Limosilactobacillus reuteri in Pediatric Oral Health: A Systematic Review

Abstract

Background: Limosilactobacillus reuteri (L. reuteri), present in the oral and intestinal microbiota, can colonize the oral cavity through breastfeeding and dairy intake, promoting oral health by balancing the microbiota, inhibiting pathogens, and modulating immune responses. This study aimed to evaluate the preventive role and therapeutic potential of L. reuteri in pediatric oral health. Methods: A literature review was conducted using PubMed, Wiley Library, and the Cochrane Library, supplemented by manual screening, according to PRISMA guidelines and covering the period from January 2011 to 31 December 2024. Results: From 835 records identified, 12 studies met the inclusion criteria. Data shows that L. reuteri strains produce antimicrobial substances that disrupt biofilms and inhibit Streptococcus mutans and other lactobacilli, leading to increased oral pH and improved periodontal indices. The effectiveness of probiotics was found to be strain-specific and transient, with continuous intake and adequate oral hygiene enhancing their ability to colonize the oral cavity. Conclusions: Probiotics show significant potential as therapeutic interventions for controlling cariogenic bacteria and supporting gum health in children. Through mechanisms including bacterial co-aggregation, competitive exclusion, antimicrobial compound synthesis, and immune modulation, probiotics may effectively reduce the risk of tooth decay and gum disease. Their effectiveness depends on the strain, regular intake, proper dosing, good oral hygiene, and suitable delivery, which enhance oral colonization and clinical benefits.

1. Introduction

The oral cavity hosts a diverse community of microorganisms, the oral microbiome, including bacteria, fungi, viruses, and small protists. However, some of these microorganisms can exhibit opportunistic behavior under certain conditions. In a healthy individual, the oral cavity microbiome maintains an equilibrium within the host, denominated, eubiosis [1,2]. When an imbalance within the resident flora occurs, for example, in patients using high-spectrum antibiotics or immunosuppressed patients, such as those with HIV or cancer, the commensalism–parasitism relationship of pathogenic microbes changes, resulting in dysbiosis [3,4,5]. The increase in opportunistic microorganisms and their intrusion into deeper tissues can lead to various oral diseases, including dental caries, gingivitis, periodontitis, and oral candidiasis [1,6,7,8]. Immunosuppression can result in the overgrowth of opportunistic pathogens, leading to deterioration of both the hard tissues (teeth and bone) and soft tissues (gums and oral mucosa) within the oral cavity [9,10].

The etiology of dental caries is multifactorial, with time, inadequate oral hygiene, and a sugary diet all contributing to disease development. The primary organisms involved are Streptococcus mutans (S. mutans) as the initial colonizer and Lactobacilli as secondary invaders, progressing through adhesion, co-aggregation, and acid production [11,12,13].

According to the World Health Organization, probiotics are live microorganisms which, when consumed in adequate amounts, confer health benefits to the host [14].

The mechanisms behind probiotic action involve more than simply regulating the intestinal bacterial flora or modulating the immune response. Probiotics such as Limosilactobacillusreuteri (L. reuteri) can interact with epithelial cells, influence cytokine production, and modulate both innate and adaptive immune pathways. Additionally, they contribute to maintaining mucosal barrier integrity and preventing colonization by pathogenic microorganisms through competitive inhibition. These actions, combined with the secretion of antimicrobial substances (e.g., reuterin and bacteriocins), highlight the multifactorial ways in which probiotics promote oral and systemic health [15,16,17].

Given that probiotics can effectively modulate local microbial communities, the oral cavity represents an ideal environment for probiotic therapeutic interventions. The primary mechanisms of probiotic action include co-aggregation, competitive exclusion, and production of antimicrobial substances such as bacteriocins (Figure 1). By transforming a pathogenic environment into a healthy microbial ecosystem, probiotics have demonstrated the capacity to both prevent and treat oral diseases [6]. Rather than indiscriminately eliminating microorganisms, probiotics selectively replace pathogenic species while preserving beneficial flora. This targeted approach allows probiotics to restore oral homeostasis more effectively than conventional antimicrobial treatments [12,18,19,20].

Lactobacilli and Bifidobacteria establish colonization in the gastrointestinal tract during early neonatal development and represent the most extensively utilized probiotic strains, commonly found in everyday fermented food products [1,9,19,21]. These beneficial microorganisms have demonstrated significant efficacy in reducing populations of S. mutans, Aggregatibacter actinomycetemcomitans, and Porphyromonas gingivalis, thereby preventing dental caries and periodontal disease progression. For successful therapeutic outcomes, these probiotics must exhibit strong adherence capabilities and environmental resilience within the harsh oral cavity conditions [6,8,22].

Their protective mechanisms encompass nutrient competition, binding site occupation, antimicrobial compound synthesis, and immune system modulation, collectively establishing an inhospitable microenvironment that inhibits pathogenic bacterial colonization [13,16].

The Lactobacilli genera encompass a diverse and heterogeneous group of bacteria, including species such as Lactobacillus acidophilus (L. acidophilus), Lacticaseibacillus rhamnosus (L. rhamnosus), L. reuteri and Lacticaseibacillus casei (L. casei) [3,23]. The primary function of these probiotics involves regulating host microbiota composition, preventing or reversing dysbiosis, and effectively managing various pathological conditions, particularly oral diseases [6,7,9,16,18,21].

L. reuteri, a probiotic bacterium naturally inhabiting the oral cavity, has been extensively investigated for its multifaceted health benefits, especially in the prevention and management of diverse diseases and gastrointestinal disorders [21,23,24]. L. reuteri strains demonstrate potent antioxidant and antifungal properties, effectively combating bacterial infections through microbiota regulation, antimicrobial metabolite secretion, and Candida spp. growth inhibition [6,7,8,13,23]. Several L. reuteri strains demonstrate remarkable tolerance to low pH environments and bile salt exposure, attributed in part to their biofilm-forming capacity, which enables them to prevent pathogenic colonization and restore microbial equilibrium. These strains interact with the host through immune system stimulation and production of antimicrobial compounds, particularly reuterin, which modulates microbial community structure and inhibits pathogenic bacterial establishment. Notably, L. reuteri strains exhibit superior resistance to reuterin toxicity compared to other bacterial species, suggesting that reuterin production functions as an adaptive survival mechanism to enhance their competitive persistence in complex microbial environments [21,23,24]. Multiple L. reuteri strains have demonstrated efficacy in inhibiting oral pathogens, highlighting their promising therapeutic potential in oral disease prevention [3,13,25].

Dental caries remains one of the most common chronic diseases in children worldwide, affecting both primary and permanent teeth, with a substantial global burden reported over the last three decades. Early childhood caries can have long-term consequences for oral health, growth, and quality of life [26,27]. Conventional plaque control and caries prevention strategies, such as fluoride application and mechanical removal, are effective but may be limited in young children or individuals with restricted access to dental care [28,29,30].

Children’s oral microbiota is in active development and is particularly susceptible to colonization by cariogenic bacteria such as S. mutans and Lactobacilli. Early modulation of the oral microbiome offers a critical window for preventing dental diseases. Probiotics, including L. reuteri, have the potential to directly target microbial causes of caries, affecting oral flora balance, inhibiting pathogenic bacterial growth, and preventing biofilm formation, thereby potentially overcoming the limitations of conventional preventive strategies [31,32,33]. While probiotics have been widely studied in adults and in gastrointestinal health, evidence specifically targeting pediatric populations remains limited and sometimes inconsistent.

This systematic review aims to evaluate the preventive and therapeutic potential of L. reuteri specifically for pediatric oral health, highlighting its benefits in the prevention and management of oral diseases as an alternative therapeutic approach.

2. Materials and Methods

2.1. Review Guidelines

This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Review and Meta-analysis (PRISMA) 2020 guidelines [34]. The study protocol was registered in the PROSPERO database (CRD420251123776) prior to the literature screening.

2.2. Selection Criteria

To be included in this study, articles had to meet the following conditions:

Inclusion criteria:

-

Articles published from January 2011 to 31 December 2024;

-

Studies involving patients under 18 years of age;

-

Studies evaluating the effect of L. reuteri in the oral cavity;

-

Randomized controlled trials (RCTs).

Exclusion criteria:

-

Articles whose abstracts do not address the research topic;

-

Studies on adult patients;

-

Studies of the effects of L. reuteri that are not in the oral cavity;

-

Systematic review, or a review, or a meta-analysis or books and documents.

2.3. Eligibility Criteria

The PICOS (Population, Intervention, Comparison, Outcomes, and Study design) strategy was employed to formulate the research question: “Does the use of probiotic bacteria, such as L. reuteri, play a preventive and therapeutic role in the oral cavity of pediatric patients?” (

Table 1. PICOS Strategy.

P	Children and adolescents under 18 years old
I	Oral administration of L. reuteri in any formulation (drops, lozenges, tablets, chewing gum, or curds)
C	Placebo or no-probiotic control group
O	Reduction in S. mutans, improvement in plaque and gingival indices, caries prevention/reduction, changes in salivary pH or buffering, oral microbial diversity, and gingival inflammation/bleeding.
S	Randomized controlled trials (RCTs)

).

2.4. Search Strategy

A bibliographic research was conducted in the PubMed, Wiley Library, and Cochrane Library databases, covering publications from January 2011 to 31 December 2024. Keyword combinations were applied to identify relevant scientific papers according to the objectives of the study. Specific keywords and MeSH terms utilized in this search are summarized in

Table 2. Databases and research strategy.

Databases	Advanced Research	Articles
PubMed	(((children[MeSH Terms]) AND (oral health[MeSH Terms]) OR (“saliva”[MeSH Terms]) AND (lactobacillus reuteri[MeSH Terms])) AND (probiotics[MeSH Terms])) OR ((lactobacillus reuteri[MeSH Terms]) AND (“child”[MeSH Terms]) AND (probiotics[MeSH Terms]))	77
Wiley Library	children AND oral health AND lactobacillus reuteri AND probiotics	714
Cochrane Library	children AND oral health AND lactobacillus reuteri AND probiotics	17

.

2.5. Selection of Articles and Data Collection

To minimize selection and extraction bias, all records were independently screened by two reviewers (R.G. and J.P.C.), and data extraction was performed in duplicate. Discrepancies between reviewers were resolved through discussion, with a third reviewer (A.R.) making the final decision when consensus could not be reached, ensuring inter-rater reliability throughout the process. The literature search was carried out both electronically and manually. The retrieved records from all databases were exported to Mendeley Reference Manager^®, where duplicates were automatically identified and removed.

2.6. Quality Assessment and Risk of Bias

The methodological quality and risk of bias of the included studies were assessed using the Joanna Briggs Institute (JBI) Critical Appraisal Checklist for Randomized Controlled Trials (2017 version) [35]. Each item in Table 3 was rated as Yes (Y), No (N), Unclear (U), or Not Applicable (NA). The risk of bias assessment was independently performed by two reviewers (J.P.C. and R.G.), and any disagreements were resolved through discussion and consensus with a third reviewer (A.R.). Seven studies were assessed as having a low risk of bias, while five studies showed a moderate risk.

3. Results

3.1. Selection of Articles

Searches of the PubMed, Wiley Library, and Cochrane Library databases identified a total of 808 articles, of which 44 were duplicates, resulting in 764 unique records. After screening titles and abstracts, 97 articles were excluded for not meeting the inclusion criteria. The remaining 27 studies were assessed in full text, of which 20 were excluded for not providing relevant information. Consequently, seven articles were selected and included. Subsequently, 5 additional studies were identified through manual searches, resulting in a total of 12 articles included in this systematic review, as shown in Figure 2.

3.2. Sample Characteristics for Study Quality

In the included randomized controlled trials, seven studies, Gizani et al. [36], 2016, Hasslöf et al. [37], 2022, Keller et al. [38], 2014, Bolla et al. [39], 2022, Cildir et al. [40], 2012, Ebrahim et al. [41], 2022, and García et al. [42], 2021, were considered to have a low risk of bias. Five studies, Stensson et al. [43], 2014, Alamoudi et al. [25], 2018, Kaur et al. [44], 2018, Tehrani et al. [45], 2016, and Alforaidi et al. [46], 2021, presented a moderate risk. Overall, the evidence included in this review is considered to have a low to moderate risk of bias. The quality assessments of the studies are presented in Table 3.

3.3. Characteristics of the Included Studies

For each eligible study included in this systematic review, data was collected on general characteristics such as author, year of publication, study design, study objective, population, duration of supplementation, bacterial strain, and probiotic vehicle. Outcomes, including caries prevalence, gingival health, bacterial counts, and salivary markers, were also recorded and analysed, as shown in Table 4.

Table 3. Joanna Briggs Institute Critical Appraisal Checklist for randomized controlled clinical trials.

	Was the Randomization Method Adequate?	Was the Allocation Method Adequate?	Were the Groups Similar at the Start of the Study?	Were the Participants Blinded?	Were the Professionals Who Administered the Interventions Blinded?	Were the Outcome Assessors Blinded?	Were the Interventions Clearly Described and Applied Equally in Both Groups?	Was the Primary Outcome Clearly Defined and Measured?	Was There an Intention-to-Treat Analysis?	Were Losses and Exclusions Described?	Were Complications or Adverse Events Reported?	Were the Study Results Accurate and Reliable?	Were the Study Results Relevant to Clinical Practice?
Stensson et al., 2014 [43]	Y	U	Y	N	U	Y	Y	Y	U	Y	N	Y	Y
Gizani et al., 2016 [36]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
Hasslöf et al., 2022 [37]	Y	U	Y	Y	Y	Y	Y	Y	U	Y	N	Y	Y
Keller et al., 2014 [38]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
Alamoudi et al., 2018 [25]	Y	U	Y	U	U	U	Y	Y	U	U	N	Y	Y
Kaur et al., 2018 [44]	Y	U	Y	U	U	U	Y	Y	U	U	N	Y	Y
Bolla et al., 2022 [39]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
Cildir et al., 2012 [40]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
Tehrani et al., 2016 [45]	Y	U	Y	U	U	U	Y	Y	U	U	N	Y	Y
Ebrahim et al., 2022 [41]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
García et al., 2021 [42]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
Alforaidi et al., 2021 [46]	Y	U	Y	U	U	U	Y	Y	U	U	N	Y	Y

(Y)—Yes, (N)—No, (U)—Unclear.

Table 3. Joanna Briggs Institute Critical Appraisal Checklist for randomized controlled clinical trials.

	Was the Randomization Method Adequate?	Was the Allocation Method Adequate?	Were the Groups Similar at the Start of the Study?	Were the Participants Blinded?	Were the Professionals Who Administered the Interventions Blinded?	Were the Outcome Assessors Blinded?	Were the Interventions Clearly Described and Applied Equally in Both Groups?	Was the Primary Outcome Clearly Defined and Measured?	Was There an Intention-to-Treat Analysis?	Were Losses and Exclusions Described?	Were Complications or Adverse Events Reported?	Were the Study Results Accurate and Reliable?	Were the Study Results Relevant to Clinical Practice?
Stensson et al., 2014 [43]	Y	U	Y	N	U	Y	Y	Y	U	Y	N	Y	Y
Gizani et al., 2016 [36]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
Hasslöf et al., 2022 [37]	Y	U	Y	Y	Y	Y	Y	Y	U	Y	N	Y	Y
Keller et al., 2014 [38]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
Alamoudi et al., 2018 [25]	Y	U	Y	U	U	U	Y	Y	U	U	N	Y	Y
Kaur et al., 2018 [44]	Y	U	Y	U	U	U	Y	Y	U	U	N	Y	Y
Bolla et al., 2022 [39]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
Cildir et al., 2012 [40]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
Tehrani et al., 2016 [45]	Y	U	Y	U	U	U	Y	Y	U	U	N	Y	Y
Ebrahim et al., 2022 [41]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
García et al., 2021 [42]	Y	U	Y	Y	Y	Y	Y	Y	U	U	N	Y	Y
Alforaidi et al., 2021 [46]	Y	U	Y	U	U	U	Y	Y	U	U	N	Y	Y

(Y)—Yes, (N)—No, (U)—Unclear.

Table 4. Data and outcomes from articles.

Authors and Year of Publication	Study Design	Objectives	Population	Duration of Supplementation	Bacterial Strain	Probiotic Vehicle	Outcomes
Stensson et al., 2014 [43]	Single-blind, placebo-controlled, multicenter trial	Assess the influence on oral health at the age of 9 with daily oral supplementation of L. reuteri ATCC 55730 given to mothers during the last month of pregnancy and to children throughout their first year of life.	113 children	For mothers: 4 weeks before birth For children: during their first year of life	L. reuteri ATCC 55730	Drops	- Reduced caries prevalence: 82% of children in the probiotic group were caries-free, compared to 58% in the placebo group. - Lower prevalence of interproximal caries lesions in the probiotic group. - Fewer sites exhibited gingivitis compared to the placebo group. - No differences between groups in bacterial counts (L. reuteri, S. mutans and lactobacilli) - No differences in salivary SIgA levels in saliva, in both groups.
Gizani et al., 2016 [36]	Double-blind RCT	Evaluate the effect of daily probiotic lozenges administration, on white spot lesion formation and salivary counts of lactobacilli and S. mutans in patients with fixed orthodontic appliances.	85 patients	17 months	L. reuteri (DSM 17938 and ATCC PTA 5289)	Lozenges	- No differences in white spot lesion incidence were observed between the groups at debonding. - Salivary lactobacilli levels significantly decreased in both groups at appliance removal compared to baseline, while S. mutans counts remained unchanged.
Hasslöf et al., 2022 [37]	Double-blind RCT	Determine the effect of probiotic-containing drops on the recurrence of dental caries in preschool children.	38 children	12 months	L. reuteri (DSM 17938 and ATCC PTA 5289)	Drops	- After 12 months, 67% of the sample showed recurrent moderate/extensive lesions, with no differences between groups. - No beneficial effects on dental plaque or gingival inflammation were observed. - Probiotic supplementation with L. reuteri did not significantly reduce the recurrence of early childhood caries compared to the placebo group.
Keller et al., 2014 [38]	Double-blind RCT	Investigate the impact of probiotic lactobacilli tablets on early caries lesions in adolescents using quantitative light-induced fluorescence.	36 adolescents	3 months	L. reuteri (DSM 17938 and ATCC PTA 5289)	Tablets	- No significant differences in fluorescence values and lesion areas were observed between the two groups at baseline or follow-up. - The test group showed a significant decrease in fluorescence over time, meaning less mineral loss in enamel.
Alamoudi et al., 2018 [25]	RCT	Investigate the role of L. reuteri probiotic lozenges on caries-associated salivary bacterial counts (S. mutans and Lactobacillus), dental plaque accumulation, and salivary buffer capacity in preschool children.	178 children	28 days	L. reuteri (DSM 17938 and ATCC PTA 5289)	Lozenges	- The experimental group showed a statistically significant reduction in S. mutans and lactobacilli. - Both groups had less plaque accumulation than at baseline. - The experimental group had a greater increase in buffer capacity than the control group, but this was not statistically significant.
Kaur et al., 2018 [44]	RCT	Assess the impact of probiotic and xylitol-containing chewing gums on salivary S. mutans counts, plaque and gingival scores, following the intervention.	40 children	3 weeks	L. reuteri (ATCC 55730 and ATCC PTA 5282)	Chewing gums	- Decrease of salivary S. mutans counts in both groups, with a significant reduction in plaque and gingival scores observed after the intervention. - Both probiotic and xylitol-containing chewing gums were equally effective in reducing S. mutans counts in children.
Bolla et al., 2022 [39]	Double-blind RCT	Evaluate the effects of L. reuteri, B. bifidum, and their combination on salivary S. mutans counts in children, and the sustainability of their action.	60 subjects	14 days	Bifidobacterium bifidum (UBBB 55, MTCC 5398) and L. reuteri (UBLRu 87, MTCC 5403)	Curds	- Significant reduction in S. mutans was observed in the L. reuteri group, lasting at least 21 days since the intervention. - No significant changes in S. mutans levels were detected in the group consuming curd with B. bifidum since the intervention to day 14 or day 21. - Significant increase in S. mutans levels was noted in the group receiving curd with a combination of B. bifidum and L. reuteri since the intervention to day 14.
Cildir et al., 2012 [40]	Double-blind, randomized crossover design	Investigate the effect of the probiotic L. reuteri on salivary S. mutans and Lactobacillus levels in children with cleft lip/palate using a novel drop containing L. reuteri.	19 operated cleft lip/palate children	25 days	L. reuteri (DSM 17938 and ATCC PTA 5289)	Drops	- No statistically significant reduction in salivary S. mutans and Lactobacillus levels was observed after 25 days in both control and test groups.
Tehrani et al., 2016 [45]	RCT	Evaluate the effect of a probiotic drop containing L. rhamnosus, Bifidobacterium infantis, and L. reuteri on salivary counts of S. mutans and Lactobacillus in children of 3 to 6 years old.	61 children	2 weeks	L. rhamnosus ATCC 15820, Bifidobacterium infantis ATCC 15697 and L. reuteri ATCC 55730	Drops	- S. mutans level decreased significantly in the probiotic group after intervention, and there were significant differences in salivary S. mutans counts after intervention between the two groups. - In the probiotic group, Lactobacillus counts decreased significantly after the intervention; however, there were no significant differences between the two groups. - The probiotic drop lowered S. mutans counts, especially in children with higher salivary counts, while Lactobacillus counts remained unchanged.
Ebrahim et al., 2022 [41]	Double-blind RCT	Determine the effectiveness of the commercially available Lorodent Probiotic Complex in reducing plaque accumulation and S. mutans levels in adolescent orthodontic patients.	60 adolescents	28 days	Streptococcus. salivarius K12, Lacticaseibacillus paracasei, Lactiplantibacillus plantarum, L. acidophilus, Ligilactobacillus salivarius and L. reuteri	Lozenges	- No significant changes were observed in the Plaque Index (PI), composite PI scores, or relative S. mutans DNA levels in saliva and plaque between the probiotic and placebo groups at any time point.
García et al., 2021 [42]	Double-blind RCT	Examine the impacts of a probiotic on oral health indices in adolescents and analyze the relationships between these indices, dietary habits, and oral hygiene.	27 adolescents	28 days	L. reuteri (DSM 17938 and ATCC 5289)	Tablets	- Improvements were noted in dental plaque, gingivitis, and bleeding in both groups although they did not reach statistical significance. - Increase of oral pH in the study group, although the variation was not statistically significant.
Alforaidi et al., 2021 [46]	RCT	Analyze the impact of probiotics on biofilm acidogenicity and the levels of salivary S. mutans and lactobacilli in orthodontic patients.	28 subjects	3 weeks	L. reuteri (DSM 17938 and ATCC PTA 5289)	Drops	- Plaque pH significantly increased in the test group after three weeks, with no significant changes in pH in the placebo group compared to baseline. - The three-week stimulated whole saliva samples showed no statistically significant difference in the number of S. mutans and lactobacilli between the two groups. - qPCR analysis demonstrated that both strains were able to colonize the dental biofilm without significantly affecting the microbial counts. - A mixture of L. reuteri reduced the pH decline at the three-week follow-up.

B. bifidum: Bifidobacterium bifidum; L. acidophilus: Lactobacillus acidophilus; L. reuteri: Limosilactobacillus reuteri; L. rhamnosus: Lactobacillus rhamnosus; PI: Plaque Index; pH: potential of hydrogen; qPCR: quantitative Polymerase Chain Reaction; RCT: Randomized Controlled Trial; SIgA: secretory IgA; S. mutans: Streptococcus mutans.

Table 4. Data and outcomes from articles.

Authors and Year of Publication	Study Design	Objectives	Population	Duration of Supplementation	Bacterial Strain	Probiotic Vehicle	Outcomes
Stensson et al., 2014 [43]	Single-blind, placebo-controlled, multicenter trial	Assess the influence on oral health at the age of 9 with daily oral supplementation of L. reuteri ATCC 55730 given to mothers during the last month of pregnancy and to children throughout their first year of life.	113 children	For mothers: 4 weeks before birth For children: during their first year of life	L. reuteri ATCC 55730	Drops	- Reduced caries prevalence: 82% of children in the probiotic group were caries-free, compared to 58% in the placebo group. - Lower prevalence of interproximal caries lesions in the probiotic group. - Fewer sites exhibited gingivitis compared to the placebo group. - No differences between groups in bacterial counts (L. reuteri, S. mutans and lactobacilli) - No differences in salivary SIgA levels in saliva, in both groups.
Gizani et al., 2016 [36]	Double-blind RCT	Evaluate the effect of daily probiotic lozenges administration, on white spot lesion formation and salivary counts of lactobacilli and S. mutans in patients with fixed orthodontic appliances.	85 patients	17 months	L. reuteri (DSM 17938 and ATCC PTA 5289)	Lozenges	- No differences in white spot lesion incidence were observed between the groups at debonding. - Salivary lactobacilli levels significantly decreased in both groups at appliance removal compared to baseline, while S. mutans counts remained unchanged.
Hasslöf et al., 2022 [37]	Double-blind RCT	Determine the effect of probiotic-containing drops on the recurrence of dental caries in preschool children.	38 children	12 months	L. reuteri (DSM 17938 and ATCC PTA 5289)	Drops	- After 12 months, 67% of the sample showed recurrent moderate/extensive lesions, with no differences between groups. - No beneficial effects on dental plaque or gingival inflammation were observed. - Probiotic supplementation with L. reuteri did not significantly reduce the recurrence of early childhood caries compared to the placebo group.
Keller et al., 2014 [38]	Double-blind RCT	Investigate the impact of probiotic lactobacilli tablets on early caries lesions in adolescents using quantitative light-induced fluorescence.	36 adolescents	3 months	L. reuteri (DSM 17938 and ATCC PTA 5289)	Tablets	- No significant differences in fluorescence values and lesion areas were observed between the two groups at baseline or follow-up. - The test group showed a significant decrease in fluorescence over time, meaning less mineral loss in enamel.
Alamoudi et al., 2018 [25]	RCT	Investigate the role of L. reuteri probiotic lozenges on caries-associated salivary bacterial counts (S. mutans and Lactobacillus), dental plaque accumulation, and salivary buffer capacity in preschool children.	178 children	28 days	L. reuteri (DSM 17938 and ATCC PTA 5289)	Lozenges	- The experimental group showed a statistically significant reduction in S. mutans and lactobacilli. - Both groups had less plaque accumulation than at baseline. - The experimental group had a greater increase in buffer capacity than the control group, but this was not statistically significant.
Kaur et al., 2018 [44]	RCT	Assess the impact of probiotic and xylitol-containing chewing gums on salivary S. mutans counts, plaque and gingival scores, following the intervention.	40 children	3 weeks	L. reuteri (ATCC 55730 and ATCC PTA 5282)	Chewing gums	- Decrease of salivary S. mutans counts in both groups, with a significant reduction in plaque and gingival scores observed after the intervention. - Both probiotic and xylitol-containing chewing gums were equally effective in reducing S. mutans counts in children.
Bolla et al., 2022 [39]	Double-blind RCT	Evaluate the effects of L. reuteri, B. bifidum, and their combination on salivary S. mutans counts in children, and the sustainability of their action.	60 subjects	14 days	Bifidobacterium bifidum (UBBB 55, MTCC 5398) and L. reuteri (UBLRu 87, MTCC 5403)	Curds	- Significant reduction in S. mutans was observed in the L. reuteri group, lasting at least 21 days since the intervention. - No significant changes in S. mutans levels were detected in the group consuming curd with B. bifidum since the intervention to day 14 or day 21. - Significant increase in S. mutans levels was noted in the group receiving curd with a combination of B. bifidum and L. reuteri since the intervention to day 14.
Cildir et al., 2012 [40]	Double-blind, randomized crossover design	Investigate the effect of the probiotic L. reuteri on salivary S. mutans and Lactobacillus levels in children with cleft lip/palate using a novel drop containing L. reuteri.	19 operated cleft lip/palate children	25 days	L. reuteri (DSM 17938 and ATCC PTA 5289)	Drops	- No statistically significant reduction in salivary S. mutans and Lactobacillus levels was observed after 25 days in both control and test groups.
Tehrani et al., 2016 [45]	RCT	Evaluate the effect of a probiotic drop containing L. rhamnosus, Bifidobacterium infantis, and L. reuteri on salivary counts of S. mutans and Lactobacillus in children of 3 to 6 years old.	61 children	2 weeks	L. rhamnosus ATCC 15820, Bifidobacterium infantis ATCC 15697 and L. reuteri ATCC 55730	Drops	- S. mutans level decreased significantly in the probiotic group after intervention, and there were significant differences in salivary S. mutans counts after intervention between the two groups. - In the probiotic group, Lactobacillus counts decreased significantly after the intervention; however, there were no significant differences between the two groups. - The probiotic drop lowered S. mutans counts, especially in children with higher salivary counts, while Lactobacillus counts remained unchanged.
Ebrahim et al., 2022 [41]	Double-blind RCT	Determine the effectiveness of the commercially available Lorodent Probiotic Complex in reducing plaque accumulation and S. mutans levels in adolescent orthodontic patients.	60 adolescents	28 days	Streptococcus. salivarius K12, Lacticaseibacillus paracasei, Lactiplantibacillus plantarum, L. acidophilus, Ligilactobacillus salivarius and L. reuteri	Lozenges	- No significant changes were observed in the Plaque Index (PI), composite PI scores, or relative S. mutans DNA levels in saliva and plaque between the probiotic and placebo groups at any time point.
García et al., 2021 [42]	Double-blind RCT	Examine the impacts of a probiotic on oral health indices in adolescents and analyze the relationships between these indices, dietary habits, and oral hygiene.	27 adolescents	28 days	L. reuteri (DSM 17938 and ATCC 5289)	Tablets	- Improvements were noted in dental plaque, gingivitis, and bleeding in both groups although they did not reach statistical significance. - Increase of oral pH in the study group, although the variation was not statistically significant.
Alforaidi et al., 2021 [46]	RCT	Analyze the impact of probiotics on biofilm acidogenicity and the levels of salivary S. mutans and lactobacilli in orthodontic patients.	28 subjects	3 weeks	L. reuteri (DSM 17938 and ATCC PTA 5289)	Drops	- Plaque pH significantly increased in the test group after three weeks, with no significant changes in pH in the placebo group compared to baseline. - The three-week stimulated whole saliva samples showed no statistically significant difference in the number of S. mutans and lactobacilli between the two groups. - qPCR analysis demonstrated that both strains were able to colonize the dental biofilm without significantly affecting the microbial counts. - A mixture of L. reuteri reduced the pH decline at the three-week follow-up.

B. bifidum: Bifidobacterium bifidum; L. acidophilus: Lactobacillus acidophilus; L. reuteri: Limosilactobacillus reuteri; L. rhamnosus: Lactobacillus rhamnosus; PI: Plaque Index; pH: potential of hydrogen; qPCR: quantitative Polymerase Chain Reaction; RCT: Randomized Controlled Trial; SIgA: secretory IgA; S. mutans: Streptococcus mutans.

4. Discussion

4.1. Limosilactobacillus Reuteri as an Alternative Approach Against Pediatric Dental Caries

4.1.1. Mechanisms of Action of L. Reuteri in Caries Prevention

The oral cavity hosts several microbial species, with hard dental tissues providing ideal spaces for colonization due to their gaps and fissures. These microorganisms colonize the oral cavity through initial adhesion, followed by biofilm maturation and acid production. Bacterial biofilms significantly contribute to caries lesion development over time [44,47,48].

Dental caries arises from tooth demineralization caused by sustained acidity in the oral environment and is influenced by multiple factors, including interactions between cariogenic bacteria, host immune responses, carbohydrate-rich diets, and duration of exposure. Acidogenic bacteria, such as S. mutans and lactobacilli, play a central role in caries formation within dental plaque by metabolizing dietary carbohydrates into acids. This acid production lowers oral pH, resulting in mineral loss through a dynamic demineralization–remineralization cycle [38,41,42,44,46]. Early detection and minimally invasive management of caries lesions are therefore essential components of modern oral healthcare [38].

Over the years, various antimicrobial strategies have been explored, with growing attention on probiotics as a natural alternative for maintaining oral health [38,44]. Probiotics are live microorganisms that confer health benefits to the host when administered in adequate amounts [37]. They act by introducing beneficial bacterial strains into the oral microbiota, inhibiting pathogenic colonization and growth, and supporting overall microbial homeostasis [36].

Among probiotics, L. reuteri is a Gram-positive, heterofermentative lactic acid bacterium naturally present in the gastrointestinal tract and oral cavity of humans and other mammals. Uniquely, it can colonize the oral mucosa and produce antimicrobial compounds, such as reuterin and bacteriocins, which inhibit cariogenic and periodontopathogenic bacteria. Its probiotic potential has been widely studied due to its dual ability to modulate the host immune system and maintain microbial balance within the oral biofilm [49,50,51]. L. reuteri Daily consumption of L. reuteri may help restore or sustain oral biofilm stability, thereby promoting both gastrointestinal and oral health [46,52].

Previous research has demonstrated that probiotics benefit oral health by pathogen colonization through multiple mechanisms: co-aggregation, antimicrobial compound production, competitive inhibition of harmful microbes for adhesion sites and nutrients, and immune modulation at both local and systemic levels [38,39,45].

From a safety perspective, L. reuteri is generally recognized as safe (GRAS status) and has demonstrated high tolerability in pediatric populations across multiple clinical trials. However, as with other probiotics, caution is recommended in severely immunocompromised individuals or those with central venous catheters, as rare cases of bacteremia have been described [53,54].

4.1.2. Early Life Interventions and Influence of Breastfeeding

Early microbial colonization of the oral cavity is essential, as the first 1000 days of life present a huge opportunity to shape the microbiota through pre- and probiotic interventions for healthy growth and development. In fact, several studies suggest that probiotic supplementation in infants, children and adolescents significantly reduces caries prevalence [37,38,43,44]. This reduction can be achieved by L. reuteri supplementation, which, through its diverse mechanisms, uses multiple mechanisms to decrease levels of S. mutans and lactobacilli, the primary bacterial species involved in dental caries development [36,39,43].

Since L. reuteri is naturally present in breast milk, breastfeeding may confer additional benefits by promoting early colonization of the oral cavity and potentially reducing S. mutans levels [37,43]. Stensson et al. [43] reported that 60 children who received L. reuteri ATCC 55730 supplementation from birth through their first year exhibited a reduced prevalence of dental caries, particularly on interproximal surfaces of primary teeth at age 9.

In another study, primary dentition was evaluated using clinical and radiographic examinations for interproximal caries, along with assessments of dental plaque, gingival health, and salivary concentrations of S. mutans, lactobacilli, and secretory IgA (SIgA). No significant differences were observed between groups in bacterial counts or salivary SIgA levels. However, a reduced prevalence of caries and interproximal lesions was reported based on clinical and radiographic findings. These discrepancies may reflect the transient nature of probiotic colonization, as probiotics administered nearly eight years earlier may not be detectable in saliva at later sampling [43].

Interestingly, the observed reduction in caries prevalence despite unchanged bacterial counts could be related to elevated salivary IgA levels in the probiotic group, which, although not statistically significant, may have contributed to inhibition of S. mutans activity [43].

4.1.3. Effectiveness of Different Probiotic Delivery Vehicles in Children

L. reuteri is administered in various formulations, including drops, lozenges, tablets, chewing gums, yogurts, and fermented dairy products. These delivery methods influence its stability, contact time, and efficacy within the oral cavity. Among drop-based interventions, Stensson et al. [43] and Hasslöf et al. [37] evaluated preschool children using supplements containing two L. reuteri strains: DSM 17938 (from human breast milk) and ATCC PTA 5289 (from the oral cavity). Despite prior evidence of these strains promoting remineralization and reducing pathogenic bacteria, the studies observed a high rate (67%) of recurrent moderate to extensive early childhood caries (ECC) after 12 months, indicating no significant reduction in ECC recurrence compared to placebo. Although this combination has been reported to induce remineralization and reduce pathogenic bacteria in school children, the lack of efficacy may be influenced by factors such as administration duration, strain selection, or individual variations in oral microbiota. Nevertheless, probiotics may still serve as co-adjuvant therapies when combined with effective oral hygiene practices [37].

Other studies in younger and adolescent populations have investigated tablets, lozenges, and curd. Keller et al. [38] reported a trend toward reduced demineralization in adolescents consuming L. reuteri tablets (DSM 17938 and ATCC PTA 5289) for three months, although differences were not statistically significant when compared with placebo. In younger populations, Bolla et al. [39] observed that curd containing L. reuteri UBLRu87 and Bifidobacterium bifidum UBBB55 unexpectedly increased S. mutans counts, possibly due to interspecies competition, whereas curd with L. reuteri alone reduced S. mutans for up to 21 days, demonstrating a transient but sustained effect requiring daily consumption.

Drop-based formulations containing L. rhamnosus ATCC 15820, L. reuteri ATCC 55730, and B. infantis ATCC 15697 significantly reduced salivary S. mutans in children aged 3–6 years, though Lactobacillus counts were variable. Similarly, Cildir et al. [40] evaluated drops with L. reuteri DSM 17938 and ATCC PTA 5289 in children aged 4–12 years with cleft lip/palate. After 25 days, no significant reductions in salivary S. mutans or Lactobacillus were observed, suggesting that efficacy may depend on population characteristics and duration of administration.

In contrast, solid formulations have generally shown more consistent benefits. Alamoudi et al. [25] demonstrated that L. reuteri lozenges significantly reduced S. mutans and Lactobacillus in preschoolers, likely due to reuterin production, with high acceptability and safety. Kaur et al. [44] reported similar benefits using chewing gum containing L. reuteri, showing reductions in S. mutans, plaque, and gingival scores, along with improved saliva buffering. García et al. [42] observed stabilization of Lactobacillus and a less pronounced increase in S. mutans in adolescents consuming probiotic tablets, with a trend toward increased oral pH.

Among the included studies, the most frequently tested L. reuteri strains were DSM 17938 and ATCC PTA 5289, used in combination in approximately 58% of trials. The strain ATCC 55730 appeared in about 25% of studies, either alone or in combination with other species, while less common variants such as ATCC PTA 5282 and UBLRu87 (MTCC 5403) were each used in about 8% of studies. One trial evaluated a multistrain commercial formulation containing L. reuteri without specifying the strain.

Drops were the most commonly used delivery vehicle (42%), followed by lozenges (25%) and tablets (17%), while chewing gums and curds were less frequent (8% each). Although drops are convenient and facilitate compliance, solid formulations, such as lozenges, tablets, and chewable gums, generally demonstrate superior efficacy [25,38,44]. A key limitation in interpreting results is the substantial heterogeneity across studies in strains, dosages, administration duration, and delivery systems, which may explain the variability in outcomes.

4.1.4. Probiotics in Pediatric Orthodontic Patients

Orthodontic patients are at higher risk of developing caries due to increased biofilm accumulation around brackets. Nowadays, even complex orthodontic movements can be performed using aligners instead of fixed braces, which facilitates better oral hygiene and consequently reduces the risk of cavities, periodontal inflammation, and dental decalcification. [55,56,57] After the placement of fixed orthodontic appliances, a rapid shift occurs in the composition of dental plaque microbiota. The biofilm becomes enriched with acidogenic species, particularly S. mutans and Lactobacillus spp. [58].

Ebrahim et al. [41] examined multistrain lozenges containing L. reuteri in adolescents with fixed appliances. Despite high compliance, no significant differences were observed in Plaque Index (PI) or S. mutans counts, likely due to the low colony-forming unit (CFU) concentration (~10⁵ per lozenge versus ~10⁹ commonly used) and the challenges of maintaining oral hygiene with fixed appliances. Similarly, Alforaidi et al. [46] evaluated L. reuteri drops (DSM17938 and ATCC PTA5289) and found no significant differences in S. mutans or lactobacilli counts between groups. However, bacterial levels remained stable in the test group while increasing in the placebo group, and plaque pH and buffering capacity improved, suggesting a temporary modulation of oral biofilm and acid stress.

Collectively, these findings emphasize that in orthodontic patients, probiotic effectiveness is limited by factors such as delivery method, dosage, and biofilm accumulation. Sustained and adequate probiotic intake, combined with proper oral hygiene, is crucial to achieving meaningful caries prevention [41,46].

4.2. Limosilactobacillus Reuteri as an Adjuvant on Periodontal Therapy

4.2.1. Effects in Children and Adolescents

The use of L. reuteri as a probiotic adjuvant in children and adolescents has shown mixed results among reports. Stensson et al. [43] reported that children treated with drops containing L. reuteri ATCC 55730 exhibited a significantly lower incidence of gingival inflammation compared to controls. In contrast, Hasslöf et al. [37] reported no beneficial effects on plaque or gingival inflammation with drops containing two strains (DSM17938 and ATCCPTA5289). These differences may possibly be due to the use of differences in strains, methodologies, study duration, or interruptions from the COVID-19 pandemic.

Alamoudi et al. [25], using the same strains, observed less plaque accumulation in both probiotic and control groups, although the probiotic group showed a greater increase in salivary buffering capacity. This suggests that observed changes may also reflect improved oral hygiene practices rather than probiotic effects alone. García et al. [42] evaluated probiotic tablets in adolescents and found trends of improvement in Plaque Index (PI) and Gingival Index (GI) in the probiotic group, but results were not statistically significant, and the Bleeding Index (BI) remained unchanged. These findings align with evidence in adults, as summarized by Butera et al. [59], who demonstrated that domiciliary probiotic protocols can enhance outcomes of non-surgical periodontal therapy. While the populations differ, these results collectively support the notion that L. reuteri contributes to microbial balance and inflammation control across age groups.

Continuous administration, parental supervision, and combination with fluoride use are recommended to optimize benefits. Evidence suggests that solid formulations, such as lozenges, tablets, or chewable gums, generally provide better contact time and clinical efficacy than liquid drops.

4.2.2. Effects During Orthodontic Treatment

Orthodontic patients are at increased risk of plaque accumulation due to fixed appliances, which can exacerbate gingivitis and caries risk. Agossa et al. [52] evaluated lozenges containing two strains (L. reuteri DSM 17938 and ATCC PTA 5289) combined with tooth brushing in adolescents with fixed orthodontic appliances. The six-month study assessed clinical oral parameters and salivary inflammation biomarkers, with the final three months evaluating lasting effects after probiotic administration, aiming to determine whether six weeks of supplementation could positively modulate the oral microbiota.

A study protocol by Seidel et al. [60] investigated the same strains in adolescents with fixed orthodontic appliances, adjusting lozenge administration to 12 weeks starting at appliance placement to optimize therapeutic efficacy. The effectiveness of the Gingival Index and Bleeding on Probing remains uncertain due to delayed administration in previous studies, the lack of short-term follow-up assessments, and insufficient post-treatment evaluation. Ebrahim et al. [41] evaluated the Lorodent Probiotic Complex in adolescents with braces and found no significant differences in PI or composite PI scores. The authors suggest that proper pre-treatment hygiene, possibly using chlorhexidine, could improve probiotic colonization and efficacy. In contrast, Kaur et al. [44] observed that chewing gums containing L. reuteri significantly reduced plaque and gingival scores, likely through a combination of increased salivary flow, mechanical cleaning, and anti-inflammatory effects, partially mediated by reuterin. Probiotics may support periodontal health in orthodontic patients, but outcomes depend on strain selection, delivery method, treatment duration, and baseline oral hygiene.

Figure 3 summarizes the main oral effects of L. reuteri, highlighting its mechanisms of action and clinical relevance in children and adolescents.

Overall, the evidence supports that L. reuteri exerts multiple benefits within the oral cavity through diverse, strain-dependent mechanisms. These include modulation of the oral microbiota, enhancement of salivary immune responses, maintenance of pH balance, and reduction of pathogenic biofilm accumulation, ultimately contributing to caries prevention and improved gingival health in pediatric populations.

5. Conclusions

L. reuteri offers multiple benefits for pediatric oral health. Naturally present in the oral cavity and gastrointestinal tract, it can colonize the mouth through breastfeeding and dairy consumption. L. reuteri reduces S. mutans and lactobacilli levels, helping prevent dental caries, and certain strains, like ATCC PTA 5289, show minimal acid production, lowering the risk of enamel demineralization. It also improves periodontal health by increasing oral pH and modulating microbial balance through co-aggregation, antimicrobial production (e.g., reuterin), competitive inhibition, and immune modulation.

However, these conclusions should be interpreted cautiously. The evidence is limited by substantial heterogeneity in probiotic strains, dosages, delivery methods, and study durations, which reduces comparability across studies. Additional limitations include small sample sizes, short follow-up periods, variations in oral hygiene practices, dietary habits, participant compliance, and potential publication bias. Some studies were also influenced by external factors, such as the COVID-19 pandemic. Larger, well-controlled, standardized trials are needed to better establish efficacy and determine optimal protocols for L. reuteri use in pediatric oral health. Despite current limitations, L. reuteri shows promising potential as a safe adjunct in pediatric oral health strategies.