The role of sugar in the aetiology of dental caries. 6. Evidence from experimental animal research

Effects of glucose and lactate on Streptococcus mutans abundance in a novel multispecies oral biofilm model

The oral microbiome plays an important role in protecting oral health. Here, we established a controlled mixed-species in vitro biofilm model and used it to assess the impact of glucose and lactate on the ability of Streptococcus mutans, an acidogenic and aciduric species, to compete with commensal oral bacteria. A chemically defined medium was developed that supported the growth of S. mutans and four common early colonizers of dental plaque: Streptococcus gordonii, Actinomyces oris, Neisseria subflava, and Veillonella parvula. Biofilms containing the early colonizers were developed in a continuous flow bioreactor, exposed to S. mutans, and incubated for up to 7 days. The abundance of bacteria was estimated by quantitative polymerase chain reaction (qPCR). At high glucose and high lactate, the pH in bulk fluid rapidly decreased to approximately 5.2, and S. mutans outgrew other species in biofilms. In low glucose and high lactate, the pH remained above 5.5, and V. parvula was the most abundant species in biofilms. By contrast, in low glucose and low lactate, the pH remained above 6.0 throughout the experiment, and the microbial community in biofilms was relatively balanced. Fluorescence in situ hybridization confirmed that all species were present in the biofilm and the majority of cells were viable using live/dead staining. These data demonstrate that carbon source concentration is critical for microbial homeostasis in model oral biofilms. Furthermore, we established an experimental system that can support the development of computational models to predict transitions to microbial dysbiosis based on metabolic interactions.IMPORTANCEWe developed a controlled (by removing host factor) dynamic system metabolically representative of early colonization of Streptococcus mutans not measurable in vivo. Hypotheses on factors influencing S. mutans colonization, such as community composition and inoculation sequence and the effect of metabolite concentrations, can be tested and used to predict the effect of interventions such as dietary modifications or the use of toothpaste or mouthwash on S. mutans colonization. The defined in vitro model (species and medium) can be simulated in an in silico model to explore more of the parameter space.

Sugars – The Arch Criminal?

Numerous lines of evidence have conclusively established the role of sugars in caries etiology and the importance of sugars as the principal dietary substrate that drives the caries process has not been scientifically challenged. While sugars appear to differ little in acidogenic potential, sucrose has been given special importance, as the sole substrate for synthesis of extracellular glucans. Water-insoluble glucans might enhance accumulation of mutans streptococci on smooth tooth surfaces and appear to enhance virulence by increasing plaque porosity, resulting in greater acid production immediately adjacent to the tooth surface. Data indicating that the sugar consumption/caries relationship is now weaker have led to suggestions that recommendations to restrict sugar consumption are no longer necessary. Clearly, fluoride has raised the threshold of sugar intake at which caries will progress to cavitation, but fluoride has its limits, and caries remains a serious problem for disadvantaged individuals in many industrialized countries and is a rising problem in many developing countries. A weakening of the sugar/caries relationship may also be explained by many technical, biological, behavioral and genetic factors. Future research should aim to determine the biologic and behavioral factors that influence caries risk. Measures to educate the public on the dangers of frequent sugar consumption, combined with recommendations for proper oral hygiene and fluoride use, are still warranted. Individual dietary counseling is highly recommended for patients at high caries risk. As dental caries is a preventable disease, each country must decide: what level of disease is society willing and able to tolerate?

The comparative sensitivity of intra-oral, in vitro, and animal models in the 'profile' evaluation of topical fluorides

The development of predictive and rapid methods for the assessment of the anticaries activity of topical fluorides has been a long-standing objective of caries researchers. These methods can provide useful benefits in a number of applications, ranging from the identification of novel agents to progress into clinical testing to the regulatory screening of commercial product variations. In the latter applications, combinations of test methods (so-called profiles) are used by manufacturers to prove that changes in formulations do not alter the efficacy of the products. Historically, combinations of in vitro and animal models have been used for basic research as well as for profile testing purposes; however, in recent years, the use of intra-oral or in situ models has increased. In this paper, in vitro, animal, and in situ methods are reviewed in terms of the historical basis for their development, protocols currently used in testing, and the primary advantages and limitations of each as applied to 'profile' applications. Recommendations are provided concerning circumstances for the appropriate use of modern test methods in formulation screening.

The effects of different concentrations of sucrose, fructose, and glucose on pH changes by Streptococcus mitior in an artificial mouth

The aim of this study was to determine the effects of the initial sucrose (S) concentration, as well as those of fructose (F), glucose (G), and invert sugar (F/G), on the pH developed by a layer of S. mitior, to represent dental plaque, in an artificial mouth which simulates the process of oral sugar clearance. At S, F. G, and F/G concentrations of 2% and with normal oral sugar clearance rates, the bacteria produced a smaller pH fall from S than from the other sugars; at initial concentrations of 20%, however, the differences were not significant. With constant S concentrations of 0.5-35%, the minimum pH reached was 4.03 +/- 0.14 (S.D.); with S concentrations of 50% and above, slightly but significantly higher values (4.41 +/- 0.34) occurred. However, with normal sugar clearance, the pH fall was much less than with a constant concentration and was dependent on the S concentration over the range of 0-10%, but was independent at higher concentrations. Exposure of the bacteria to S for as short a period as two min during normal sugar clearance gave a nearly maximum pH fall. This suggests that rinsing the mouth with water more than two min after consumption of sucrose in liquid form will have very little effect in reducing the pH fall in dental plaque. A more appropriate method for reducing acid formation by dental plaque would be consumption of a salivary stimulant which would increase the flow rate and buffer capacity of the saliva.

Differential sensitivity of normal human pulp and transformed mouse fibroblasts to cytotoxic challenge

Six different lines of diploid cells from human pulp and one commonly used transformed-cell line, L929 (a continuous fibroblast line of mouse lung connective tissue origin), were challenged by sera changes, an agar-overlay toxicity test and transfilter-histochemistry-toxicity test. The normal diploid cells showed greater sensitivity than transformed cells in each test. Although a different parameter of cell toxicity was measured in each test, the data indicate greater toxic response in diploid cells by all measurements. These normal diploid human cells are more appropriate cells for toxicity testing materials for human use.