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

<b>Background</b>: Early childhood caries is closely associated with oral dysbiosis and the proliferation of <i>Streptococcus mutans</i>. Oral probiotics, particularly <i>Lactobacillus reuteri</i> and <i>Lactobacillus rhamnosus</i>, have been proposed as ecological modulators capable of reducing cariogenic microorganisms. <b>Objective</b>: To evaluate the efficacy of orally administered <i>L. reuteri</i> and <i>L. rhamnosus</i> in reducing salivary <i>S. mutans</i> levels in children aged 6 months to 12 years through a systematic review and meta-analysis. <b>Methods</b>: 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. <b>Results</b>: 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 <i>S. mutans</i> levels (MD = -0.65 log₁₀ CFU/mL; 95% CI: -0.97 to -0.34; <i>p</i> < 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; <i>p</i> = 0.10; I² = 35%). Short-term interventions using high oral-retention formulations demonstrated the most consistent microbiological effects. <b>Conclusions</b>: Oral probiotics may significantly reduce salivary <i>S. mutans</i> 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.