## 4 Discussion  
### Impact of Probiotics in the Oral Microbiota  
The community of microorganisms that colonizes the mouth and forms the dental plaque mainly plays a protective role against pathogens. Dental plaque consists of bacterial cells (mainly streptococci and lactobacilli), bacterial metabolites/products/toxins, salivary polymers/proteins, and food debris [1]. Probiotic lactobacilli may inhibit the adhesion of pathogenic bacteria to the oral tissues, reducing the amount of biofilm formed (Figure 1). On the other hand, lactobacilli probiotics may potentiate dental plaque acidogenicity and increase the load of acid tolerant bacteria such as _S. mutans_ and viridans streptococci, making the dental biofilm more pathogenic [18]. The following 9 trials studied the impact of probiotics in the oral ecosystem (Table S3). In [PERSON] et al. [19], dental plaque accumulation in children was significantly reduced after 14 days of tablet consumption containing _L. acidophilus, L. rhamnus_, _Bifdobacterium longum_ and _S. boulardii_, and after three weeks of intervention. In a study by [PERSON] et al. [20], administration of _Streptococcus salivuring_ M18 to children for 3 months caused a significant decrease in dental plaque scores. Non-target microorganisms, _S. salivarius_, _Lactobacillus_ spp., hemolytic streptococci and _Candida_ spp. levels were not changed during the study. Despite a high adhesion rate (>80%), only 22% of the children were colonized by _S. salivarius_ M18 and this lasted until 4 months after discontinuation. _S. mutans_ counts were reduced, especially in colonized children, suggesting that _S. salivarius_ M18 may have anti-carie activity and that colonization helps the probiotic effect.  
A significant reduction of salivary _Aggregatibacter actynomyctancomitans_, _P. gingialis_ and _Streptococcus mutans_ counts, and total number of microorganisms was achieved in children after 2-weeks ingestion of Petit-Suisse (cream cheese) with _L. casei_[21]. However, the reduction of total number of microorganisms and _Streptococcus mutans_ was attributed to Petit-Suisse alone as it occurred in both the probiotic and control groups.  
A significant decrease in _S. mutans_ counts and in the total number of microorganisms occurred after once daily consumption of fermented milk containing _L. rhamnus_ SD11 in adults during 4-weeks and after 4-weeks discontinuation [22]. In contrast, _Lactobacillus_ spp. counts increased significantly at week 4 in all patients, and the probiotic was detected up to 8 weeks in 80% of the individuals. In a study by [PERSON] et al. [23], a combination of _L. rhamnus_ GG and _B. lactis_ BB-12 administered over 4 weeks to adults caused no change in the salivary _S. mutans_ or _Lactobacillus_ spp. counts. Similarly, _L. reuteri_ DSM 17938 and ATCC PTA 5289 administered during 2-12 weeks to adults had no impact on salivary _S. mutans_ count and microbial profile and diversity [24; 25; 26]. Administration of _L. rhamnus_ GG or _L. reuteri_ D2112 and PTA 5289, also did not impact salivary _S. mutans_ count [18].  
However, there was a significant increase in lactobacilli in volunteers taking \(L\). _reuteri_ but not \(L\). _rhannosus_.  
In summary, most probiotic strains analyzed in these trials are safe, as they do not significantly and definitively affect the commensal oral microbiota, and some can transiently colonize the dental surfaces and have the potential to prevent dental caries.