The effect of sucrose application frequency and basal nutrient conditions on the calcium and phosphate content of experimental dental plaque

Biofilm reactor calibration for in vitro investigation of composite biodegradation

Introduction: The majority of biodegradation studies of composite materials use simplified models of microbial biofilm despite the apparent diversity of the oral microbiota. The use of in vitro systems of “artificial mouth” design is a step towards clarifying the synergistic effect that microbial plaque and human saliva have on composite degradation.

Aim: Establishment of functional parameters for in vitro reproduction of oral biofilms via biofilm reactor systems.

Materials and methods: The CDC Biofilm Reactor system consists of eight polypropylene sticks. The rod cover and the retaining plates are mounted in a 1-dm glass cylinder with an outlet side opening. The laboratory bioreactor has a working volume of 340 ml. The device is equipped with a four-blade magnetic stirrer. The system also includes gauging appliances and executive mechanisms for controlling and adjusting the basic parameters of the process.

Results: Determination of the operating volume of the reactor is performed prior to the experiment along with the time of reach and stabilization of the set temperature in the design which is 60 min at 120 rpm. A mathematical model is used to calculate the rate of delivery of growth medium - 11 millilitres per minute. The bioreactor is sterilized by 0.3% neomycin solution for 24 hours. Prior to the experiment the system is cleansed (via passage) with sterile water for 60 minutes.

Conclusions: The pre-calibration of a bioreactor system allows specification and refinement of its working parameters, thus engaging for accurate reproduction of the environmental conditions in the oral cavity.

Characterization of polymicrobial biofilms obtained from saliva or carious lesions in dentin

This study aimed to compare the formation of polymicrobial biofilms using carious dentin or saliva as inoculum for application in in vitro microbiological studies on caries research. For biofilm growth, combined samples of infected dentin or saliva from three donors were used. The biofilms were grown on glass coverslips, under a regimen of intermittent exposure (6 h day^-1) to 1% sucrose for 4 days. Total bacterial loads, as well as specific aciduric bacteria and mutans streptococci loads were quantified and correlated with biofilm acidogenicity and susceptibility to chlorhexidine. The data were evaluated using the Student's-t, Mann Whitney and Kruskal-Wallis tests. The two biofilms showed similar microbial loads (total bacteria, aciduric bacteria and mutans streptococci) on day 4, and high acidogenicity after 48 h and were susceptible to chlorhexidine at different time intervals. In conclusion, both dentin and saliva can be used as an inoculum in in vitro studies of processes related to biofilm formation.

Exopolysaccharides regulate calcium flow in cariogenic biofilms

Caries-associated biofilms induce loss of calcium from tooth surfaces in the presence of dietary carbohydrates. Exopolysaccharides (EPS) provide a matrix scaffold and an abundance of primary binding sites within biofilms. The role of EPS in binding calcium in cariogenic biofilms is only partially understood. Thus, the aim of the present study is to investigate the relationship between the calcium dissolution rates and calcium tolerance of caries-associated bacteria and yeast as well as to examine the properties of EPS to quantify its binding affinity for dissolved calcium. Calcium dissolution was measured by dissolution zones on Pikovskaya's agar. Calcium tolerance was assessed by isothermal microcalorimetry (IMC) by adding CaCl2 to the bacterial cultures. Acid-base titration and Fourier transform infrared (FTIR) spectroscopy were used to identify possible functional groups responsible for calcium binding, which was assessed by isothermal titration calorimetry (ITC). Lactobacillus spp. and mutans streptococci demonstrated calcium dissolution in the presence of different carbohydrates. All strains that demonstrated high dissolution rates also revealed higher rates of calcium tolerance by IMC. In addition, acidic functional groups were predominantly identified as possible binding sites for calcium ions by acid-base titration and FTIR. Finally, ITC revealed EPS to have a higher binding affinity for calcium compared, for example, to lactic acid. In conclusion, this study illustrates the role of EPS in terms of the calcium tolerance of cariogenic microbiota by determining the ability of EPS to control free calcium concentrations within the biofilms as a self-regulating mode of action in the pathogenesis of dental caries.

In vitro biofilm models to study dental caries: a systematic review

The aim of this systematic review is to characterize and discuss key methodological aspects of in vitro biofilm models for caries-related research and to verify the reproducibility and dose-response of models considering the response to anti-caries and/or antimicrobial substances. Inclusion criteria were divided into Part I (PI): an in vitro biofilm model that produces a cariogenic biofilm and/or caries-like lesions and allows pH fluctuations; and Part II (PII): models showing an effect of anti-caries and/or antimicrobial substances. Within PI, 72.9% consisted of dynamic biofilm models, while 27.1% consisted of batch models. Within PII, 75.5% corresponded to dynamic models, whereas 24.5% corresponded to batch models. Respectively, 20.4 and 14.3% of the studies reported dose-response validations and reproducibility, and 32.7% were classified as having a high risk of bias. Several in vitro biofilm models are available for caries-related research; however, most models lack validation by dose-response and reproducibility experiments for each proposed protocol.

Dental Biofilm and Laboratory Microbial Culture Models for Cariology Research

Dental caries form through a complex interaction over time among dental plaque, fermentable carbohydrate, and host factors (including teeth and saliva). As a key factor, dental plaque or biofilm substantially influence the characteristic of the carious lesions. Laboratory microbial culture models are often used because they provide a controllable and constant environment for cariology research. Moreover, they do not have ethical problems associated with clinical studies. The design of the microbial culture model varies from simple to sophisticated according to the purpose of the investigation. Each model is a compromise between the reality of the oral cavity and the simplification of the model. Researchers, however, can still obtain meaningful and useful results from the models they select. Laboratory microbial culture models can be categorized into a closed system and an open system. Models in the closed system have a finite supply of nutrients, and are also simple and cost-effective. Models in the open system enabled the supply of a fresh culture medium and the removal of metabolites and spent culture liquid simultaneously. They provide better regulation of the biofilm growth rate than the models in the closed system. This review paper gives an overview of the dental plaque biofilm and laboratory microbial culture models used for cariology research.

In situ Effect of Frequent Sucrose Exposure on Enamel Demineralization and on Plaque Composition after APF Application and F Dentifrice Use

Since the effect of the combination of methods of fluoride use on enamel demineralization and on plaque composition is not clearly established, this study examined the effect of the combination of acidulated phosphate fluoride (APF) application and F dentifrice on enamel demineralization and on plaque composition. In this crossover study, 16 volunteers, wearing a palatal appliance containing bovine enamel blocks, were subjected to 4 treatment groups: non-fluoridated dentifrice (PD), FD, APF+PD, and APF+FD. The APF was applied to the enamel before the 14-day experimental period. During the experimental period, test dentifrices were applied 3×/day, and a 20% sucrose solution was applied 4× and 8×/day by being dripped on the blocks. Although APF application was able either to increase F concentration in plaque or to reduce the % of mutans streptococci, its combination with F dentifrice use neither reduced enamel mineral loss nor changed any other measured plaque variable with respect to the FD group alone.

Ca, Pi, and F in the Fluid of Biofilm Formed under Sucrose

Calcium (Ca), inorganic phosphorus (Pi), and fluoride (F) concentrations are low in the whole plaque biofilm formed under exposure to sucrose. It was hypothesized that this would be reflected in the biofilm fluid, where these low values should greatly influence the de/remineralization process. Dental biofilms were formed in situ over enamel blocks mounted in palatal appliances and exposed 8 times/day to distilled water, glucose+fructose, or sucrose solutions for 14 days. While Ca, Pi, and F concentrations in the whole biofilms were significantly lower in the glucose+fructose and sucrose groups, no effect on biofilm fluid was observed, even after a cariogenic challenge. An increase in whole biofilm mineral ions was observed 24 hrs after the carbohydrate treatments were suspended, but this effect was also not observed in the fluid. These results suggest that there is a homeostatic mechanism that maintains biofilm fluid mineral ion concentration, regardless of its total concentration in the whole biofilm.

Influence of the Inoculum Source on the Cariogenicity of in vitro Microcosm Biofilms

This study investigated the cariogenic potential of biofilms originating from different types of inoculum (saliva and dental plaque) from caries-active and caries-free individuals. Ten volunteers were selected from each caries condition for the paired collection of saliva and dental plaque. Microcosm biofilms were grown in triplicate from each inoculum on enamel specimens in 24-well plates under cariogenic challenge. After 10 days, the biofilms were collected for analysis of outcome variables: percentage of surface hardness change (%SHC) and microbiological composition of biofilms. Statistical analysis was performed using the t test, the linear multivariate analysis model and Pearson's correlation coefficients (α = 0.05). A comparative analysis between microbiological baseline data showed higher counts of mutans streptococci in plaque samples within caries-active individuals; a comparative analysis of colony-forming unit (CFU) counts between individuals with different caries status showed higher counts of acid-tolerant microorganisms and mutans streptococci in dental plaque and of acid-tolerant microorganisms in saliva. After 10 days of biofilm growth, the CFU values for total microorganisms, lactobacilli, mutans streptococci and acid-tolerant bacteria, as well as for SHC, were not statistically significant, considering the type of inoculum and caries condition (p > 0.05). A positive correlation was found for %SHC and CFU counts of acid-tolerant bacteria (r = 0.406) and lactobacilli (r = 0.379). Under the limits of this study, the cariogenic potential of biofilms, formed under identical conditions in vitro, is similar, regardless of baseline differences between the source and type of inoculum.

Caries-related plaque microcosm biofilms developed in microplates

In vivo dental plaque biofilms consist of complex communities of oral bacteria that are a challenge to replicate in vitro. The aim of this investigation was to establish human dental plaque microcosms in microplates to reflect conditions that are relevant to dental caries. Microcosm plaque biofilms were initiated from the saliva of two different donors, grown for up to 10 days in 24-welled microplates on Thermanox coverslips in various types of artificial saliva with and without sucrose, which were replaced daily. Microbiota composition of 40 species associated with oral health and dental caries was monitored in the plaques using Checkerboard DNA-DNA hybridization analysis. pH was measured as an indicator of cariogenic potential. The composition of the saliva inocula was different, and yielded plaque microcosms with different composition and growth responses to sucrose. Artificial saliva type and presence of sucrose, and the resulting growth and pH conditions, modified the growth of individual species and hence the ecological profile of the microplate plaques during development. Complex population shifts were observed during development, and older plaques comprised predominantly facultative anaerobic species. Sucrose supplementation limited the decline of Streptococci over time but did not increase the abundance of mutans Streptococci. Sucrose at 0.15% increased levels of caries-associated species including Lactobacillus fermentum, Lactobacillus acidophilus and Actinomyces gerensceriae; these were further increased with sucrose at 0.5%, in addition to Actinomyces israelii, Rothia dentocariosa and Capnocytophaga gingivalis. The microplate plaques demonstrated complex community dynamics that appeared to reflect the maturation of natural plaques, and sucrose induced a cariogenic plaque composition and pH.

Effect of Frequency of Sucrose Exposure on Dental Biofilm Composition and Enamel Demineralization in the Presence of Fluoride

It has been suggested that enamel would resist higher frequencies of sucrose exposure if fluoride from water or dentifrice is being used. However, the effect of increasing frequencies of sugar on dental biofilm composition is not well known. Ten volunteers living in a fluoridated area wore palatal appliances bearing human enamel slabs during 14 days. The slabs were exposed to 20% sucrose solution 0 (control), 2, 4, 6, 8 or 10 times/day and the volunteers used fluoride dentifrice 3 times/day. Enamel demineralization was significantly greater than control for sucrose frequencies higher than 6 times/day. However, biofilm mass, total microbiota, total streptococci, lactobacilli counts and insoluble extracellular polysaccharide concentration increased, while Ca, P(i) and F concentration in whole biofilm decreased significantly, with frequencies of sucrose exposure lower than 6 times/day. The findings confirm that fluoride can reduce enamel demineralization if sucrose consumption is not higher than 6 times/day, but changes in the biochemical and microbiological composition of the biofilm are observed with lower frequencies of sucrose use.

The role of sucrose in cariogenic dental biofilm formation--new insight

Dental caries is a biofilm-dependent oral disease, and fermentable dietary carbohydrates are the key environmental factors involved in its initiation and development. However, among the carbohydrates, sucrose is considered the most cariogenic, because, in addition to being fermented by oral bacteria, it is a substrate for the synthesis of extracellular (EPS) and intracellular (IPS) polysaccharides. Therefore, while the low pH environment triggers the shift of the resident plaque microflora to a more cariogenic one, EPS promote changes in the composition of the biofilms' matrix. Furthermore, it has recently been shown that the biofilm formed in the presence of sucrose presents low concentrations of Ca, P(i), and F, which are critical ions involved in de- and remineralization of enamel and dentin in the oral environment. Thus, the aim of this review is to explore the broad role of sucrose in the cariogenicity of biofilms, and to present a new insight into its influence on the pathogenesis of dental caries.

Composition of dental plaque formed in the presence of sucrose and after its interruption

Since dental plaque reservoirs of fluoride (F), calcium (Ca) and inorganic phosphorus (Pi) are susceptible to decreases in pH, this in situ crossover study was conducted to test the hypothesis that the low concentration of these ions in plaque, formed in the presence of sucrose, could be attributed merely to the fermentation of this sugar. Eleven volunteers wore palatal appliances containing 6 human enamel blocks during two stages. In each stage, the treatments were either 20% sucrose solution or distilled deionized water, which were dripped onto the blocks 8 times a day. After 28 days, in each stage, the dental plaque formed on two blocks was collected, the treatment was inverted and after a further 24 and 48 h, the biofilm formed was collected from the other blocks. The concentration of acid-soluble F, Ca and Pi, and the concentration of insoluble polysaccharide (IP) were determined in the dental plaque. Statistically lower concentrations of F, Ca and Pi, and a higher concentration of IP were found in the 28-day biofilm formed in the presence of sucrose than in its absence; after the treatment inversion the change in F, Ca and Pi was not statistically significant, but the IP concentration changed significantly. The hypothesis was rejected because change in concentration of F, Ca and Pi is not due to fermentation of the sucrose.

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.

Relationship among dental plaque composition, daily sugar exposure and caries in the primary dentition

The relationship among daily sugar exposure, dental plaque composition and caries patterns in the primary dentition is not known. Three groups of 20 children (18-48 months old) were studied: (1) caries-free (CF), (2) presenting pit and fissure caries (PFC), and (3) with nursing caries (NC). Dental caries index (dmfs) was assessed and the frequency of daily sugar exposure (TS) and its forms were estimated by questionnaires. Fluoride (F), calcium (Ca), inorganic phosphorus (P(i)), insoluble polysaccharide (IP) and mutans streptococci were quantified in dental plaque. The three groups of children were statistically different regarding daily sugar exposure. Concentrations of F, Ca and P(i) were lower and the IP and mutans streptococci statistically higher in dental plaque from NC children than those from CF children. PFC children also showed statistically lower F and P levels in their plaque than CF children. Statistically significant correlations were found between IP, TS and dmfs from NC and PFC children. A statistically significant correlation between mutans streptococci in plaque from anterior teeth and dmfs for NC children was also observed. These results suggest that the dietary sugar experience changes the biochemical and microbiological composition of dental plaque, which could partly explain the different caries patterns observed in primary dentition.