from saliva samples include L. paracasei, L. plan- has often been considered essential for them tarum, L. rhamnosus, and L. salivarius.7-10 Culture- to exert oral effects; however, the possibility of based studies suggest that bifidobacteria are systemic effects cannot be excluded, although among the first anaerobes in the oral cavity.11 In- the total sIgA levels in saliva seem unaffected deed, both lactobacilli and bifidobacteria can be by probiotic use.23,24 Interestingly, maternal use found in breast milk, suggesting early exposure of of some probiotic strains seems to influence the the oral cavity to these bacteria.12,13 Bifidobacterial composition of breast milk.25 species isolated from oral samples include B. bifi - In respect to commensal oral microbes, sever- dum, B. dentium, and B. longum.8,14,15 al aspects support the idea that it may be possible Lactobacilli and bifidobacteria are generally to find bacteria that could be useful in preven- regarded as safe, and, since the early writing of tion or treatment of oral diseases. In fact, it has Metchnikoff,16 even more fermented food products been suggested that some observed probiotic ef- have been associated with health benefits. In re- fects are not just properties of a few well-studied spect to normal microbiota and oral health, there strains but common to several species.26-28 The seem to be differences in the ability of lactobacilli ecological plaque hypothesis suggests that se- isolated from caries-active or healthy subjects to lective pressure in environmental conditions can inhibit Streptococcus mutans in vitro.10 In addition, change the balance between oral health and dis- the species composition of both Lactobacillus and ease.29 As bacteria can also influence their envi- Bifidobacterium microbiota is different between ronment, and both synergistic and antagonistic patients with periodontitis and those who are interactions are suggested for bacteria in dental periodontally healthy.17,18 On the other hand, both plaque, the environmental pressure described in lactobacilli and bifidobacteria are also associated the ecological plaque hypothesis could be intro- with dental caries.19,20 In addition, caries-associat- duced partly by bacteria. Second, it is well recog- ed lactobacilli and bifidobacteria have been char- nized that the normal microbiota protect the oral acterized as exogenous and opportunistic coloniz- cavity from infections. Finally, just as there are ers possibly acquired from food.15,21 bacterial species associated with oral diseases, During the last few years, several authors there are also species that seem to be associated have suggested that probiotic bacteria originally with oral health;20,30,31 however, it is questionable planned for gut health could also be beneficial to whether bacteria administered in food could influ- oral health. The aim of this review is to examine ence relatively stable oral microbiota, in particular potential mechanisms of probiotic bacteria in the in adults. In addition, it must be kept in mind that oral cavity and to summarize observed effects all the mechanisms described in this section are of probiotics with respect to oral health. Finally, based on in vitro results, and the clinical evidence some future aspects are briefly discussed. This is based mainly on short clinical pilot studies. review focuses on probiotic lactobacilli and bifi- OBSERVED EFFECTS ON dobacteria, genera that are most used in various ORAL HEALTH probiotic products. Caries and caries-associated microbes POTENTIAL MECHANISMS OF Several studies suggest that consumption of PROBIOTIC EFFECTS IN THE products containing probiotic lactobacilli or bi- ORAL CAVITY fidobacteria could reduce the number of mutans streptococci in saliva.32-40 The tendency toward a The general mechanisms of probiotics can be decreased number of mutans streptococci in the divided into three main categories: normalization saliva seems to be independent of the product or of the intestinal microbiota, modulation of the strain used; however, such effect has not been immune response, and metabolic effects.22 The observed in all studies.41 The discrepancies be- mechanisms of probiotic action in the oral cavity tween results cannot be explained by only the use could be analogous to those described for the of different probiotic strains, as different results intestine. Possible ways that probiotics might have also been obtained using the same strains. In affect oral health are summarized in Figure 1. most of these studies, the levels of salivary lacto- Thus far oral colonization by probiotic bacteria July 2010 - Vol.4 349 European Journal of Dentistry Haukioja