A comprehensive review of the application of probiotics and postbiotics in oral health

1The World Health Organization estimates that about half of people suffer from oral diseases. About 2.4 billion people have permanent teeth decay and, 532 million children are also affected by primary teeth decay. Oral health care is expensive, and in high-income countries, about 5% of the total health costs are spent on it. In other countries, the required costs are higher than the capacity. Tooth decay is a disease related to several different factors. Factors such as a cariogenic diet, poor oral health, high counts of cariogenic bacteria, dental plaque, inadequate saliva flow, and lack of sufficient fluoride exposure are among the environmental risk factors that cause tooth decay. As a general fact confirmed by various studies, the composition of oral microbiota is closely associated with oral health, and its disruption is an important step in oral disease. S. mutans, one of the bacteria in the oral microflora, is known as the basic cause of dental decay and one of the biofilm-forming bacteria. Many clinical studies conducted on different people in the community have reported the correlation between S. mutans levels and caries. It should be noted that the association between S. mutans levels and the occurrence of caries in people is different for various reasons, including genetic factors in the etiopathogenesis of tooth decay, and certain patients may be more vulnerable or resistant to caries. Although the intervention of various factors is effective in causing tooth decay, microbiological factors are the main factor. Therefore, it is believed that bacteriotherapy can be an effective way to prevent oral disease. Since the oral cavity is the first part of the gastrointestinal system, it is logical that probiotics exert their effects on the oral and affect its microbiota just like the gut. probiotics are live microorganisms that, if consumed in enough amounts, are advantageous to host health. In vitro studies have shown the advantageous effects of different strains of probiotics against oral pathogens. In vivo studies have also mentioned the effects of probiotics in the prevention or cure of periodontal diseases and tooth decay. However, some trials have reported conflicting results. The possible mechanisms of probiotics’ action on oral health are the same mechanisms mentioned in gastrointestinal studies ( Figure 1 ). Evidence suggests that part of the antimicrobial effects of probiotics is associated with the substances they produce, including bacteriocins, organic acids, fatty acids, and hydrogen peroxide. Considering some of the challenges and limitations that exist for using probiotics in food products, the use of metabolites or their non-living components (postbiotics) can be a new and appropriate solution. Among the challenges that are related to the use of probiotics and can be solved by postbiotics is the creation of clinical problems, especially in people with a weakened immune system, the probability of transmission of resistance genes to pathogenic microorganisms and the emergence of antibiotic resistance, and the need for high costs to provide an ongoing cold chain from production to consumption. Accordingly, in this review, while examining the effects of different strains of probiotics on oral and dental health, we will also look at the impact of postbiotics.

2Microbes live in different parts of the body, and the ratio of the count of cells in the human body to the count of microbes is about 1:10. Advances in biological technologies have led to many studies to understand better the composition and impact of microbiota on human diseases. There are four main locales of microbial colonization in the body: the oral, intestines, skin, and vagina; therefore, the oral cavity is one of the largest microbial sites, and the microbiome is stored on the teeth, tongue, soft and hard palates, gingival sulcus, and tonsils, and is an important factor in health and diseases related to the oral and teeth ( Figure 2 ). Using whole metagenome sequencing methods, it has been determined that bacteria such as Actinobacteria, Proteobacteria, Fusobacteria, Bacteroidetes, and Firmicutes constitute about 80-95% of the total oral microbiome. The most prevalent species identified in the oral cavity using different methods including Actinomyces, Prevotella, Streptococcus, Fusobacterium, Leptotrichia, Veillonella, Rothia, Corynebacterium, Capnocytophaga, Selenomonas, Treponema, Haemophilus, and TM7 Genes 1 and 5. When oral homeostasis is disrupted, and the dominance of good bacteria is lost, the conditions for dysbiosis and the growth of a diverse population of pathogenic bacteria with parasitic lifestyles are created. showed that dysbiosis occurs before the clinical symptoms of periodontal disease are evident, so examining the balance of oral microbiota can help as a tool for the initial detection of periodontitis. Also, in disease conditions, oral bacterial diversity decreases, and fewer bacterial genera predominate. For example,, after collecting saliva samples from 12-year-old children, found that the two bacteria, Porphyromonas and Provetella, show an increasing percentage in children with caries compared to healthy people, and as the severity of the disease increases, bacterial diversity decreases. Two factors play a role in tooth decay: the immune system and diet. The oral microbiota creates an acidic environment on the teeth as a result of a frequent carbohydrate diet, and in this situation, acid-tolerant and acidogenic bacteria probably cause demineralization and cause tooth decay. The study by also showed that caries risk assessment often involves diet and bacteria and showed a correlation between the detection and abundance of S. mutans with the existence of caries. A study of 39 healthy and 51 caries people showed that the bacterial profiles of intact enamel in healthy people were significantly different from the bacterial profiles of intact enamel in sick people. Species like Staphylococcus intermedius, Eubacterium saburreum clone GT038, Kingella oralis, Streptococcus cristatus, and Gemella morbillorum were found at a high level in healthy people. They were at significantly decreased levels in the healthy enamel of diseased people in permanent teeth. Other health-related species in permanent teeth, like Streptococcus sp. group H6, Streptococcus sp. clone CH016, Eubacterium sp. clone EI074, Campylobacter showae, Capnocytophaga sputigena, Leptotrichia sp. clone DT031, and Fusobacterium nucleatum subsp. polymorphum were found at decreased levels in plaque from white spot or dentin lesions of diseased people compared to the levels detected in healthy people. On the other, S. mutans, Streptococcus salivarius, Lactobacillus fermentum, L. gasseri, Prevotella clone AO036, Propionibacterium FMA5, and Atopobium genomospecies C1 were found in the teeth of healthy subjects at low levels, while in dentin cavities and deep-dentin cavities were found at high levels, so caries-associated species were considered. In The study by, a clear correlation was observed between the frequency of S. mutans and caries. In this study, Romanian teenagers with an average age of 14.4 and a history of minimal dental care were found to have abundant dental plaque and a mean decayed, missing filling surface (DMFS) score of 20.1. In contrast, Swedish teenagers with an average age of 17 and a mean DMFS of 7.5 had regular dental care and also had a minimum observable dental plaque. On top of having more caries experience, Romanian teenagers showed a higher abundance of S. mutans diagnoses than Swedish ones (85% compared to 50%). On the other hand, there is evidence of microbiotas associated with non-mutans caries and not identifying S. mutans in about 10-15% of people with active caries. These data suggest that potential acid-producing species are involved in caries, some of which have not yet been cultivated. For example, Scardovia wiggsiae was detected in about 40% of children with active caries who did not have S. mutans. Therefore, this species may be one of the important factors of caries in the absence of S. mutans. In a study by, different parts of the microbiota were cultured in rich blood containing agar and in an acid isolation medium with low pH. The greatest difference between the microbial population of children without tooth decay and ECC (Early childhood caries) children was in the acid isolation medium. The results of this study showed that there was a significant difference between health and caries in the acid-resistant population. Acid-resistant species included were the S. mutans, Actinomyces odontolyticus, and several having no name Actinomyces spp., Bifidobacterium, Scardovia, and Lactobacillus species. Also, two species related to caries, i.e., S. mutans and S. wiggsiae, had a significant difference between disease and health conditions in both types of bacterial isolation in acid or blood agar conditions. In a study, researchers cultivated several new species of Actinomyces that were resistant to acidic conditions. Therefore, the potential for acid production at a low pH was considered to Discover the caries potential. For a long time, Actinomyces have been known as part of the caries microbiome and have been a significant group in the induction and development of caries. Several studies have also associated Veillonella spp. with dental decay. reported that Veillonella was the most populous organism, especially in low pH after all glucose-pulsing regimens. reported that acid production was higher in bacterial plaques mixed with V. alcalescens and S. mutans than plaques that consist of only one of these two species. Also, confirmed an active relationship between acid-producing bacteria and Veillonella species. On the other hand, Veillonella spp. may be consequential for acid-producing bacteria by reducing nitrate. reported that low concentrations of nitrite and pH levels of less than 7 (0.2 mM) killed S. mutans. These findings indicate that nitrite in saliva affects the growth and survival of cariogenic bacteria. and found that Prevotella-like and Prevotella bacteria master the varied polymicrobial community in some cases of caries, which suggests that Prevotella play a role in the development of dental decay. In addition, co-aggregation between six species of S. constellatus ND10-13A, S. bovis II/2 ND2-2, L. acidophilus ND7-2A, S. sanguinis II ND7-3, C. sputigena ND2-12A, P. intermedia ND8-9A, and F. nucleatum NT6-6A, showed that these strains can co-aggregate with many oral bacteria and while forming dental plaque and bringing it to maturity, they will also control the composition of the microbial community. Evidence suggests that a wide range of bacterial strains are involved in tooth decay. In the study conducted by, also it was stated that there were 75 species or phylotypes in 10 caries lesions. Up to 31 species have been reported separately in each sample. Various species of Lactobacilli make up about 50% of the species, and about 15% are Prevotellae. Other species with high affluence are F. nucleatum, Pseudoramibacter alactolyticus, Dialister spp., Selenomonas spp., Eubacterium spp., Olsenella spp., Bifidobacterium spp., members of the Lachnospiraceae family, and Propionibacterium sp.. In more studies, it was stated that Lactobacillus and Mutans spp. are dominant in advanced caries. In a study, it was stated that the number of S. mutans decreased after the cure compared to before the cure. While, in another study, it was indicated that the treatment was associated with a change in the microbial profile, but this change was not significant for S. mutans. The object of the treatment is to keep the tooth structure and intercept its further decay, but since there is evidence that dental decay at first is a reversible disease that has a multifactorial cause, dentists are no longer seeking to use interventional treatment at a specific starting point of the appearance and severity of the disease. Nowadays, it is quite explicit that the restorative treatment of dental caries alone does not treat caries, and the caries process should be controlled with the patient’s participation throughout his life. Also, considering that the treatment of dental diseases is not possible for all members of society due to the need to spend a lot of money and time, prevention is a more economical way. Therefore, personal hygiene and diet modification should be considered. It has been said about children that their oral microbiota originates from the oral microbiota of the child’s main caregiver, the mother. Therefore, it is very important to focus preventive and therapeutic measures on child caregivers to prevent tooth decay in children.

3Tooth decay is a local demineralization of the hard tissues of the crown and root surface of the tooth, which happens in a bacterial accumulation that adheres to the tooth surfaces, called dental plaque, which is made of a gelatinous substance. S. mutans has been introduced as the main species of caries-causing bacteria due to its strong acid production and high tolerance to acidic conditions. Research has shown that the number of S. mutans in the saliva of people without caries is normally between 104 and 105 CFU/mL, But at 106 CFU/mL, the risk of caries increases significantly. In this regard, 105 CFU/mL has been considered the threshold of caries in clinical studies.

4Probiotic bacteria may cause chemical and physical alterations in the microbial flora of people’s oral cavity. Theoretically, probiotics have stronger adhesion to oral tissues than pathogens and can compete for adhesive surfaces. This leads to bacterial aggregation and co-aggregation and the formation of a new biofilm. Probiotics compete with the oral microbial flora and pathogens for adhesion sites, nutrients, and growth factors, thus protecting oral health. These bacteria aggregate after sticking to the oral cavity and prevent the adhesion of pathogenic bacteria through the production of antimicrobial components like acids, bacteriocins, and peroxides. Therefore, probiotic bacteria may prohibit the growth of caries-causing bacteria and periodontal diseases, create an immune response against pathogens, and prevent oral tissue destruction and inflammation in the oral cavity. Various researchers have surveyed the effect of the consumption of probiotics on tooth decay. For example,;, concluded that the use of probiotics could decrease the count of S. mutans and thus have a prohibitory effect on tooth decay. concluded during a meta-analysis that probiotics, including Lactobacillus, Bifidobacterium, and Streptococcus, could significantly reduce the count of S. mutans compared to control groups and prevent tooth decay. The results of this meta-analysis are consistent with systematic reviews that showed a significant decrease in the count of S. mutans after probiotic consumption.

5Strains such as Bifidobacterium spp. and L. rhamnosus, L. reuteri, and L. casei have all shown the capability to change the colonization of carious bacteria, which can prevent tooth decay. examined Lactobacillus strains in their research and reported that 69% of the strains could inhibit the growth of S. mutans, and 82% of the strains can inhibit the growth of Porphyromonas gingivalis. Both L. salivarius and L. fermentum have been found to have antagonistic activity against the growth of S. mutans. Also, L. salivarius TI 2711 isolated from a healthy human has shown an inhibitory effect on P. intermedia, P. nigrescens, and P. gingivalis after co-cultivation. In a study by, the ability to inhibit the growth of various oral pathogens by 3790 lactic acid bacteria (LAB) was investigated, and it was stated that L. rhamnosus and L. paracasei have a strong antibacterial effect on several oral pathogens. Considering the digestive benefits of Lactobacilli and the fact that the L. rhamnosus GG strain is not cariogenic due to its inability to consume sucrose or lactose (Homofermentative Lactobacilli), became the first researcher to show the inhibitory effect of this bacterium on a cariogenic pathogen. Therefore, it is expected that this bacterium has been conducive to oral health. In this regard, it has been stated that L. rhamnosus GG ATCC 53103 produces a growth-prohibitory compound against Streptococcus sobrinus and is suggested to decrease the risk of caries. Lactococcus lactis has been introduced as another effective strain of Lactobacilli in eliminating the colonization of pathogenic oral bacteria and modulating the oral microflora. This bacterium has shown the ability to eliminate the colonization of bacteria such as Actinomyces, V. dispar, S. oralis, and cariogenic S. sobrinus.

6On the other hand, some studies have claimed that Lactobacillus themselves are correlated to the initiation and development of caries. During their metabolism, Lactobacilli produce acids that may increase the risk of caries. found that the number of Lactobacilli was significantly higher in children who had tooth decay than in children without decay. Therefore, they concluded that the number of Lactobacilli in the oral cavity is associated with the progression of caries. In other studies, evidence suggests that Lactobacilli are more correlated with caries development than with initiation of caries and play a momentous impress in the development of tooth decay. Also, it has been reported that the count of Lactobacilli after probiotic consumption was not significantly different between the control and experimental groups. In a study, it was stated that the count of Lactobacilli decreased in one of the two probiotic groups after oral probiotic consumption. In contrast, it was noted in two studies that the count of Lactobacilli increased significantly. Therefore, according to the reports of most studies, it can be claimed that the consumption of probiotics does not cause the initiation of tooth decay. In this regard, it was reported in an intervention study that the consumption of Lactobacilli by children decreased the risk of tooth decay and primary caries in them. Until now, the effect of the consumption of different strains of probiotics on tooth decay has been investigated in several studies [ Table 1 ]. Regarding the literature investigation, most studies stated that the consumption of probiotics efficiently prevents tooth decay development, and they advise a controlled consumption of probiotics to reach advantageous effects.

7Interest in using probiotic Bifidobacteria to prevent and treat oral microbial diseases is increasing. Nevertheless, the results reported from clinical studies aimed at investigating the effect of Bifidobacteria on oral microbiota are conflicting and controversial. Some studies have reported that Bifidobacteria have anti-caries effects, and others have published the opposite results. Few studies have surveyed the effect of Bifidobacteria on S. mutans, and some of these studies have also surveyed the effects of Bifidobacteria along with Lactobacilli. In the meta-analysis conducted by, it was shown that Bifidobacterium does not effectively reduce the count of Lactobacilli in saliva. Still, it has a significant reduction effect on the count of S. mutans. Also, studies that used supplements of Bifidobacterium and Lactobacillus spp., reported a reduction in the count of S. mutans in the oral cavity. Similar results have been reported in people who did not use fluoride. While in some studies, the use of Bifidobacterium animalis BB-12 and other species of Bifidobacterium genus in people who did not use fluoride failed to show its therapeutic effects on tooth decay. A meta-analysis was conducted to investigate the difference in the count of S. mutans in saliva before and after consuming Bifidobacterium. The pooled results of four studies did not show significant differences. In a study, it has been stated that Bifidobacteria did not have a significant effect on reducing the count of S. mutans but significantly reduced the number of P. gingivalis in the biofilm. On the other hand, showed that Bifidobacterium can make an acidic environment and enhance the formation of biofilm so that Bifidobacterium may have a cariogenic effect on teeth. stated that Bifidobacterium could cause tooth demineralization because of the production of organic acids. Also, Dual-species biofilms formed by S. mutans and Bifidobacteria produce more acid than S. mutans biofilm alone, and as a result, more pH drops will occur. investigated the distribution of Bifidobacterium in the oral cavity and the association between Bifidobacterium and tooth decay in children. In this clinical study, the results indicated that the rate of Bifidobacterium was 47.5% in patients with severe ECC and 0% in healthy people without caries. Table 2 shows the results of clinical studies conducted to investigate the effect of Bifidobacterium on oral health. The number of studies that have used Bifidobacteria alone and separately from Lactobacilli is limited. This has made it challenging to decide on the effect of Bifidobacteria on the count of S. mutans and tooth decay. Therefore, in the future, more studies should be conducted with food products containing only Bifidobacteria.

8Candida albicans fungus is one of the prevalent infectious agents in the oral cavity. At older ages and in immunodeficiency conditions, the risk of yeast infections increases. Candida has different species, and the species that are usually isolated from the oral cavity include C. krusei, C. glabrata, C. tropicalis, C. parapsilosis, and C. albicans. Increasing antifungal resistance is becoming a new issue worldwide, and new methods are needed to fight pathogenic fungi. Probiotic Lactobacilli have demonstrated different inhibitory effects against oral Candida during in vitro studies. Meanwhile, L. Rhamnosus GG has shown a strong prohibitory effect. conducted a placebo-controlled study for the first time to examine the effect of probiotics on the abundance of oral Candida. Consumption of cheese containing Propionibacterium freudenreichii ssp. shermanii JS and L. rhamnosus GG probiotics decreased the abundance of C. albicans by 75% in the elderly. Yakult drink containing B. breve and L. casei increases salivary secretory protein levels and causes a significant decrease in Candida and non-Candida species in the oral cavity of the elderly. Also, the consumption of L. reuteri lozenge and probiotic cheese containing L. rhamnosus LC 705 and L. rhamnosus GG has reduced the number of oral yeasts correlated with caries. During a randomized clinical trial that was conducted on 60 children, it was shown that a probiotic rinse for one week was as efficient as chlorhexidine digluconate 0.2% in decreasing the count of C. albicans. It has also been reported that probiotic bacteria have an anti-biofilm effect against C. albicans and against the ability to produce biofilms of two species of C. albicans and S. mutans.According to these promising studies, it can be estimated that the expansion of research on the relationship between probiotics and oral yeast infections and the investigation of molecular means of probiotic activity may expand their potential applications in the future.

9Periodontal disease is inflammation of dental support tissues, which comprises gums, the bony socket, the outer layer of the roots of teeth, and the associated connective tissue. This disease begins with the formation of plaque. Symptoms of periodontal disease are bleeding on probing, color alterations, swelling, pain, and in advanced stages, dental mobility. Probiotics prevent plaque formation by reducing the pH of saliva and producing antioxidants that use free electrons used in plaque mineralization because, in this condition, carcinogenic bacteria are not able to form plaque. Therefore, in this way, probiotics prevent periodontal disease. Also, encouraging results of the consumption of probiotics in the cure of plaque level, gingivitis, periodontitis, and significant reduction of periodontopathogens have been reported in various studies. Today, according to new knowledge, three factors are known to cause plaque-related periodontitis. These three factors are a vulnerable host, the existence of a pathogen, and the low extent or absence of advantageous microbiota. An unbalance between the pathogenic and saprophytic flora of the oral cavity in a vulnerable individual may cause periodontal disease. Through scaling and root planing, and deep pocket debridement, which are considered the primary cure, it is possible to reduce invasive periopathogens significantly. Debridement treatment of periodontal disease is possible in two ways, surgical or non-surgical. Sometimes, systemic antimicrobial administration is also necessary. Although plaque removal using mechanical surface instrumentation is an effective treatment method, pocket recolonization is highly unpredictable. Considering that the administration of these drugs gradually creates bacterial resistance, new alternatives are needed for them. Since a probiotic can modify the oral microbial flora, it is probably a purposive strategy in the clinical control of periodontitis in addition to other beneficial advantages it provides to the host. On the one hand, probiotics can compete with periodontal pathogens and modulate dysbiosis conditions, therewith decreasing the overall immunogenicity of the oral microbiota, and also, they can modulate immune/inflammatory pathways to decrease the destructive inflammation of periodontitis and result in immune homeostasis that can be retained by the individual for a long time. It has been shown that the existence of probiotics with a suitable concentration of 108 CFU/mL in periodontal dressings reduces the number of periodontal pathogens, including Actinomyces sp., Bacteroides sp., S. intermedius, and C. albicans. The impact of probiotic Lactobacilli on inhibiting the growth of periodontopathogens in the oral cavity has also been shown. There is a straight relationship between periodontal inflammation and destruction and the reduction of LAB levels. It has been reported that bacteria such as L. fermentum and L. gasseri in the oral cavity of patients with chronic periodontitis had lower levels than in healthy people. The usage of L. reuteri has also shown a decrease in gum bleeding and a decrease in gum inflammation. In another study, L. reuteri was used as a supplement to conventional treatments for patients with periodontitis, and it caused a decrease in the number of P. gingivalis, P. intermedia, and Aggregatibacter actinomycetemcomitans. In addition to reducing the count of periodontal pathogens, the activity of probiotics has resulted in a decrease in the concentration of IL-17, IL-1β, and TNF-α in the gingival crevice fluid. It has been shown in mouse models that gut-based probiotics can have a protective effect on periodontitis through immune modulation. Therefore, this characteristic of probiotics can make them an alternative to antibiotics in periodontal cure to help decrease the overall burden of antibiotic resistance. Meta-analyses also support the consumption of probiotics in the management of periodontitis. However, before recommending the routine consumption of probiotics for the cure of periodontitis and gingivitis, various aspects and points should be considered. For example, the length of the treatment and how to do it so that the pathogenic microbiota does not prevail again after it ends, and also pay attention to the possible risks that the consumption of probiotics can cause in subjects with a weak immune system.

10Halitosis, or the nasty odor that comes out of the mouth, is a disease that depends on various factors, and its origin may be oral or non-oral. Halitosis is usually attributed to biofilm in the interdental spaces, the back of the tongue, and chronic inflammatory diseases. The incidence of this disease in different populations is estimated between 22 and 50 percent. Sulfuric gases such as dimethyl sulfide, hydrogen sulfide, and methyl mercaptan play an important role in causing bad breath. These gases are released in the oropharynx (tongue coating, tonsillitis, gingivitis, periodontitis) through bacterial degradation of sulfur-containing amino acids ( Figure 3 ). F. nucleatum, Treponema denticola, P. intermedia, and P. gingivalis can be mentioned among the diverse range of bacteria that contribute to this disease. On the other hand, the levels of bacterial species that form the dominant microbiota in the oral of healthy people are not significant in people with halitosis. Current treatments seek to eliminate these pathogenic bacteria using chemical or physical antibacterial agents. Antimicrobial treatment indiscriminately reduces the population of pathogenic bacteria and bacteria that are not involved in causing halitosis but are probably effective in maintaining a normal oral microenvironment. However, the result of this cure is to reduce the bad smell temporarily, and after some time, the bacteria causing halitosis appear again. Probiotics that are effective in maintaining periodontal health may also be useful for eliminating bad breath by helping to keep a healthy ecology of the tongue because in oral health, access to certain areas of the tongue, such as the dorsal posterior surface to the circumvallate papillae, which acts as a shelter for the large number of gram-negative bacterial species correlated with bad breath, is more difficult. Nevertheless, it is known that the tongue is a more palpable recess than the periodontal recesses in terms of species that normally colonize, suggesting the necessity of recess-specific conformity, and probiotic strains intended to colonize the periodontal recesses may not freely colonize the tongue to exert health-promoting impacts. For the first time, sought a probiotic to prevent or treat halitosis using a scientific approach. In children, after gargling the rinsing solution containing Weissella cibaria, a significant decrease in H2S (48.2%) and CH3SH (59.4%) levels, and as a result, halitosis has been observed. A possible candidate to compete with bacterial species that increase the level of volatile sulfur compounds is S. salivarius, which has shown an inhibitory effect on volatile sulfur compounds (VSC). Rinsing with chlorhexidine followed by lozenges containing S. salivarius K12 strain decreased the respiratory levels of VSCs in most subjects and maintained their levels for at least 2 weeks.. This strain produces two lantibiotic bacteriocins that inhibit gram-positive strains involved in halitosis. S. salivarius K12 seems to be able to reduce bad breath for a long time by replacing the bacteria involved in halitosis. In some other studies, the effect of this strain in eliminating bad breath has been reported. In a study, it was shown that L. salivarius WB21 has a positive effect on the organoleptic test score and bad breath. Chewing gum containing probiotics can also reduce the levels of non-sulfur odor-producing bacteria. In the study by, this was mentioned, and it was shown that the organoleptic scores were significantly decreased after the consumption of chewing gum containing L. reuteri compared to the placebo group. In contrast, there was no improvement in breath VSCs concentration or organoleptic scores after being treated with L. brevis CD2 lozenges. Also, no significant decrease in breath VSCs concentration or organoleptic scores was observed after consuming milk containing L. casei Shirota. Anti-VSC effects by non-oral bacteria such as S. thermophilus, E. faecium, and W. cibaria have also been reported in laboratory studies. It seems that the effect of probiotics, mostly used in commercial foods, has been less investigated in past studies. Therefore, it is recommended that future researches focus on such strains.

11During the past years, researchers have usually used different idioms such as cell-free supernatant, biogenic, abiotics, metabiotics, pseudoprobiotics, ghost probiotics, paraprobiotics, and postbiotics to introduce non-viable parts or metabolites of probiotic bacteria. In the meantime, postbiotic has been used the most. By definition, postbiotics are cell wall fragments, cytoplasmic extracts, or metabolites produced by gut-resident probiotics and fermented foods. However, because of the uncertain definition and broad bioactivity of metabolites produced by probiotics, the term postbiotic can be defined as follows: any soluble agent (products or metabolic by-products of microbial metabolisms or compounds produced by the action of LAB on culture or food ingredients) that is released by live probiotics or after their cell lysis during fermentations is secreted in food, microbiological cultures or gut. During the fermentation process, probiotics feed on prebiotics and produce a wide range of postbiotics. The functional mechanism of probiotics is related to the production of compounds like bacteriocins, organic acids, fatty acids, and hydrogen peroxide ( Figure 4 ). These compounds are currently produced by laboratory methods, and if they are consumed in sufficient amounts, they will have health effects. Also, due to their favorable antimicrobial effects, they are considered a promising alternative to antibiotics. Organic acids are acknowledged as one of the most important and impressive components of postbiotics. Lactic acid produced during bacterial fermentation effectively controls pathogenicity. Bacteriocins are peptides that have antimicrobial activity. These compounds resist heat and pH and can prevent the development of pathogens in the gastrointestinal. The mechanism of function of bacteriocins is in the cytoplasmic membrane of bacteria. Also, fatty acids, as one of the components of postbiotics, have shown good antimicrobial effects. Long-chain fatty acids like eicosapentaenoic acid are active against gram-positive bacteria. Fatty acids exhibit various antimicrobial mechanisms against bacteria, including increasing membrane permeability, lysing bacterial cells, disrupting the structure and activity of enzymes, disrupting the electron transport chain, and inducing morphological and functional changes in susceptible components like proteins. H2O2 (Hydrogen peroxide) is another antimicrobial agent whose antimicrobial effect is said to be due to its strong oxidizing action on the bacterial cell and the destruction it causes to the structure of proteins in its cytoplasm. Earlier, the advantageous effects of some probiotics in improving oral health and preventing oral diseases are mentioned. It is thought that the antimicrobial properties of probiotics are because of the production of antibacterial compounds, such as hydrogen peroxide and organic acids, and antifungal compounds, such as bacteriocins and fatty acids. Also, it has been reported that L. reuteri produces water-soluble antimicrobial compounds like reuterin and reutericyclin, which have antagonistic activity. These substances are resistant to lipolytic and proteolytic enzymes and maintain their function in a wide range of pH. The strain of S. dentisani isolated from the mouth of people without caries also can produce bacteriocin and can be considered another useful probiotic spp.. Also, in a three-month clinical study, the results showed that the S. salivarius M18 probiotic strain, which also produces bacteriocin, can reduce the development of tooth decay in children. It has been found that some bacterial spp., such as S. sanguinis and S. uberis, are not present in the subgingival plaque samples of people with refractory periodontitis and people with localized juvenile periodontitis. At the same time, they are present in the subgingival plaque samples of healthy individuals. It was shown that these strains prevent the growth of A. actinomycetemcomitans and other periodontal pathogens through the production of hydrogen peroxide.

12It has been stated that the decrease in pH level and tooth demineralization by bacteria like S. mutans, S. sobrinus, and Lactobacillus spp. is because of the organic acids produced in the biofilm as metabolic byproducts produced during fermentation. On the other hand, as mentioned, one of the mechanisms of probiotics to prevent the growth of pathogenic bacteria is the production of organic acids. Therefore, the question may arise whether the presence of these acids causes a further reduction in pH and an increase in tooth decay. In different studies, it was shown that salivary pH levels were significantly higher in the probiotic treatment group than in the control group. The results of this study are consistent with some other studies that have reported an increase in pH level after the use of yogurt and curd-containing probiotics (a). The increase in the pH level after the consumption of probiotics can be considered due to their competition with pathogenic bacteria and reducing their number. This reduction in the number of acid-producing pathogens causes the production of acid to decrease and, as a result, the pH level of saliva increases. Since tooth decay is affected by pH imbalance, these findings can be important. The issue of protection and security has been noticed in recent years due to the prevalence of probiotic food products. A probiotic strain to treat a bacterial disease must have some characteristics. For example, the probiotic bacteria should not cause disease or otherwise susceptible the individual to other disease states by disrupting the ecosystem in which it resides. They should also be examined and screened for general safety, such as antibiotic resistance genes and those related to the potential invasion. Considering these concerns about the consumption of live bacteria, we should consider a suitable and safe alternative that has the same biological health benefits as probiotics. The evidence achieved from in vitro and in vivo studies indicates that postbiotics are clinically safe and have beneficial and therapeutic effects if used in appropriate amounts and duration. With features such as a safe profile, long shelf life (up to 5 years), no toxic effects, standardization, and easier transportation, postbiotics can be safe and cost-effective alternatives for probiotics in the pharmaceutical and food industries.

13Probiotics and their therapeutic effects in the control, prevention, and treatment of some diseases have been the subject of attention for many years. Therefore, people’s desire to use probiotic food and pharmaceutical products is increasing day by day. Due to the problems that exist in the survival of probiotics in food until the time of consumption, as well as under adverse environmental and gastrointestinal conditions, the interest in using probiotic side products and secretions, especially postbiotics, has increased. Because there is no need for survival of probiotics, and also these compounds are kept unchanged for a long time and are easily included in different food formulations, therefore, postbiotics can be a suitable alternative to replace live probiotics. One of the new topics is the use of postbiotics in maintaining and improving the health of the oral cavity. Various studies have shown that the presence of probiotics in the oral cavity has significant effects in reducing abnormalities such as tooth decay, oral cavity fungus, infection, and swelling of the gums and palate. It seems that the use of postbiotics instead of probiotics has a similar effect and can be effective in the prevention and cure of many oral and dental infectious diseases caused by the presence of pathogens such as S. mutans, and the results of existing studies confirm this impact. However, there are still some challenges as to whether postbiotics are as durable as probiotics. Also, in order to prove these preventive and therapeutic effects, many animal studies and human clinical trials are needed, especially in the case of postbiotics of different types and species of probiotics, to be able to prove their effectiveness finally. Since probiotics are acidogenic like cariogenic bacteria such as S. mutans, future studies also could examine the issue of what differences exist between the amount and type of acids produced by probiotic and pathogenic strains. The results of these studies can provide clearer evidence and reasons for whether there is a difference in the cariogenic of these acids and the strains that produce them.

14EM and AHR conceived the idea. EM wrote the first draft of the manuscript. HP wrote sections of the manuscript. All authors contributed to the manuscript revision, read, and approved the submitted version.