Effects of Oral Probiotics on Streptococcus mutans in Children: A Systematic Review and Meta-Analysis

Background: Early childhood caries is closely associated with oral dysbiosis and the proliferation of Streptococcus mutans. Oral probiotics, particularly Lactobacillus reuteri and Lactobacillus rhamnosus, have been proposed as ecological modulators capable of reducing cariogenic microorganisms. Objective: To evaluate the efficacy of orally administered L. reuteri and L. rhamnosus in reducing salivary S. mutans levels in children aged 6 months to 12 years through a systematic review and meta-analysis. Methods: This review followed the PRISMA 2020 guidelines and was prospectively registered in PROSPERO (CRD420251086304). Searches were conducted in MEDLINE/PubMed, CENTRAL, Embase, Scopus and LILACS without language or date restrictions. Randomized controlled trials administering the target probiotic strains for ≥30 days were included. Risk of bias was assessed using RoB 2, and certainty of evidence using GRADE. Random-effects meta-analyses were performed for continuous and dichotomous outcomes. Results: Six randomized controlled trials were included (N = 1362). Only two trials reported continuous outcomes in comparable log₁₀ CFU/mL format and could therefore be pooled for the continuous meta-analysis. This analysis showed a significant reduction in salivary S. mutans levels (MD = -0.65 log₁₀ CFU/mL; 95% CI: -0.97 to -0.34; p < 0.0001; I² = 19%), although the pooled estimate was largely driven by one study and should be interpreted cautiously. Four trials contributed to the dichotomous meta-analysis, which showed a non-significant trend toward risk reduction (OR = 0.73; 95% CI: 0.51-1.06; p = 0.10; I² = 35%). Short-term interventions using high oral-retention formulations demonstrated the most consistent microbiological effects. Conclusions: Oral probiotics may significantly reduce salivary S. mutans in the short-term, especially when delivered through slow-dissolving formulations. However, their effects vary according to strain, vehicle, and intervention duration. Larger, standardized, and longer-term clinical trials are needed to determine the sustainability and clinical relevance of these effects.