Background: Probiotic bacteria are suggested to play a role in the maintenance of oral health. Such health promoting bacteria are added to different commercial probiotic products. The aim of the study was to investigate the ability of a selection of lactobacilli strains, used in commercially available probiotic products, to inhibit growth of oral mutans streptococci andC. albicans in vitro.Methods: Eight probiotic lactobacilli strains were tested for growth inhibition on three reference strains and two clinical isolates of mutans streptococci as well as two reference strains and three clinical isolates ofCandida albicanswith an agar overlay method.Results: At concentrations ranging from 10⁹to 10⁵CFU/ml, all lactobacilli strains inhibited the growth of the mutans streptococci completely with the exception ofL. acidophilusLa5 that executed only a slight inhibition of some strains at concentrations corresponding to 10⁷and 10⁵CFU/ml. At the lowest cell concentration (10³CFU/ml), onlyL. plantarum299v andL. plantarum931 displayed a total growth inhibition while a slight inhibition was seen for all five mutans streptococci strains byL. rhamnosusLB21,L. paracaseiF19,L. reuteriPTA 5289 andL. reuteriATCC 55730. All the tested lactobacilli strains reduced candida growth but the effect was generally weaker than for mutans streptococci. The twoL. plantarumstrains andL. reuteriATCC 55730 displayed the strongest inhibition onCandida albicans. No significant differences were observed between the reference strains and the clinical isolates.Conclusion: The selected probiotic strains showed a significant but somewhat varying ability to inhibit growth of oral mutans streptococci andCandida albicans in vitro.

The World Health Organization has defined probiotics as “Live microorganisms which, when administered in adequate amounts, confer a health benefit to the host.” Traditionally, probiotic microorganisms (mainly Lactobacillus ssp. and Bifidobacterium ssp.) have been used to prevent or treat diseases in the gastrointestinal tract. In the past 20 years, there has been an increased interest in possible oral health effects of probiotics. In vitro studies have shown promising results with growth inhibition of mutans streptococci (MS) and Candida albicans. There are only a few clinical studies with caries development as the primary outcome while more studies have been focusing on control of caries risk factors or so-called surrogate outcomes. Several studies have evaluated the effects of probiotic bacteria on MS in saliva and/or plaque, and a number of probiotic strains show ability to reduce the number of MS. Probiotic bacteria have not been shown to permanently colonize the oral cavity; in early-in-life interventions or in subjects with a mature microbiota. To date investigated strains are transiently present in saliva during and shortly after an intervention. There are eight randomized controlled clinical trials with dental caries as outcome and probiotic strains, administration, duration of the intervention, and target group varied. In a majority of the studies (75%), the interventions resulted in caries reduction in the treatment groups. Although a majority of these studies suggest a caries-preventive effect of probiotic bacteria, more long-term clinical studies are needed in this field before probiotics could be recommended for preventing or treating dental caries.