaggregation of A. naeslundii versus Fusobacterium nucleatum and A. naeslundii versus P. gingivalis. Concomitant challenge of constant-depth film fermenter-grown plaques with the enzymes did not result in detectable ecological perturbations (assessed by differential culture and denaturing gradient gel electrophoresis). Similar dosing and analysis of multiple Sorbarod devices did not reveal increases in bacterial dispersion which could result from disaggregation of extant plaques. A short-term hydroxyapatite colonization model was therefore used to investigate possible enzyme effects on early-stage plaque development. Whilst culture did not indicate significant reductions in adhesion or plaque accumulation, a vital visual assay revealed significantly increased aggregation frequency following enzyme exposure. In summary, although hydrolytic enzymes negatively influenced binary coaggregation, they did not cause statistically significant changes in bacterial viability within plaque microcosms. In contrast, enzyme exposure increased aggregation within extant plaques.

Assessment of inhibitory effects of fluoride-coated tubes on biofilm formation by using the in vitro dental unit waterline biofilm model

This study aimed to establish an in vitro model to simulate biofilms formed in dental unit waterlines (DUWLs) and to investigate the ability of polyvinylidene fluoride (PVDF)-coated tubes to inhibit biofilm formation using this model. The water and biofilm samples were obtained from DUWLs which had been clinically used for 2.5 years, and the predominant bacteria were identified. A conventional polyurethane tube was incubated for 24 to 96 h in the mixed flora of isolated bacteria, and the optimal incubation conditions to simulate a clinically formed biofilm were determined by observation with a scanning electron microscope. Biofilm formation on a PVDF-coated tube was observed using this in vitro model, and the adherence of different bacterial species to conventional and PVDF-coated tubes was assessed. Sphingomonas paucimobilis, Acinetobacter haemolytics, and Methylobacterium mesophilicum were predominantly isolated from contaminated DUWLs. Incubation of the polyurethane tube with the mixed flora containing these three species for 96 h resulted in the formation of a mature biofilm similar to the one clinically observed. The PVDF-coated tube was significantly less adhesive to all three bacterial species than the polyurethane tube (P < 0.05 by the Mann-Whitney U test), and the attachment of small amounts of rods was observed even after incubation with the mixed flora for 96 h. In conclusion, an in vitro biofilm model was obtained by using a mixed flora of bacteria isolated from DUWLs, and the PVDF-coated tube was found to be effective in preventing biofilm formation using this model.

In vitro models for oral malodor

A model is a representation of some real phenomena and contains aspects or elements of the real system to be modeled. The model reflects (or duplicates) the type of behavior (or mechanisms) seen in the real system. The main characteristic of any model is the mapping of elements or parameters found in the system being studied (e.g. tongue dorsum biofilm in situ) on to the model being devised (e.g. laboratory perfusion biofilm). Such parameters include correct physico-chemical (abiotic) conditions as well as biotic conditions that occur in both model and reality. The main purpose of a model is to provide information that better explains the processes observed or thought to occur in the real system. Such models can be abstract (mental, conceptual, theoretical, mathematical or computational) or 'physical', e.g. in the form of a real disaggregated in vitro system or laboratory model. A wide range of different model systems have been used in oral biofilm research. These will be briefly reviewed with special emphasis on those models that have contributed most to knowledge in breath odor research. The different model systems used in breath odor research are compared. Finally, the requirements for developing an overall 'bad breath model' from considering the processes as a whole (real oral cavity, substrates in saliva, biotransformation by tongue microflora, odor gases in the breath) and extending this to the detection of malodor by the human nose will be outlined and discussed.

Raman spectroscopic measurement of relative concentrations in mixtures of oral bacteria

Near-infrared Raman spectroscopy has been used for species identification of pure microbial specimens for more than a decade. More recently, this optical method has been extended to the analysis of specimens containing multiple species. In this report, we demonstrate rapid, reagent-free quantitative analysis of a simplified model of oral plaque containing three oral bacteria species, S. mutans, S. sanguis, and S. gordonii, using near-infrared Raman spectroscopy. Raman spectra were acquired from bacterial mixtures in 200 seconds. A prediction model was calibrated by the partial least squares method and validated by additional samples. On a scale from 0 to 1, relative fractions of each species could be predicted with a root mean square error of 0.07. These results suggest that near-infrared Raman spectroscopy is potentially useful in quantification of microbial mixtures in general and oral plaques in particular.

Characterization of a three bacteria mixed culture in a chemostat: Evaluation and application of a quantitative terminal-restriction fragment length polymorphism (T-RFLP) analysis for absolute and species specific cell enumeration

Growth dynamics of Pseudomonas aeruginosa, Burkholderia cepacia, and Staphylococcus aureus in a batch and chemostat, were investigated as a laboratory model system for persistent infections in cystic fibrosis. Most species-specific enumeration methods for mixed cultures are laborious or only qualitative, and therefore impede generation of quantitative data required for validation of mathematical models. Here, a quantitative T-RFLP method was evaluated and applied for specific and absolute cell number enumerations. The method was tested to be unbiased by quantitative sample composition and allowed reproducible enumerations of mixed cultures. For assay validation, samples of defined concentration containing one, two or three species were quantified. Logarithmically transformed absolute cell numbers of single-species dilutions were linear within a lower working range of 10(4)-10(6) cfu/mL (species-dependent) and an upper working range of 10(10) cfu/mL. Quantifications of single species (10(6)-10(10) cfu/mL) spiked with one or two other species agreed well with single species controls. Differences between slopes of first order linear regression of spiked and pure dilution series were insignificant. Coefficient of variation of defined mixed replicates was maximum 4.39%, of a three-species chemostat it was maximum 1.76%. T-RFLP monitoring of pure cultures in parallel shake flasks and of a three-species mixed chemostat gave very consistent results. Coexistence of at least two species after a time period equivalent to more than 33 volume exchanges was found. This result was not predicted from pure cultures clearly indicating the need for quantitative mixed culture experiments to better understand microbial growth dynamics and for mathematical model validation.

Individual microflora beget unique oral microcosms

AIMS

To examine the efficacy of the multiple Sorbarod device (MSD) for the reproduction of inter-individual variations in oral microbiotas. The MSD supports sessile growth on parallel cellulose filters, perfused with artificial saliva. This enables biofilms (BF) to be grown and sampled, together with released cells in eluted medium (perfusates, PAs).

METHODS AND RESULTS

Two sets of triplicate MSDs were established. One set was inoculated using fresh saliva from three separate volunteers; the second set was inoculated from one saliva donor. Both were incubated in an anaerobic cabinet. BF and PA were analysed at 24-h intervals by PCR-denaturing gradient gel electrophoresis (DGGE) of 16S rDNA. Hierarchical dendrograms were constructed in order to sort community fingerprints over time, based on community relatedness. The MSD supported complex oral communities, as evidenced by DGGE (>20 distinct DGGE bands) and confocal scanning laser microscopy. DGGE band sequencing revealed bacteriological diversity and a high incidence of anaerobic species, including Prevotella sp. Dendrograms demonstrated marked inter-individual variation in the relative species abundance within salivary inocula from different volunteers (DV) and each associated MSD (all >45%, majority c. 85% concordance). Less variation was shown between triplicate models established using saliva from a single volunteer (SV) (all >58%; majority c. 95% concordance). PAs clustered together with the associated biofilms and inocula in the majority of cases for the DV MSDs whilst SV MSD community profiles clustered between replicate MSDs.

CONCLUSIONS

Data indicate that marked inter-individual variations in human salivary composition can be partially replicated in individualized MSD microcosms.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrates the in vitro reproduction of individual oral microbiotas and suggests that taking inter-individual variability into account will increase the relevance of microcosm studies.

Development and characterization of a simple perfused oral microcosm

AIMS

To validate perfused, inline, filter-based fermentation systems (multiple Sorbarod devices, MSD) for their ability to maintain stable oral bacterial communities. MSD enable replicate (n=5) microcosm biofilms (BF) to be established and sampled, together with their perfusates (PA, cells in eluted medium).

METHODS AND RESULTS

Fresh saliva from human volunteers was used to inoculate MSD, incubated in an anaerobic cabinet and perfused with artificial saliva at 7 ml h(-1). BF within Sorbarod filters and cells eluted in the PA were analysed at 24-h intervals by differential bacteriological culture and checkerboard DNA-DNA hybridization (CKB, 40 oral species). Dynamic stability was apparent after 2-3 days within both BF and PA as evidenced by culture, CKB data and pH measurements. BF harboured large numbers of anaerobic species and facultative anaerobes [ca 10-11 log10 colony-forming units (CFU)/filter] comprising considerable numbers of streptococci and Gram-negative species. PA contained ca 9-10 log(10) CFU ml(-1) suggesting an apparent mean growth rate of 0.1 h(-1) for the BF, as a whole corresponding to a mean generation time of 10 h. CKB analysis revealed considerable bacterial diversity within the respective MSD. Inter-individual variations in the relative species abundance of inocula was broadly reproduced in the MSD (BF and PA), although considerable variation was apparent between triplicate models established using saliva from one saliva donor or from three individual donors. The dominance of Gram-negative species, indicated by culture was supported by CKB analysis (major species, Prevotella melaninogenica and Fusobacterium nucleatum).

CONCLUSIONS

Data obtained from the various analytical approaches showed a high degree of congruence. The MSD enables the maintenance of complex, stable salivary microcosms and represents a simple, reproducible tool for modelling individual oral bacterial ecosystems.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrates the utility of the MSD for studying the micro-ecology of the oral cavity.

Measurement of bacterial concentration fractions in polymicrobial mixtures by Raman microspectroscopy

Relative concentrations of Streptococcus mutans and Streptococcus sanguis are important parameters in the study of dental caries, but current methods of measuring these concentrations are time consuming and prone to inaccuracies. We investigate the use of Raman spectroscopy for measuring relative concentrations of these two bacterial species in solid mixtures. To our knowledge, this is the first time Raman spectroscopy has been used to analyze bacterial mixtures rather than to identify the species of a pure colony. Mixtures of the two streptococcal species in various ratios are measured for 200 s using a home-built Raman microscope. Spectral correlations with bacterial content were identified via partial least-squares analysis. The relative concentrations of S. mutans in subsequent samples are predicted with a root mean squared error below 5%. In clinical plaque samples, this sort of accuracy would enable discrimination between normal and dangerously elevated levels of S. mutans. Samples with and without salivary proteins are predicted with equal accuracy. This result shows the potential of Raman spectroscopy for analyzing mixed populations of bacteria, such as those that occur in oral plaques.

Effects of triclosan-containing rinse on the dynamics and antimicrobial susceptibility of in vitro plaque ecosystems

Dental plaque microcosms were established under a feast-famine regimen within constant-depth film fermentors and exposed four times daily postfeeding to a triclosan (TR)-containing rinse (dentifrice) (TRD). This was diluted so that the antimicrobial content was 0.6 mg/ml. Microcosms were characterized by heterotrophic plate counts and PCR-denaturing gradient gel electrophoresis (DGGE) with primers specific for the V2-V3 region of the eubacterial 16S rRNA gene (rDNA). Dominant isolates and PCR amplicons were identified by partial sequencing of 16S rDNA. TRD caused considerable decreases in the counts of both gram-negative organisms and total anaerobic cells, transiently lowered the numbers of streptococci and actinomycetes, and markedly increased the proportion of lactobacilli. DGGE indicated the presence of putatively unculturable bacteria and showed that a Porphyromonas sp. and Selenomonas infelix had been inhibited by TRD. Pure culture studies of 10 oral bacteria (eight genera) showed that Neisseria subflava, Prevotella nigrescens, and Porphyromonas gingivalis were highly susceptible to TR, while the lactobacilli and streptococci were the least susceptible. Clonal expansion of the lactobacilli in the pulsed microcosm could be explained on the basis of TR activity. The mean MICs of TR, chlorhexidine, erythromycin, penicillin V, and vancomycin for the population before and after 5 days of exposure to TRD showed few significant changes. In conclusion, changes in plaque microcosm populations following repeated exposure to TRD showed inhibition of the most susceptible flora and clonal expansion of less susceptible species.

Antimicrobial effects of a novel Triclosan/zinc citrate dentifrice against mixed culture oral biofilms

AIM

To compare the antimicrobial efficacy in vitro of a new fluoride system containing 0.3% Triclosan and 2.0% zinc citrate with a standard fluoride toothpaste against bacteria commonly isolated from dental plaque.

METHOD

Mature biofilms of fixed depth containing ten oral bacterial species were developed in a model system and then exposed to the formulations both in situ and ex situ.

RESULTS

The standard fluoride toothpaste had limited activity against the Gram positive species following 1 and 5 minutes ex situ exposure (p = 0.004 and p = 0.02, respectively) but none against the Gram negative species; also, no overall effect following repeated pulsing in situ was observed. In contrast, viable counts of Gram positive and Gram negative species were markedly reduced following ex situ exposure for 1 and 5 minutes to the new formulation (Gram positive: p = 0.0007 and p = 0.04; Gram negative: p = 0.04 and p = 0.00001, respectively). Counts of Gram positive bacteria were reduced by 99.9% over a 4-day in situ pulsing period, while no Gram negative species were recoverable from the biofilm by day 4.

CONCLUSION

The new toothpaste formulation had a broad spectrum of antimicrobial activity against biofilm consortia containing bacteria associated with both dental caries and periodontal diseases.

Effects of a chlorhexidine gluconate-containing mouthwash on the vitality and antimicrobial susceptibility of in vitro oral bacterial ecosystems

Oral bacterial microcosms, established using saliva inocula from three individuals, were maintained under a feast-famine regime within constant-depth film fermenters. Steady-state communities were exposed four times daily, postfeeding, to a chlorhexidine (CHX) gluconate-containing mouthwash (CHXM) diluted to 0.06% (wt/vol) antimicrobial content. The microcosms were characterized by heterotrophic plate counts and PCR-denaturing gradient gel electrophoresis (DGGE). CHXM caused significant decreases in both total anaerobe and total aerobe/facultative anaerobe counts (P < 0.05), together with lesser decreases in gram-negative anaerobes. The degree of streptococcal and actinomycete inhibition varied considerably among individuals. DGGE showed that CHXM exposure caused considerable decreases in microbial diversity, including marked reductions in Prevotella sp. and Selenomonas infelix. Pure-culture studies of 10 oral bacteria (eight genera) showed that Actinomyces naeslundii, Veillonella dispar, Prevotella nigrescens, and the streptococci were highly susceptible to CHX, while Lactobacillus rhamnosus, Fusobacterium nucleatum, and Neisseria subflava were the least susceptible. Determination of the MICs of triclosan, CHX, erythromycin, penicillin V, vancomycin, and metronidazole for microcosm isolates, before and after 5 days of CHXM exposure, showed that CHXM exposure altered the distribution of isolates toward those that were less susceptible to CHX (P < 0.05). Changes in susceptibility distributions for the other test agents were not statistically significant. In conclusion, population changes in plaque microcosms following repeated exposure to CHXM represented an inhibition of the most susceptible flora with a clonal expansion of less susceptible species.

Growth and molecular characterization of dental plaque microcosms

AIMS

(i) To compare the effects of feeding protocols upon the composition and stability of dental plaque microcosms formed in constant-depth film fermenters (CDFF). (ii) To evaluate the utility of denaturing gradient gel electrophoresis (DGGE) and culture methodologies for the investigation of such models.

METHODS AND RESULTS

Microcosms were established anaerobically in the CDFFs from freshly collected saliva. These were fed either with artificial saliva alone (famine) or combined with discontinuous feeding (feast-famine). Culture and 16s rDNA sequencing indicated that supplemental feeding gave ca. 2 log increases in Lactobacillus rhamnosus and Prevotella buccae. Feast-famine microcosms were then further characterized by DGGE using primers specific for the V2-V3 region of eubacterial rDNA. These gave single major bands with pure cultures (eight species) and resolved all strains apart from Lact. rhamnosus and Actinomyces naeslundii. Whilst culture with selective media indicated a degree of stability and reproducibility between replicate microcosms, DGGE showed a considerable degree of variability that related to several putatively uncultured bacteria.

CONCLUSIONS

Feast-famine regimes altered community composition. DGGE analyses identified putatively unculturable species and demonstrated variability between replicate fermenters.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrates the utility of DGGE for the analysis of dental plaque, especially with respect to unculturable bacteria. Results question the assumptions of reproducibility of plaque microcosms established in non-replicated CDFFs made on the basis of selective media. Feeding regimes, particularly those involving complex nutrients, will dramatically affect population dynamics.

Are dental diseases examples of ecological catastrophes?

Dental diseases are among the most prevalent and costly diseases affecting industrialized societies, and yet are highly preventable. The microflora of dental plaque biofilms from diseased sites is distinct from that found in health, although the putative pathogens can often be detected in low numbers at normal sites. In dental caries, there is a shift towards community dominance by acidogenic and acid-tolerant Gram-positive bacteria (e.g. mutans streptococci and lactobacilli) at the expense of the acid-sensitive species associated with sound enamel. In contrast, the numbers and proportions of obligately anaerobic bacteria, including Gram-negative proteolytic species, increase in periodontal diseases. Modelling studies using defined consortia of oral bacteria grown in planktonic and biofilm systems have been undertaken to identify environmental factors responsible for driving these deleterious shifts in the plaque microflora. Repeated conditions of low pH (rather than sugar availability per se) selected for mutans streptococci and lactobacilli, while the introduction of novel host proteins and glycoproteins (as occurs during the inflammatory response to plaque), and the concomitant rise in local pH, enriched for Gram-negative anaerobic and asaccharolytic species. These studies emphasized (a). significant properties of dental plaque as both a biofilm and a microbial community, and (b). the dynamic relationship existing between the environment and the composition of the oral microflora. This research resulted in a novel hypothesis (the 'ecological plaque hypothesis') to better describe the relationship between plaque bacteria and the host in health and disease. Implicit in this hypothesis is the concept that disease can be prevented not only by directly inhibiting the putative pathogens, but also by interfering with the environmental factors driving the selection and enrichment of these bacteria. Thus, a more holistic approach can be taken in disease control and management strategies.

A continuous culture biofilm model of cariogenic responses

AIMS

To validate an in vitro model for the analysis of physiological and ecological responses to sugar challenge in bacterial populations, and subsequent changes in enamel mineralization.

METHODS AND RESULTS

A seven-organism bacterial consortium was grown in a biofilm mode on enamel and hydroxyapatite (HA) surfaces in a continuous culture system and exposed to repeated sucrose challenges. This produced 'pH-cycling' conditions within the system. Populations on HA surfaces were enumerated. Changes in relative proportions of the different populations, and in the total viable count, were observed, between different treatments. Microradiography of the enamel sections showed increasing demineralization with increasing sucrose concentration. The lesions formed were similar to 'white-spot' lesions found in vivo. Differences in the quality of biofilms formed were also observed using Confocal Laser Scanning Microscopy.

CONCLUSION

An in vitro model has been validated for the analysis of both physiological and ecological responses to sucrose challenges in bacterial populations, and subsequent changes in enamel mineralization.

SIGNIFICANCE AND IMPACT OF THE STUDY

This model should facilitate the study of changes in bacterial populations in response to application of putative anticaries agents and concomitant changes in enamel mineralization.

The effects of histatin-derived basic antimicrobial peptides on oral biofilms

Susceptibility of bacteria to antimicrobial agents is strongly reduced by the formation of complex biofilms. We investigated whether synthetic histatin analogs with broad-spectrum antibacterial activity in vitro were also active against these complex mixtures of bacteria, as present in saliva and plaque. In a simplified model system for dental plaque, hydroxyapatite discs were placed in a continuous culture system comprised of Streptococcus mutans, S. sanguis, S. salivarius, Actinomyces naeslundii, Veillonella parvula, Fusobacterium nucleatum, and Prevotella intermedia. Ex situ treatment of the biofilms formed on these discs with 100 microg/mL of peptide dhvar4 significantly reduced facultative anaerobic, total anaerobic, and obligate anaerobic Gram-negative counts with 0.8, 0.5, and 0.5 log units, respectively. Ex vivo treatment of salivary bacteria gave reductions of 0.4, 0.7, and 1.5 log units, respectively. For ex vivo treatment of plaque bacteria, reductions of 0.4, 0.4, and 1.4 log units, respectively, were found. In both saliva and plaque samples, obligate anaerobic Gram-negative bacteria were significantly more susceptible to dhvar4 than facultatively anaerobic or anaerobic bacteria as a whole (p=0.013 and p=0.018, for salivary bacteria, and p=0.021 and p=0.020 for plaque bacteria, respectively). Although the oral bacteria are protected by biofilm formation, the synthetic histatin analog caused a significant reduction of viable counts in a model for oral biofilm as well as in isolated oral biofilms.

Role of Fusobacterium nucleatum and coaggregation in anaerobe survival in planktonic and biofilm oral microbial communities during aeration

Coaggregation is a well-characterized phenomenon by which specific pairs of oral bacteria interact physically. The aim of this study was to examine the patterns of coaggregation between obligately anaerobic and oxygen-tolerant species that coexist in a model oral microbial community. Obligate anaerobes other than Fusobacterium nucleatum coaggregated only poorly with oxygen-tolerant species. In contrast, F. nucleatum was able to coaggregate not only with both oxygen-tolerant and other obligately anaerobic species but also with otherwise-noncoaggregating obligate anaerobe-oxygen-tolerant species pairs. The effects of the presence or absence of F. nucleatum on anaerobe survival in both the biofilm and planktonic phases of a complex community of oral bacteria grown in an aerated (gas phase, 200 ml of 5% CO2 in air x min-1) chemostat system were then investigated. In the presence of F. nucleatum, anaerobes persisted in high numbers (>10(7) x ml-1 in the planktonic phase and >10(7) x cm-2 in 4-day biofilms). In an equivalent culture in the absence of F. nucleatum, the numbers of black-pigmented anaerobes (Porphyromonas gingivalis and Prevotella nigrescens) were significantly reduced (P </= 0.001) in both the planktonic phase and in 4-day biofilms, while the numbers of facultatively anaerobic bacteria increased in these communities. Coaggregation-mediated interactions between F. nucleatum and other species facilitated the survival of obligate anaerobes in aerated environments.

Oral microbial ecology and the role of salivary immunoglobulin A

In the oral cavity, indigenous bacteria are often associated with two major oral diseases, caries and periodontal diseases. These diseases seem to appear following an imbalance in the oral resident microbiota, leading to the emergence of potentially pathogenic bacteria. To define the process involved in caries and periodontal diseases, it is necessary to understand the ecology of the oral cavity and to identify the factors responsible for the transition of the oral microbiota from a commensal to a pathogenic relationship with the host. The regulatory forces influencing the oral ecosystem can be divided into three major categories: host related, microbe related, and external factors. Among host factors, secretory immunoglobulin A (SIgA) constitutes the main specific immune defense mechanism in saliva and may play an important role in the homeostasis of the oral microbiota. Naturally occurring SIgA antibodies that are reactive against a variety of indigenous bacteria are detectable in saliva. These antibodies may control the oral microbiota by reducing the adherence of bacteria to the oral mucosa and teeth. It is thought that protection against bacterial etiologic agents of caries and periodontal diseases could be conferred by the induction of SIgA antibodies via the stimulation of the mucosal immune system. However, elucidation of the role of the SIgA immune system in controlling the oral indigenous microbiota is a prerequisite for the development of effective vaccines against these diseases. The role of SIgA antibodies in the acquisition and the regulation of the indigenous microbiota is still controversial. Our review discusses the importance of SIgA among the multiple factors that control the oral microbiota. It describes the oral ecosystems, the principal factors that may control the oral microbiota, a basic knowledge of the secretory immune system, the biological functions of SIgA, and, finally, experiments related to the role of SIgA in oral microbial ecology.

Methods for microscopic characterization of oral biofilms: analysis of colonization, microstructure, and molecular transport phenomena

Assessment of the role of biofilm microstructure in biofilm-specific activities requires non-destructive measurement techniques for parameterization of structural characteristics in parallel with relevant biochemical and physiological data. This paper briefly reviews some current methods for biofilm structural analysis, with emphasis on new developments in optical imaging and mathematical modeling methods. Fluorescence imaging studies of bacterial colonization events occurring on exposed model tooth surfaces indicated that bacterial adhesion to sessile organisms was of central importance to the early colonization process and that this occurred in a non-random manner. Structural studies of mature biofilms by confocal microscopy demonstrated the spatial distribution of individual species using fluorescent antibodies. Biofilms grown under different physiological conditions exhibited differences in structure, and methods were developed for parameterizing the spatial orientations of the bacteria. Diffusive processes within biofilm microstructures were studied using a random walk model in both 2-D and 3-D. Modeling of convective flow within biofilm microstructures was achieved by application of lattice Boltzmann methodology.

Nutritional influences on biofilm development

The amounts and types of nutrients in the environment influence the development and final bacterial and chemical composition of biofilms. In oligotrophic environments, organisms respond to nutrient stress by alterations in their cell morphology and cell surfaces, which enhance adherence. Little is known of the responses to stress by bacteria in the animal oral cavity. The environment in the oral cavity is less extreme, and saliva provides a constant source of nutrients. Catabolic cooperation among oral bacteria allow carbon and nitrogen from salivary glycoproteins to be utilized. Modification of growth environments of oral bacteria can influence their cell surfaces and adhesion. Studies in experimental animals have shown that feeding either glucose or sucrose diets or fasting has little effect on the initial stages of development of oral biofilms. However, diet can influence the proportions of different bacterial species later in biofilm development. Studies of competition among populations in communities of oral bacteria in vitro and in vivo have shown the significance of carbon limitation and excess and changes in environmental pH. Relatively few studies have been made of the role of a nitrogen metabolism in bacterial competition in biofilms. In keeping with biofilms in nature, oral biofilms provide a sequestered habitat, where organisms are protected from removal by saliva and where interactions among cells generate a biofilm environment, distinct from that of saliva. Oral biofilms are an essential component in the etiologies of caries and periodontal disease, and understanding the biology of oral biofilms has aided and will continue to aid in the prevention and treatment of these diseases.

Artificial dental plaque biofilm model systems

Difficulties with in vivo studies of natural plaque and its complex, heterogeneous structure have led to development of laboratory biofilm plaque model systems. Technologies for their culture are outlined, and the rationale, strengths, and relative uses of two complementary approaches to microbial models with a focus on plaque biodiversity are analyzed. Construction of synthetic consortia biofilms of major plaque species has established a variety of bacterial interactions important in plaque development. In particular, the 'Marsh' nine-species biofilm consortia systems are powerful quasi steady-state models which can be closely specified, modified, and analyzed. In the second approach, microcosm plaque biofilms are evolved in vitro from the natural oral microflora to the laboratory model most closely related to plaque in vivo. Functionally reproducible microcosm plaques are attainable with a biodiverse microbiota, heterogeneous structure, and pH behavior consistent with those of natural plaque. The resting pH can be controlled by urea supply. Their growth patterns, pH gradient formation, control of urease levels by environmental effectors, and plaque mineralization have been investigated. Microcosm biofilms may be the only useful in vitro systems where the identity of the microbes and processes involved is uncertain. Together, these two approaches begin to capture the complexity of plaque biofilm development, ecology, behavior, and pathology. They facilitate hypothesis testing across almost the whole range of plaque biology and the investigation of antiplaque procedures yielding accurate predictions of plaque behavior in vivo.

Investigations of bifidobacterial ecology and oligosaccharide metabolism in a three-stage compound continuous culture system

BACKGROUND

Several different types of in vitro fermentation systems are currently employed to investigate pro- and prebiotic activities in the human large intestinal microbiota, ranging from simple batch cultures, with or without stirring and pH control, to more complex models involving pH controlled single and multiple-component continuous culture systems.

METHODS

In this investigation, we used a three-stage continuous culture model to study the activities of colonic bacteria. This fermentation system reproduces several of the nutritional and environmental characteristics of the proximal large intestine (vessel 1) and the distal colon (vessels 2 and 3), and was validated using bacteriological, metabolic and chemical measurements made with intestinal material obtained from different regions of the large bowel. In this paper, we report studies on prospective probiotic effects of Bifidobacterium longum NCFB 2259 in relation to other bacterial populations, production of tyrosine and phenylalanine metabolites, and bacterial synthesis of enzymes involved in the formation of putatively genotoxic metabolites, including beta-glucosidase (GS), arylsulphatase (AS), beta-glucuronidase (GN), nitroreductase (NR) and azoreductase (AR).

RESULTS

Bacterial activities at two different retention times were studied (31.1 and 68.4 h), which correspond to large intestinal transit times. At R = 31.4 h, significant probiotic effects were observed with respect to reductions in GS and GN, upon adding B. longum. However, despite the fact that this organism does not ferment aromatic amino acids or produce significant amounts of genotoxic enzymes, dysbiotic manifestations occurred in that both NR synthesis and dissimilatory tyrosine metabolism were stimulated. In contrast, at R = 68.4 h, GS formation increased between five and 20-fold, while GN and NR activities increased by a factor of two after adding the bifidobacterium. These data are reviewed in relation to potential health hazards that may be encountered with long-term probiotic administration. In the prebiotic experiments, the three-stage fermentation system was operated at R = 65 h. Oligofructose was added to V1 to give an initial concentration of 30 grams per litre, when the system was in steady state, to study its effects on a number of experimental parameters including bifidogenicity, bacterial growth, fermentation product formation and mutagenicity. After addition of the oligosaccharide, a multiplicity of effects were observed in V1, where synthesis of NR and AR, bifidobacterial populations and overall fermentation processes were stimulated, although these influences progressively diminished in V2 and V3.

CONCLUSIONS

These studies indicate that bacterial metabolism and putative beneficial consequences associated with the breakdown of readily fermentable prebiotics in the large intestine may in some circumstances be spatially and temporally limited to the proximal bowel.

The establishment of a community of oral bacteria that controls the growth of Candida albicans in a chemostat

The purpose of this study was to establish and identify a community of oral bacteria that controls the growth of Candida albicans in the chemostat. The chemostat was operated under glucose-limiting conditions at a dilution rate of 0.05 h-1 and inoculated with a yeast-free suspension of a tongue scraping. After a steady state had been reached, it was inoculated with C. albicans to establish the yeast and determine whether its growth could be contained. The steady-state community comprised the species Streptococcus sanguis, Streptococcus sobrinus, Streptococcus mitis, Lactobacillus casei, Eubacterium saburreum, Veillonella dispar and Fusobacterium nucleatum. Bacteroides gracilus and Haemophilus segnis were also detected but infrequently. Yeast growth was suppressed and yeast cells were lost at the same rate as the theoretical washout rate. It is concluded that this mixed community of oral bacteria can be used to identify the parameters that maintain the equilibrium between oral bacteria and C. albicans in the oral cavity.

Effect of oxygen, inoculum composition and flow rate on development of mixed-culture oral biofilms

The effect of aeration on the development of a defined biofilm consortium of oral bacteria was investigated in a two-stage chemostat system. An inoculum comprising 10 species, including both facultatively anaerobic and obligately anaerobic bacteria, and species associated with oral health and disease, was inoculated into an anaerobic first-stage chemostat vessel. The effluent from this chemostat was linked to an aerated [200 ml CO2/air (5:95, v/v) min-1] second-stage vessel, in which removable hydroxyapatite discs were inserted to allow biofilm formation. Comparisons were made of planktonic and biofilm communities in the aerated second-stage vessel by means of viable counts. Both planktonic and early biofilm communities were dominated by Neisseria subflava, comprising > 40% of total c.f.u. in the fluid phase, and > 80% of c.f.u. in 2 h biofilms. Obligate anaerobes persisted in this mixed culture, and succession in biofilms led them to predominate only after 7 d. Despite the continuous addition of air, the dissolved oxygen tension (dO2) within the culture remained low (< 5% of air saturation), and the redox potential (Eh) was -275 mV. In order to assess the significance of the presence of N. subflava in community development, a subsequent experiment omitted this aerobe from the inoculum, to produce a nine-species culture. The planktonic phase was predominated by three streptococcal species, Prevotella nigrescens and Fusobacterium nucleatum. Biofilms again underwent successional changes, with anaerobes increasing in proportion with time. In contrast to the culture including N. subflava, dO2 was 50-60% of air saturation, and the Eh was +50 mV. In the final experiment, the rate of addition of first-stage culture was reduced to 1/10 of that in the previous experiment, in order to determine whether anaerobes were growing, rather than merely persisting in the aerated culture. The data for the planktonic phase indicated that the anaerobes were growing in aerated (dO2 40-50%, Eh +100 mV) conditions. Once again, anaerobes increased in proportion in older biofilms. The study indicates that mixed cultures can protect obligate anaerobes from the toxic effects of oxygen, both in the biofilm and planktonic modes of growth.

Development of a steady-state oral microbial biofilm community using the constant-depth film fermenter

The complexity of biofilm communities like dental plaque suggests that laboratory model biofilm growth systems may help to understand their structure and function. This study describes the use of a constant-depth film fermenter (CDFF) to investigate biofilm formation by a nine-membered community of oral bacteria. The community was grown to steady state in a chemostat incubated anaerobically. The chemostat output was fed into the CDFF incubated aerobically. Viable counts for each species from the chemostat and the CDFF at steady state showed major differences; however, all nine organisms were present under both conditions. There was evidence of succession during biofilm formation with obligately anaerobic species only establishing after several days. A steady-state biofilm community was achieved which remained stable over time. Electron microscopy showed evidence of spatial differentiation with what appeared to be Neisseria subflava dominant near the upper surface and Fusobacterium nucleatum largely confined to the middle portion.

A modified chemostat system to study the ecology of oral biofilms

Previously, we developed a chemostat system to study the behaviour and properties of a community of up to 10 species of oral bacteria. The present study describes modification of this system to incorporate removable and replaceable hydroxyapatite (the major mineral in human dental enamel) disks on which biofilms could develop. Hydroxyapatite disks were immersed in the chemostat for known time periods, and the bacterial content of biofilms determined by viable counting. Initial deposition rates were rapid, with all 10 species detected after 1 h, and the numbers of bacteria in biofilms continued to increase for 21 d. The species composition of biofilms reflected that of the surrounding fluid phase, and showed only limited signs of the type of 'species succession' which is observed in developing dental plaque in vivo, although anaerobic species increased in proportion in older biofilms. Four-day biofilms showed the least variability and were chosen as the 'standard biofilm' for more detailed study. Variability in the bacterial composition of 4-d biofilms was comparable both within a single chemostat run and between independent chemostat runs. Glucose pulsing in the absence of pH control resulted in the selection of cariogenic species; the disruption of the biofilm community was less marked than that of the equivalent planktonic culture. The model system has considerable potential in studying the effects of a variety of factors on biofilm development, as well as in comparing the efficacy of antimicrobial systems against biofilms.

Inhibition of acid production in Streptococcus mutans R9: inhibition constants and reversibility

End-product inhibition of acid production in Streptococcus mutans R9 was investigated by measuring effects of varying concentrations of H+ and of undissociated lactic or acetic acids on acid production rates in the pH stat. H+ caused purely uncompetitive inhibition (inhibition constant Kiu 0.018 mmol 1(-1). Lactic acid caused mixed inhibition with inhibition constants of Kiu 4.24 mmol 1(-1) and Kic 4.55 mmol 1(-1). Reversibility of inhibition by H+ showed only a statistically significant reduction only at pH < 4.5. Reversibility of inhibition by lactic and acetic acids decreased with increasing inhibitor concentration. In all cases, reversibility correlated with the extent to which viability was retained, suggesting that loss of reversibility was due to cell death. These results suggest that, after a low-pH episode in dental plaque, caused by fermentation of dietary carbohydrate, the ability of plaque organisms to produce acid in subsequent exposures to carbohydrate may be reduced.

Oxygen metabolism, oxidative stress and acid-base physiology of dental plaque biofilms

Dental plaque is a natural biofilm which has been a focus of attention for many years because of its known roles in caries and periodontal diseases. Acid production by plaque bacteria leads to the erosion of tooth mineral in caries, and the cariogenicity of plaque is related to population levels of acid-tolerant organisms such as mutans streptococci. However, the biofilm character of plaque allows for survival of a diverse flora, including less acid-tolerant organisms, some of which can produce ammonia from arginine or urea to counter acidification. Plaque is often considered to be relatively anaerobic. However, evidence is presented here that both supragingival and subgingival plaque have active oxygen metabolism and that plaque bacteria, including anaerobes, have developed defenses against oxidative stress. Even in subgingival plaque associated with periodontitis, measured residual oxygen levels are sufficient to allow for oxygen metabolism by organisms considered to be extremely anaerobic such as Treponema denticola, which metabolizes oxygen by means of NADH oxidases and produces the protective enzymes superoxide dismutase and NADH peroxidase. The finding that plaque bacteria produce a variety of protective enzymes is a good indicator that oxidative stress is a part of their everyday life. The biofilm character of plaque allows for population diversity and coexistence of aerobes, anaerobes and microaerophiles. Overall, agents that affect oxidative metabolism offer possibilities for reducing the pathogenic activities of plaque.

Chemostat flow cell system: an in vitro model for the evaluation of antiplaque agents

We developed an experimental in vitro model of dental plaque to assess the potential efficacy of antiplaque agents. The model used a chemostat, which provided a continuous source of 5 species of oral bacteria grown in an artificial "saliva-like" medium. This mixture was pumped through six flow cells, each containing two types of surfaces on which plaque formed and was subsequently measured. Formation of bacterial plaque on hydroxyapatite surfaces was assessed by measurement of the DNA and protein content of the plaque film. The amount of bacterial plaque formed on germanium surfaces was measured by attenuated total reflectance (ATR/FT-IR) spectroscopy. Plaque viability was also assessed by a fluorescent staining technique. The quantity of plaque formed on both types of surfaces gradually increased with the duration of flow (from 24 to 72 h) through the cells during a 72-hour experimental period. The flow cells were then pulsed with experimental treatment solutions for 30 s, twice daily. Parallel to results of human clinical studies, the model was capable of discriminating among water, a placebo mouthrinse, and an active antimicrobial mouthrinse formulation containing 0.03% triclosan. It therefore offers a valuable alternative to animal model testing and allows for more rapid evaluations under well-controlled experimental conditions.

The effects of triclosan and zinc citrate, alone and in combination, on a community of oral bacteria grown in vitro

A mixed-culture chemostat system has been used as a more stringent laboratory system for evaluation of the antimicrobial effects of Triclosan and zinc citrate. The inhibitors were added alone, and in combination, as a pulse (a high initial inhibitor concentration which decreased with time) or as a dose (concentration of inhibitor increased with time) to give maximum concentrations of 34.5 mumol/L Triclosan and 39.8 mumol/L zinc citrate. When dosed, Triclosan inhibited A. viscosus and all five Gram-negative species, whereas zinc citrate had less effect, probably due to complexation by media components. Similar effects were seen when Triclosan was pulsed, except that S. mutans was the most sensitive Gram-positive species and V. dispar was unaffected. However, when the inhibitors were dosed or pulsed in combination, marked complementary and additive inhibitory effects were observed, particularly against Gram-negative species, although S. gordonii and S. oralis were relatively unaffected. The data confirm that increased effects can be obtained with suitable combinations of antimicrobial agents and suggest that, under certain conditions, apparently broad-spectrum antimicrobial agents may be acting more selectively than hitherto suspected.

Dentifrices containing new agents for the control of plaque and gingivitis: microbiological aspects

Antimicrobial agents have been proposed as playing an important role in controlling plaque and gingivitis. Unfortunately, a large number of potential compounds are unsuitable for use in dentifrices because they lack "substantivity", produce undesirable side-effects, or are incompatible with toothpaste ingredients. New agents that have been successfully incorporated into dentifrices include plant extracts, phenolic compounds and metal salts. Several products are currently being based on the phenol, Triclosan. Triclosan has a broad spectrum of antimicrobial activity against yeasts and oral bacteria. To enhance its clinical efficacy, Triclosan has been combined either with a co-polymer or with another compatible antimicrobial agent, zinc citrate. The co-polymer acts to increase the oral retention of Triclosan, and has resulted in further reductions in salivary bacterial counts in vivo. Zinc salts also have antimicrobial activity, and at low concentrations, can inhibit glycolysis and bacterial proteases. In mixed culture chemostat studies, Triclosan selectively inhibited Gram-negative periodontopathic bacteria; additive effects were obtained when zinc citrate and Triclosan were combined. In an experimental human gingivitis study, a zinc citrate/Triclosan dentifrice reduced plaque accumulation and gingivitis compared to a placebo paste; the ratio of anaerobic/aerobic bacteria and the proportions of Actinomyces species in plaque were also reduced. The prolonged use of a zinc citrate/Triclosan dentifrice neither significantly altered the ecology of supragingival plaque nor led to the selection of Triclosan-resistant bacteria. The data suggest that dentifrices containing new antimicrobial agents could be of clinical relevance in the prevention and control of plaque and gingivitis.

Effect of dietary components on the indigenous oral bacterial flora of BALB/c mice

Diet is recognized as one of the most important factors affecting the equilibrium of the oral bacterial flora. Studies related to dental caries development have focused mainly on the effect of carbohydrates. In the present work, using swabbing coupled with an immuno-colony-blot assay, we followed the indigenous oral bacterial populations of BALB/c mice fed diets with different concentrations of sucrose, starch, proteins, lipids, vitamins, and minerals for a period of 14 days. The results indicate that although high-sucrose diet favored an increase of the proportion of S. faecalis, only variations in the protein and the starch concentrations significantly influenced the composition of the indigenous oral bacterial populations of BALB/c mice. With low-protein diets, the proportion of Lactobacillus murinus decreased, that of Streptococcus faecalis increased, while that of Staphylococcus aureus was relatively stable. A diet containing a high proportion of starch (65%) resulted in a significant increase in the population of S. faecalis, while that of S. aureus decreased proportionally. With the same diets used as in vitro culture media, growth of L. murinus was the fastest at high protein concentrations, while that of S. faecalis was not affected by the protein concentrations. These results indicate a direct effect of dietary protein content on the indigenous oral bacterial populations of BALB/c mice.

The effect of fluoride on the stability of oral bacterial communities in vitro

An in vitro model system has been used to study the combined influences of fermentable carbohydrate, pH, and fluoride on the stability of complex oral microbial communities. The pH generated from carbohydrate pulses rather than the availability of substrate per se was responsible for the enrichment of the cariogenic species S. mutans and L. casei. The addition of sub-MIC levels of sodium fluoride (1 mmol/L; 19 ppm) reduced both the rate of acid production and the fall in terminal pH from glucose pulses, thereby enabling pH-sensitive bacteria, including many Gram-negative species, to persist. Furthermore, the combination of even a moderately-low environmental pH (ca. pH 5.0) with a low level (1 mmol/L) of fluoride was able to prevent S. mutans from out-competing other species and resulted in its proportions within the bacterial community remaining low. By this mechanism, fluoride could make a significant contribution to preventing dental caries.

Prevention of population shifts in oral microbial communities in vitro by low fluoride concentrations

A continuous culture system has been used to study the effects of low (sub-MIC) levels of sodium fluoride on the stability and metabolism of a defined oral microbial community. The microflora was also subjected to glucose pulses at pH 7.0, with and without subsequent pH control. At pH 7.0, a continuous supply of 1 mmol/L NaF reduced slightly the viable counts of the oral microflora, although their proportions were relatively unaffected. At pH 7.0, during glucose pulsing, 1 mmol/L NaF prevented the rise in proportions of A. viscosus and reduced the levels of B. intermedius. Glucose pulsing without pH control and in the absence of fluoride markedly inhibited the growth of many species, and L. casei, V. dispar, and S. mutans predominated in the culture. Fluoride (1 mmol/L), either pulsed with the glucose or provided continuously, reduced both the rate of change and the degree of fall in pH, and in doing so prevented the enrichment of S. mutans in the culture. Fluoride also reduced the pH-mediated inhibition of other members of the oral community, although S. sanguis was inhibited even further. Thus, even sub-MIC levels of fluoride may have a beneficial anti-bacterial effect on dental plaque by interfering with acid production. This would reduce the pH-mediated disruption to the balance of the microflora and suppress the selection of S. mutans.