Dental caries is demineralisation of the tooth induced by microbial production of acid in the dental plaque. Hence, modulation of the dental plaque by probiotic bacteria has naturally been of interest.

Since the first study on the topic by Näse et al. in 2001, there have been an increasing number of clinical trials with caries-related end points. However, the majority of these studies have other end points than caries such as microbial counts or plaque index. Mutans streptococci are the most common microbial end point, but some studies also look at lactobacilli. Most of these studies show an inhibitory effect of probiotics on salivary mutans streptococci, but a few does not find any difference after the intervention period (Stecksen-Blicks et al. 2009; Lexner et al. 2010; Taipale et al. 2012). Nonetheless, two recent systematic reviews conclude that probiotic bacteria decrease the mutans streptococci counts as long as there is a regular intake of the probiotic bacteria (Cagetti et al. 2013; Laleman et al. 2014). The strains used in the clinical intervention trials vary among different lactobacilli strains and a few different bifidobacteria and even some streptococci strains. There does not seem to be any clear-cut difference between the outcomes of the trials based on the strains used in the study.

Since the lactobacilli used in most studies have strong acidogenic abilities, a natural reservation in relation to caries would be that addition of lactobacilli to the oral cavity would involve an increased risk of an acidogenic shift of the oral microbiota. However, two clinical trials found no increase in biofilm acidurity after exposure to probiotic lactobacilli strains (L. reuteri SD2112, L. reuteri DSM 17938 and L. reuteri PTA 5289) (Keller and Twetman 2012; Marttinen et al. 2012).

There are fewer studies with caries incidence, root caries arrest or remineralisation of carious lesions as outcome. The six studies with caries incidence as end points are listed in Fig. 12.1, which displays the prevented fraction of the studies. The calculated mean of preventive fraction is 33 %. The most popular vehicle for the probiotic bacteria was milk which was supplemented with 2.5 ppm fluoride in one study. The first study from Finland (Näse et al. 2001) also used milk as the vehicle for L. rhamnosus GG during a 7-month intervention period. They found a preventive fraction of 56 % among the children being 3–4 years of age but only 21 % when looking at the entire study population of 1–6 years of age. Besides the dental health benefits, the study also showed a reduction in antibiotic treatments for respiratory infections in the test group (Hatakka et al. 2001). The promising results encouraged a similar study in Sweden where preschoolers aged 1–5 years were provided with milk containing L. rhamnosus LB21 and 2.5 ppm fluoride on a daily basis (Stecksen-Blicks et al. 2009). Their results showed a 75 % preventive fraction but due to the study design, it was not possible to determine how much of this can be attributed to the fluoridation or the addition of probiotic bacteria. Secondly, this study also found an additional beneficial effect in fewer prescriptions of antibiotics to the children in the test group. A more recent study from Finland chose another vehicle based on the young age of the participants (0–12 months) and used a pacifier with room for probiotic tablets to obtain longer oral exposure to the probiotic strain L. rhamnosus LB21 (Taipale et al. 2013). At age four, however, the caries preventive fraction was rather modest (5 %) which might be due to uncertain compliance.

Two Swedish studies are follow-up studies on trials conducted 6 and 7 years ago, respectively. After having eaten gruel with added L. paracasei F19 during infancy, the children in the test group had statistically significant less eczema at age 6 but not statistical significant results on caries incidence (Hasslof et al. 2013). The dropout rate was rather high and together with generally low level of caries, it may account for failure to achieve statistically significant results. In comparison, the results of the other study were more promising. Stensson et al. (2014) reports a preventive fraction of 29 % 7 years after the original intervention with five drops of L. reuteri ATCC a day during the first 12 months of age.

It has been discussed whether colonisation of the probiotic strains is necessary to gain an effect. The studies with microbial end points show a change in levels of mutans streptococci as long as the probiotic strains are administered but mutans streptococci return to previous levels when the probiotic bacteria are ceased. Hence, the effect seems to be dependent of a continuous distribution of the probiotic bacteria. However, the follow-up study by Stensson reported on an effect 7 years after the original intervention. This could point towards the importance of intervention in early childhood to secure a long-term effect. Despite the promising results from the existing studies, there is still insufficient evidence to give any clinical recommendations, and more long-term studies with caries as an end point are needed.

The yeast Candida can cause a number of disorders in the oral cavity, known as oral candidiasis. Seven Candida species are the clinically most important, of which C. albicans, C. tropicalis and C. glabrata are most frequently isolated (80 %). Candida species are commensal microorganisms in the oral cavity in 40–60 % of the population and only cause disease when disturbances in the oral microbial balance occur (Teughels et al. 2008). Oral candidiasis is therefore often seen in elderly people and is frequently associated with antibiotic treatment, hyposalivation, impaired local or systemic immune system, neglected oral hygiene, dentures and smoking (Anil et al. 2014; Pires et al. 2002; Shay et al. 1997; Torres et al. 2002). A number of antifungal agents are available for the treatment of oral candidiasis, e.g. the polyenes (nystatin) and the azoles (fluconazole). However, since oral candidiasis is caused by an ecological imbalance (dysbiosis) in the oral biofilm that favours fungal overgrowth, a certain interest has been addressed to a bioecological approach for prevention and management.

In the past decade, several clinical studies have investigated the ability of probiotic bacteria to hamper the growth of Candida in the oral cavity. Hatakka and co-workers (2007) were the first to conduct a randomised double-blind, placebo-controlled trial on the effect of probiotics on the prevalence of oral Candida