Dental Caries Prevention: Ecological Approaches

Introduction

• Oral health is vital for overall well-being, impacting individuals and society beyond craniofacial functions.
• Dental caries are a major, preventable public health issue with substantial costs.
• Traditional methods don't fully capture the social and economic impact of caries.
• Current prevention relies on mechanical oral hygiene with fluoride toothpaste, a method used for over 50 years.
• Ecological strategies are emerging, suggesting a future beyond just fluoride treatments.
• This review explores the rationale for alternative caries control and discusses ecological approaches to biofilm modification and prevention.

Abstract

• Contemporary caries understanding focuses on dental plaque biofilm ecology and environmental factors influencing disease.
• A healthy oral microbiome is increasingly recognized for preventing caries and promoting oral health.
• Ecological preventive approaches aim to broaden available preventive measures.
• These approaches target cariogenic virulence factors without harming bacterial viability.
• They also enhance the growth of health-associated, diverse microbial communities.
• The paper supports developing ecological measures beyond conventional methods.
• These approaches can reduce caries severity by promoting stable, health-associated oral biofilm communities.

Dental Caries: A Biofilm-Mediated Multifactorial Disease

• Dental caries is a multifactorial disease lacking a single cause, making simple solutions ineffective.
• It's an endogenous, biofilm-mediated disease where acidogenic/aciduric bacteria gain an advantage, disrupting biofilm balance.
• Modern analyses show various bacteria, not just mutans streptococci (MS) or lactobacilli (LB), contribute at different stages.
• Fungi like Candida albicans can enhance cariogenic virulence of biofilms.
• MS- or LB-dominated microbiome is typical of advanced stages with frequent biofilm acidification, reducing microbial diversity.
• The shared phenotypic characteristics (acidogenicity and aciduricity) are key in driving the ecological shift leading to caries.
• While ionic aspects have been researched for decades, new insights into microbial aspects have led to novel prevention and control approaches.
• Preventive measures should correct environmental pressures causing biofilm dysbiosis and maintain a healthy, diverse microbiome.
• These approaches acknowledge the role of potentially unidentified bacteria in the caries process, advocating for comprehensive prevention.

Rationale for Newer Methods of Dental Caries Control

Ecological Strategies

• Historical focus was on eliminating plaque, but current evidence highlights benefits of a healthy oral microbiome.
• Commensal plaque microflora has a symbiotic relationship with the host, acting as a barrier to opportunistic pathogens and carrying out beneficial metabolic processes.
• Host immune system and microbial symbionts interplay, making acute oral mucosa infections rare.
• Resident bacteria's pro- and anti-inflammatory activities maintain oral cavity homeostasis.
• Moderate amount of healthy plaque can prevent erosive enamel lesions and hypersensitivity.
• Caries-preventive strategies should take an ecological approach to the holobiont.
• Indiscriminate suppression of the entire oral biota is unlikely to have long-term success.
• Preventive products should target cariogenic bacteria specifically or inhibit virulence factors without affecting bacterial viability.
• Enhancing colonization of health-promoting microbial communities can correct ecological imbalance in cariogenic biofilms.
• This approach counterbalances low pH from acidogenic organisms with ammonia production from other bacteria.
• Ecological prevention preserves beneficial effects from the resident oral microbiome while reducing cariogenic virulence factors responsible for plaque biofilm dysbiosis.

Aetiological Factors

• Control should focus on aetiological factors: periodic disorganization of plaque biofilm via mechanical oral hygiene, and dietary modification to reduce fermentable carbohydrate exposure.
• Individual oral hygiene has limited impact.
• Toothbrushing's effectiveness stems from fluoride delivery, not plaque disruption.
• Dietary modification, restricting sucrose and fermentable substrates, is difficult due to the availability of cariogenic foods.
• Sucrose restriction alone may not prevent caries if other starches are frequently consumed.
• While minimizing aetiological factors is critical, additional preventive measures based on individual risk are required.

Caries Epidemiology

• Dental caries was once a universal pandemic.
• Fluoride's discovery and widespread use in preventive programs significantly reduced caries prevalence in developed countries in the late 20th century.
• However, untreated caries in permanent teeth remains the most common human disease worldwide.
• Doubts exist whether the previous decline in caries prevalence has continued into this century.
• Recent studies in the USA and Australia suggest tooth decay may be increasing again.
• A similar trend is seen in Norway and Iceland, which initially showed the biggest improvements.
• The flattening of prevalence rates, even in dentally aware populations using fluoridated toothpaste, is concerning and highlights the need for additional synergistic or complementary preventive measures to fluoride.

Fluoride Alone May Not Be Sufficient

• Fluoride is effective and economical, remaining the mainstay of caries prevention, but it may not be sufficient alone.
• Carious lesions can still develop with more than six daily sugar exposures, even with regular fluoride use.
• Ready availability of cariogenic snacks and drinks can overwhelm the benefits of community water fluoridation and daily fluoride toothpaste use.
• The limit to fluoride's repair potential could explain the reversal of caries decline in contemporary epidemiological reports.
• Fluoride's cariostatic actions inhibit enamel demineralization and enhance remineralization.
• Fluoride ions can also influence MS virulence factors, reducing acidogenicity, aciduricity, and glucan formation.
• Metabolome analysis shows fluoride can repress acid production in vivo.
• However, the extent to which antibacterial mechanisms contribute to preventive effects is still under question.
• Brief fluoride exposure from toothpaste or mouthwash may not sustain anti-acid production activity, with biofilms recovering acidogenicity over time.
• Concerns exist regarding the emergence of microorganisms more resistant to fluoride's effects on microbial metabolism.
• Preventive approaches combining fluoride with other protective agents are advocated to enhance its ability to modify biofilms and diminish the cariogenic bacterial challenge.
• Fluoride-antimicrobial combinations are recommended based on the rationale that fluoride can reduce the critical pH at which dissolution starts, while effective antimicrobials can decrease the depth of the Stephan curve pH drop following consumption of fermentable carbohydrates.
• Randomized controlled trials (RCTs) have shown fluoride-chlorhexidine interventions significantly reduced bacterial load and pH drop from glucose metabolism and lowered caries increment in high-risk groups.
• Systematic reviews have also found high-quality evidence that fluoride-triclosan toothpastes give a small improvement in reducing coronal caries compared to conventional fluoride toothpaste.
• However, broad-spectrum antimicrobials may not be ideal; treatment should modify plaque biofilm ecology by weakening virulence factors rather than eliminating microflora.
• An alternative is combining fluoride with agents that promote community-wide microbial shift by encouraging health-associated bacteria growth, rebalancing the biofilm ecology for better long-term caries control.
• Sufficient evidence supports fluoride-arginine combinations as a new standard of care.
• A toothpaste containing enzymes and proteins shifted the oral microbiome toward a community with a stronger association to health.

Caries Risk

• Caries distribution has shifted from a pandemic to endemic in specific risk groups.
• A cross-sectional study concluded that 75% of the caries-risk burden tends to reside in 25–40% of the population.
• Genetic differences exist in caries susceptibility, with not everyone benefiting equally from traditional preventive programs.
• Caries pattern has changed from a rapidly progressing childhood disease to a slowly progressing disease occurring or persisting throughout adulthood and old age.
• Contemporary studies reveal carious lesions are increasingly localized to specific tooth sites.
• Occlusal surfaces are most likely to experience caries in children and adolescents.
• Increasing life expectancy means older adults with exposed root surfaces are at greater risk of root caries.
• Fluoride's primary cariostatic action is on smooth enamel surfaces, leaving susceptible occlusal/root surfaces predisposed to acid dissolution.
• Caries-preventive strategies must go beyond conventional methods to better protect at-risk surfaces in at-risk patients.

Fluoride Safety

• The US Centers for Disease Control and Prevention ranked community water fluoridation as one of the 10 great public health achievements of the 20th century.
• Despite broad support within the dental profession, the safety of fluorides used for caries prevention continues to be debated.
• Recent reports add to the alarmist picture for those not informed on the scientific literature.
• A 2012 systematic review and meta-analysis concluded that high fluoride exposure may lower IQ levels in children, attracting media attention and triggering the inclusion of fluoride among chemicals classified as developmental neurotoxicants.
• Concerns with the 2012 review: all selected studies were from China, fluoride exposure came from multiple sources not just drinking water, the definition of “high” fluoride levels varied widely, and the probability of confounding as covariates was not controlled.
• The authors were conservative, concluding that “our results support the possibility of adverse effects” and advocating future research to evaluate dose-response relations based on individual-level measures of exposure over time.
• These caveats are overlooked in media reports, fanning extremist views on fluoride safety.
• Dental fluorosis from excessive fluoride intake during tooth formation is a persisting concern raised by anti-fluoride lobbyists.
• While fluoride is safe at recommended levels for community water fluoridation or toothpaste, swallowing large toothpaste amounts is not recommended.
• There are opportunities for “halo” exposure from other fluoride sources, triggering downward revision of recommended community water fluoridation levels (from 0.7–1.2 to 0.7 mg/L) to ensure the risk for dental fluorosis does not increase.

Ecological caries-preventive approaches synergistic with fluoride can potentially allow dental products to be designed with lower concentrations of fluoride and these could be useful for caries prevention in infants and young children, as well as for patients reluctant to use oral care products with high fluoride concentrations.

Ecological Approaches to Caries Prevention

Antimicrobial Peptides

• Antimicrobial peptides (AMPs) are a heterogeneous group of molecules with unique antimicrobial characteristics that have great potential for controlling bacterial infections and modifying biofilms.
• AMPs have a broad range of antibacterial, antiviral, and antifungal activity mediated by selectively interacting electrostatically with negatively charged components of cell membrane phospholipids, resulting in membrane permeabilization and disruption, leading to cell death.
• Escaping the actions of bilayer-disruptive AMPs would entail changing membrane composition and organization, a “costly” process in evolutionary terms, meaning that AMPs have very low resistance rates compared to common antibiotics.
• Besides naturally secreted salivary AMPs (lactoferrin, cathelicidins, histatins, defensins), a number of AMPs have been synthesized in the laboratory, and these include specific anticaries peptides that have shown the potential to modify plaque biofilms and inhibit dental caries.

Specifically Targeted Antimicrobial Peptide (STAMP)

• The specifically targeted antimicrobial peptide (STAMP) is a synthetic fusion peptide with 2 independent functional domains, consisting of a Streptococcus mutans-selective “targeting domain” designated as C16, and a “killing domain” designated as G2.
• C16 is derived from a fragment of the S. mutans competence stimulating peptide (CSP), while G2 is derived from a broad-spectrum antimicrobial peptide.
• C16G2 had antimicrobial mechanisms similar to traditional AMPs, and critically, its membrane-disrupting activity specifically targets S. mutans from multispecies biofilms without affecting closely related non-cariogenic oral streptococci.
• More recently, an in vitro study on human saliva-derived polymicrobial biofilms was able to demonstrate that treatment with C16G2 not only eliminated S. mutans, but also resulted in a more benign oral microbial community with increased populations of health-associated bacteria and fewer harmful Gram-negative bacteria.
• C16G2 is recognized by the US Food and Drug Administration as an investigational new drug for dental caries prevention and has successfully completed phase II clinical trials, where it was delivered to patients in the form of a dental gel loaded in trays.

α-helical antimicrobial decapeptide KSL-W

• Another promising anticaries AMP is a synthetic α-helical antimicrobial decapeptide designated KSL-W, which can selectively destabilize the cell membranes of cariogenic bacteria including S. mutans, S. sobrinus, and L. acidophilus.
• This peptide resits enzymatic degradation in human saliva for 1 h, and the potential use of KSL-W as an antibiofilm agent in a chewing gum formulation has been suggested.
• More recently, a hydroxyapatite-binding AMP was designed, based on the fusion of specific hydroxyapatite-binding heptapeptide (HBP7) with KSL-W, and this bioconjugate was shown to have improved oral retention and antibacterial efficacy.
• Other AMPs that have shown in vitro antimicrobial activity against cariogenic bacteria and inhibited oral biofilm formation include a synthetic peptide called L-K6 (derived from the naturally occurring peptide temporin-1CEb), a short synthetic amphiphilic peptide known as 1018, and an amphipathic α-helical peptide containing only 12 amino acids named GH12.
• Limitations of AMPs include their potential toxicity, susceptibility to proteases, high cost of peptide production, and the reduced cationic activity of most AMPs in physiological fluids like saliva.
• With regard to their use for dental caries prevention, questions still remain on whether anticaries AMPs will be able to function against a background of excessive acid production often seen in high caries-risk individuals.
• Before any clinical recommendations can be made, it will be essential to test such agents in a caries-conducive oral environment, and to evaluate whether the desired outcome of reduced caries increment in at-risk population groups can be achieved.

Probiotics

• The term probiotics refers to “live micro-organisms, which, when administered in adequate amounts, confer a health benefit on the host”, and is based on the Noble-prize-winning pioneering work of Metchnikoff [1907] for maintaining a healthy gut flora.
• This concept of implanting a harmless effector strain into the host’s microflora to maintain or restore a natural microbiome by inhibition of pathogenic micro-organisms is attractive and has also been used to support health-associated microbes or restore diversity in the oral plaque biofilm.
• The mechanisms by which probiotics re-establish ecological balance in oral biofilms are not fully understood, but probiotic bacteria are believed to have both local and systemic effects.
• The local anti-caries effects may include competitive inhibition with cariogenic bacteria for nutrition or adhesive surfaces, selective co-aggregation of MS without disturbing other oral flora, and bacteriocin-producing probiotics targeting MS.
• The most commonly used and studied probiotics belong to the Lactobacillus and Bifidobacterium bacterial genera, although not all their strains have the same efficacy in the inhibition of S. mutans growth or biofilm formation.
• Evidence supporting the application of probiotics for preventing dental caries is controversial, with recent reports suggesting potential harmful effects for some probiotic bacterial strains.
• One of the problems identified in using probiotics for caries prevention is that the commonly available Lactobacillus and Bifidobacterium probiotic bacteria are themselves acidogenic and aciduric, and could contribute to the caries process if such bacteria are allowed to colonize the oral cavity.
• Recent in vitro biofilm studies have confirmed this apprehension with different strains such as Lactobacillus salivarius W24, Lactobacillus rhamnosus GG, Bifidobacteria animalis lactis BB12, Lactobacillus rhamnosus LB21, and Lactobacillus acidophilus LA-5 which have all been shown to lower biofilm pH.
• In fact, some Lactobacillus and Bifidobacterium strains have greater cariogenic attributes than even MS.
• Thus, while displacing S. mutans from plaque biofilms may in principle be desirable, substituting them for even more cariogenic bacteria will not be useful.
• Some of the non-acidogenic alternatives to Lactobacillus and Bifidobacterium that have shown promising early results include S. salivarius M18, heat-inactivated BB12, and Weissella cibaria CMU.
• Another limitation with the traditional use of gut-associated Lactobacillus and Bifidobacterium probiotic species to promote oral health is that these non-oral bacterial strains may not efficiently colonize the oral niche, which is vital for the long-term success of probiotics.
• Even the use of a bacteriocin-producing strain of S. salivarius may not succeed, as S. salivarius, while a typical member of the oral soft tissue flora, has limited ability to colonize tooth surfaces.
• However, very recently 2 natural oral commensal species, Streptococcus dentisani and Streptococcus A12 that were isolated from the supragingival plaque of caries-free individuals, have demonstrated promising probiotic effects against dental caries.
• Both these “active colonizers” have a double probiotic action, as they can not only inhibit the growth of MS, but also moderate plaque pH through their arginolytic actions.
• The current decade has seen a number systematic reviews and meta-analyses evaluating the effectiveness of using traditional gut-associated probiotics for caries prevention.
• A meta-analysis of studies with surrogate caries markers (MS and/or LB counts) concluded that these probiotics significantly decreased MS counts, but there were insufficient data on whether caries increment was reduced as well.
• A similar inference was also reached by a qualitative systematic review of probiotic caries studies, concluding that clinical recommendations would be premature without more comprehensive RCTs showing an actual reduction in individual caries experience.
• Relatively few RCTs of oral probiotics have used clinical dental caries indicators to prove the efficacy of probiotics in preventing or treating dental caries.
• A comprehensive systematic review utilizing evidence from these RCTs concluded that current evidence is insufficient for recommending probiotics in controlling dental caries.
• Taken together, currently available data indicate that while traditional probiotic bacteria may have a beneficial effect on the gut flora and systemic health, a beneficial and clinically significant effect on the oral flora is yet to be demonstrated with sufficient rigor.
• However, observations from 2 major clinical trials in children support the intriguing concept of a “metabolic domino effect,” with reductions in caries risk seen to be accompanied by improvements in general health.
• Particularly promising for caries prevention is the move away from gut-associated probiotic bacteria to resident oral probiotic strains, such as the 2 S. mutans-antagonistic bacterial species S. dentisani and A12.
• Further evidence on the ability of these oral probiotics to inhibit dental caries is awaited with interest.

Prebiotics

• The prebiotic approach involves feeding resident microbiota with specific nutrients to create conditions that favour the growth and dominance of healthy bacteria in the biofilm.
• The nutritional stimulation of endogenous beneficial oral flora to restore microbial balance and promote oral health has been validated in mixed species models.
• Oral prebiotic substrates that are especially valuable to prevent caries include arginine, arginine-rich peptides, and urea, as these foods when metabolized create alkalizing effects that counteract the acidogenic environment created by cariogenic bacteria.
• Many commensal bacteria are able to use arginine or urea to generate ammonia by the arginine deaminase system or urease enzymes, respectively.
• Multiple studies have shown that bacterial production of alkaline metabolites such as ammonia can play a major role in biofilm pH homeostasis and beneficially alter the de-/remineralization equilibrium.
• A substantial body of evidence from microbiological, genetic, biochemical analyses, and clinical studies has now accumulated confirming that the modulation of the alkalinogenic potential of dental biofilms is a promising strategy for caries control.
• The preventive potential of oral alkali production has resulted in the development of commercial oral care products that utilize arginine to promote a healthy resident oral microbiome.
• In vitro biofilm experiments found that fluoride-arginine combinations synergistically inhibited S. mutans but enriched S. sanguinis growth within multispecies biofilms, while maintaining a “streptococcal pressure” against the potential growth of oral anaerobe Porphyromonas gingivalis in the alkalized biofilm.
• Fluoride-arginine combinations can also suppress exopolysaccharide production, thus targeting another critical virulence factor for cariogenic biofilms.
• Human in situ studies and several double-blinded RCTs using a fluoride dentifrice containing 1.5% arginine and an insoluble calcium compound have shown significantly greater protection against carious lesions than a fluoride dentifrice alone.
• A systematic review and meta-analysis of the anticaries effects of arginine-containing dentifrice formulations concluded that arginine products provided a superior preventive effect over matched formulations containing fluoride alone.
• Other authors have been more conservative in their conclusions, either citing insufficient evidence in support of a caries-preventive effect for arginine, or expressing concerns over the higher cost of arginine-fluoride dentifrices versus any additional caries-preventive effect these may provide.
• However, the preponderance of evidence does seem to suggest that the arginolysis is an effective approach to improve oral health and balance the microbial ecology.

Sugar Polyols

• Sucrose has been designated as the “arch criminal” in the caries process for a long time, and the search for alternative non-fermentable sweeteners has attracted much attention.
• Data collected from in vitro and in vivo studies indicate that such sugar substitutes can exhibit potential anticaries effects through a number of different mechanisms.
• Xylitol, a naturally occurring 5-carbon sugar polyol, is the non-nutritive sweetener that has been most extensively researched over the past 4 decades for its potential cariostatic effects.
• Xylitol inhibits MS growth by disrupting their energy production processes, leading to a futile energy cycle and cell death.
• Although not all MS strains were inhibited by xylitol in this manner, even xylitol-resistant bacteria were found to be less virulent after xylitol treatment.
• The predominant delivery vehicle for xylitol has been chewing gums, with xylitol dentifrices, candies, lozenges, and mouthrinses also having been used with varying degrees of success.
• A substantial body of evidence suggests that 5–6 g of xylitol per day delivered over 3 exposures are needed for worthwhile anticaries effects.
• Among other sugar polyols, erythritol has been attracting increasing attention as it is has been shown to be more effective than xylitol and sorbitol, and importantly, its anticaries effects were shown to persist for up to 3 years.
• Despite an immense body of literature, the caries-preventive effects of xylitol products remain inconclusive because of inconsistent study outcomes.
• While numerous studies have indicated xylitol has beneficial effects on surrogate end points (MS levels, plaque pH, acid production), evidence for worthwhile reductions in caries experience remains equivocal, and there is a need for more double-blind placebo-controlled RCTs, focussing on optimal dosage, delivery vehicle, and possible synergism with other preventive agents.
• In fact, the more recent data conclude that there is limited evidence to show xylitol is effective in the fight against dental caries.
• A double-blind cluster RCT using xylitol gummy bears found that polyol consumption did not provide any additional benefit over other caries-preventive measures.
• Recent Cochrane systematic reviews also found only very-low- to low-quality evidence on xylitol effectiveness which was not sufficient to determine whether xylitol-containing products can prevent caries in infants, children or adults.
• Other issues that limit the usefulness of sugar polyols include their high costs and low compliance in high-risk patients because of the need for daily long-term use.

Quorum-Sensing Targets

• Another approach that may maintain and support a healthy oral plaque ecology is to interfere with the fundamental cell-cell communications system between biofilm bacteria.
• This process of quorum sensing (QS) is mediated through small diffusible hormone-like molecules (pheromones) and their specific receptors.
• For MS, the stress-dependent QS system is primarily comprised of the CSP and its ComD/ComE 2-component signal transduction system for communication between biofilm cells of the same species, while interspecies signalling is mediated via the autoinducer-2 molecule produced by LuxS.
• The CSP-mediated QS system in S. mutans affects biofilm formation, acidogenicity, aciduricity, genetic transformation, bacteriocin production, stress response, and the ability to produce persister phenotypes.
• Targeting QS signalling pathways could provide a promising avenue in the development of novel therapeutics to alter cariogenic biofilms.
• As QS is not directly involved in processes essential for bacterial growth, targeting QS will allow less virulent bacteria to remain in the biofilm and will also not impose selective pressures that can lead to the development of antibiotic resistance.
• Interfering with CSP signalling systems has been shown to inactivate a wide range of bacteriocins and mutacins that play an important role in the sustained existence of S. mutans in the dental plaque.
• The addition of an exogenous CSP can disrupt signalling events of S. mutans and induce cell death.
• Another novel QS-modifying compound, 3-Oxo-N, was seen to significantly minimize lactic acid accumulation without affecting biofilm growth even in the presence of fermentable sugars, representing a promising agent for maintaining a healthy, non-cariogenic microbial ecology in dental plaque.

Natural Products

• Natural products include secondary metabolites or phytochemicals derived from plants, fruits, herbs, or spices.
• They offer a rich source of structurally diverse molecules with a wide range of biological activities and could prove useful as alternative or adjunctive anticaries agents.
• Potential cariostatic mechanisms identified include inhibition of bacterial growth or acid production, inhibition of glucan synthesis by interfering with glucosyltransferase (Gtf) activity, and inhibition of bacterial adhesion.
• Polyphenols from propolis (apigenin and tt -farnesol) and cranberry proanthocyanidins have been shown to exert useful ecological effects on the plaque biofilm.
• Apigenin is a potent inhibitor of water-insoluble glucan synthesis, while tt -farnesol disrupts S. mutans membrane permeability and acid production.
• An animal study found a combination of these 2 phytochemicals with fluoride suppressed dental caries without affecting the viability of normal oral flora, being as potent as a fluoride-chlorhexidine control in caries inhibition, but without the broad antibacterial action of the control.
• Similarly, cranberry proanthocyanidins, which lack significant biocidal activity, can modify plaque biofilms by reducing acidogenicity and glucan synthesis, and these surrogate end points were also translated into cariostatic effects in vivo.
• A number of other polyphenol compounds have been found to be effective in killing S. mutans, with the minimal inhibitory concentrations of some bioactive molecules like xanthorrhizol (from Curcuma xanthorrhiza) or macelignan (from Myristica fragrans) almost comparable to chlorhexidine.
• While most of the tested anticaries phytochemicals showed growth-inhibitory or anti-adhesive effects, a potentially interesting natural agent in caries prevention is Galla chinensis, which was able to beneficially regulate the de-/remineralization balance of dental hard tissues.
• An analysis of how phytochemical research has impacted oral care in the period from 2000 to 2015 found that despite many in vitro, in vivo, and clinical studies testing natural products derived from plants, only 11% of studies were in phase IV clinical trials
• Similar conclusions were reached in a systematic review of the anticaries effects of essential oils and their isolated constituents, which found that most studies were conducted in the laboratory, and did not provide botanical characterization or compositional data on the natural product being tested.
• Natural products remain a largely unexplored source of effective and non-toxic antibiofilm molecules that could potentially be used in combination with fluoride as useful alternatives to traditional microbiocides like chlorhexidine or triclosan.
• However, future research needs to focus on translational approaches to advance the development of effective anticaries products containing phytochemicals or essential oils.

Replacement Therapy with “Designer” Bacteria

• Modifying dental plaque by replacing S. mutans, a member of the normal microbiota, with a less virulent effector strain has been an established concept for many years.
• The rationale for bacterial replacement therapy against dental caries is that relatively avirulent strains of MS are most likely to occupy the same ecological niche in plaque as their more cariogenic counterparts thereby reducing the overall cariogenicity of the plaque biofilm.

Designer Bacteria

• A number of “designer” bacteria have been studied for bacteriotherapy against cariogenic biofilms including a glucan synthesis-defective mutant of S. mutans, variants of S. salivarius (TOVE-R), and a recombinant alkali-generating ureolytic S. mutans strain.
• The most extensive research in using genetically modified bacteria for preventing dental caries used a wild-type S. mutans strain that naturally produces an antibiotic called mutacin 1140 capable of killing all other strains of S. mutans.
• This strain was genetically modified by deleting the open reading frame for lactate dehydrogenase, to yield a viable strain called BCS3-L1 that still produced wild-type levels of mutacin 1140, but notably produced no lactic acid.
• In laboratory and animal models, the BCS3-L1 strain proved to have significantly reduced cariogenic potential.
• It persistently and pre-emptively colonized tooth sites normally occupied by wild-type S. mutans strains, with no reported adverse effects.
• To overcome safety issues and to enable the altered strain to be implanted into human oral biofilms for clinical trials, additional genetic modifications of BCS3-L1 were made, which could facilitate its rapid elimination should an adverse event manifest itself.
• This strain was designated as A2JM and was extensively tested to assure its safe use in phase I clinical trials.

gcrR-deficient S. mutans strain

• It has also been suggested that hypocariogenic strains exhibiting only defects in acid production are unlikely to compete successfully with wild-type strains for initial plaque locations, and on this principle, an S. mutans strain that was deficient in the gcrR gene was genetically engineered for bacterial replacement therapy.
• The gcrR gene functions as a negative transcriptional regulator of the gbpC gene, which encodes the glucan-binding lectin, an adhesin that is ubiquitous on S. mutans surfaces and plays an important role in initial bacterial aggregation and adhesion to tooth surfaces.
• The MS- gcrR-def mutant bacteria showed reduced acid production, out-competed wild S. mutans strains with its strong early colonization ability, and lowered caries incidence in vivo.
• Modulation of oral plaque biofilms with genetically engineered “designer” bacteria has great potential through fostering a healthy oral environment, which prevents the dominance of cariogenic bacteria.
• A single treatment regimen could lead to persistent colonization by the effector strain affording lifelong protection, with minimal need for patient compliance.
• Whilst there have been encouraging results with genetically modified strains, the concept of replacement therapy needs to be tested for effectiveness in highly cariogenic environments.
• Even if successful, the widespread acceptance of genetically engineered “designer” bacteria may prove to be difficult for emotional, ethical, and legal reasons.

Other Approaches

• In addition to the strategies discussed above, several alternative approaches for modifying plaque biofilm ecology are under investigation including using compounds that specifically affect bacterial virulence proteins, calcium phosphate-osteopontin particles that can inhibit biofilm formation and reduce the fall in pH without affecting bacterial viability, nanoparticles, graphene oxide, and ceramic water.

Conclusions

• There is no doubt that fluoride will continue to be the mainstay of any caries prevention protocol as it still remains the most effective and economical protective agent against dental caries.
• However, fluoride alone may not offer complete protection against the disease, and it is generally recognized that the effectiveness of fluoride could be enhanced when combined with additional cariostatic agents.
• Moreover, current paradigms emphasize the importance of maintaining a healthy and stable oral plaque biofilm for long-term disease control.
• One way to do this is to limit or exclude refined sugars from the diet; however, within the constraints of present-day consumer culture, behavioural dietary changes are difficult to achieve and sustain.
• Adopting ecological preventive measures can help in correcting the disturbed plaque ecology and drive the advent and persistence of a symbiotic oral microbiome.
• These could be valuable tools in achieving long-term dental caries control, allowing the clinician to shift to a biological model for the management of the disease.
• It is imperative that the effectiveness of ecological preventive approaches be evaluated for success in individuals who consume a conventional diet containing a fairly high level of sugars before any clinical recommendations are made.
• Furthermore, rather than surrogate end points like lower MS levels or reduced acid production, the critically important outcome for all new caries-preventive measures will be whether they can ensure a significant reduction in individual caries experience.