Background
Beneficial microorganisms, or probiotics, have various functions including antibacterial activities [
1], modulation of the host immune response [
2], antiallergic effects [
3], and cancer prevention in the human intestine [
4]. A number of studies using lactic acid bacteria for the prevention of oral diseases have been reported [
5‐
10]. Probiotic bacteria in the human oral cavity include
Bifidobacterium species and
Lactobacillus species [
11]. Oral administration of
Lactobacillus salivarius WB21-containing tablets and oil reduced plaque accumulation, periodontal pocket depth, bleeding on probing, and oral malodor [
7‐
10].
L. salivarius TI 2711 showed antibacterial activity against
Porphyromonas gingivalis in mixed culture experiments [
12], and a clinical trial indicated that the number of
P. gingivalis in subgingival plaque was reduced by oral administration of
L. salivarius TI 2711-containing tablets but recovered following cessation of tablet administration [
13]. However, there have been no reports regarding caries prevention and control by
L. salivarius.
Lactobacillus species are microbial markers of dental caries risk [
14]. Some species of lactobacilli have been reported to occur in large numbers in both superficial and deep caries [
15,
16]. However, several
Lactobacillus species have been isolated from healthy mouths [
17,
18]. Increased production of organic acids in the dental plaque would be considered a side effect of probiotic lactobacilli. Most studies of caries prevention and control by lactic acid bacteria were performed over the last few years.
Lactobacillus species, including
Lactobacillus rhamnosus,
Lactobacillus reuteri, and
Lactobacillus paracasei, that were investigated in previous clinical trials did not alter or reduce mutans streptococci levels [
19‐
21]. No adverse effects or potential risks of these bacteria were reported. In the present study, the potential effects of tablets containing
L. salivarius WB21 or TI 2711 on caries risk factors were compared with tablets containing an antibody against
Streptococcus mutans and those containing only xylitol as controls. In addition, oral administration of
L. salivarius WB21-containing tablets for 2 weeks was performed to evaluate their effect on the levels of mutans streptococci.
Discussion
Oral consumption of L. salivarius WB21 and TI 2711 did not show side effects related to advance of caries in the oral cavity. In fact, those tablets provided better resistance to caries risk factors compared to the xylitol tablet. Mutans streptococci levels increased and salivary flow and salivary buffering capacity decreased after taking the xylitol tablet in the present study. In contrast, the mutans streptococci level decreased in the group taking oral L. salivarius WB21-containing tablets. A reduction in mutans streptococci level was seen in 29.4% of subjects in the L. salivarius WB21 group (5/17), 18.8% in the L. salivarius TI 2711 group (3/16), 38.5% in the Ovalgen® DC group (5/13), and 16.7% in the xylitol group (3/18). An increase in mutans streptococci level occurred in 11.8% of subjects in the L. salivarius WB21 group (2/17), 31.3% in the L. salivarius TI 2711 group (5/16), 30.8% in the Ovalgen® DC group (4/13), and 44.4% in the xylitol group (8/18). In addition, the exploratory short-term administration trial with L. salivarius WB21-containing tablets showed a reduction in levels of mutans streptococci in saliva at 2 weeks.
Salivary flow, salivary pH, and salivary buffering capacity did not change or increased after taking the
L. salivarius WB21 and TI 2711 tablets. No changes were noted in salivary buffering capacity in the
L. salivarius WB21 and TI 2711 groups, although a reduction was seen in the xylitol group. Medians salivary buffering capacity was the same in the
L. salivarius WB21 and TI 2711 groups, but less variability was observed in the
L. salivarius TI 2711 group than in the
L. salivarius WB21 group (Table
3). A significant difference was detected in buffering capacity between the
L. salivarius TI 2711 group and the xylitol group.
Salivary flow did not differ among the four groups in the present study, although it increased in the
L. salivarius TI 2711 group [from 7.3 (5.8 to 8.8) to 8.0 (6.1 to 9.3) mL per 3 min]. The trend toward increased salivary pH in the
L. salivarius TI 2711 group may have been caused by the increase in salivary flow (Table
3). No changes in salivary flow, salivary pH, and salivary buffering capacity were noted in the
L. salivarius WB21 group. In previous studies, salivary flow was significantly increased by oral administration of
L. salivarius WB21-containing products for 2 weeks [
9,
24]. Salivary pH and buffering capacity did not change in those studies. The increase in saliva in those studies may have been caused by continuous ingestion of the tablets.
Our results suggest that
L. salivarius strain-containing tablets can be used safely without increasing the caries risk, and that their oral administration may contribute to caries control in healthy adults. An open-label trial is insufficient to confirm a positive effect of lactobacilli on caries prevention because placebo effects were evident in previous double-blind trials [
7,
9,
10]. Therefore, a double-blind randomized placebo-controlled trial is required to confirm the effect of
L. salivarius-containing tablets on caries risk factors.
Tablets containing egg yolk antibodies against
S. mutans cell-associated glucosyltransferase (anti-CA-gtf IgY, Ovalgen® DC) and xylitol tablets were used for comparison in the present study. All products used contained xylitol, a non-fermentable sugar alcohol that cannot be used as an energy source in the metabolism of cariogenic bacteria [
24]. In this study, the level of mutans streptococci in the saliva increased in 44.4% of subjects in the xylitol group. The gum test was performed twice, and therefore more mutans streptococci may have been removed from the surface of teeth. The flow rate, pH, and buffering capacity of saliva were reduced or remained unchanged by oral consumption of the xylitol tablets. A clinical trial using xylitol gum found that the effects of chewing are essential for the stimulation of salivary flow and the resulting recovery of pH levels and reduction of
S. mutans levels in saliva [
25]. It was suggested that licking a xylitol tablet does not affect the quality of saliva.
The Ovalgen® DC tablet significantly increased salivary buffering capacity compared with the xylitol tablet, although the cause is unclear. The rate of reduction in mutans streptococci levels in the Ovalgen DC® group (38.5%) was greater compared with the xylitol group (16.7%), although the differences between the groups were not significant. It has been reported that anti-CA-gtf IgY suppressed the development of caries lesions in rats [
26], and tablets containing anti-CA-gtf IgY significantly reduced the numbers of mutans streptococci in a 5-day double-blind placebo-controlled trial [
27]. The mean anaerobic bacterial counts were not significantly different before and after the trial [
27], indicating that Ovalgen® DC is specific for dental cariogenic species. In contrast, lactic acid bacteria affect various bacteria residing in the oral cavity and have various functions that differ among species, including aggregation, competition for adhesion sites, nutrients, and growth factors, production of antimicrobial compounds such as acid, hydrogen peroxide, and bacteriocins, and effects on the immune response [
28]. The mechanisms by which
L. salivarius WB21 and TI 2711 reduce caries risks are unclear, but coaggregation, growth inhibition of mutans streptococci, and reduced plaque acidogenicity have been reported as potential mechanisms by which lactic acid bacteria prevent dental caries [
29‐
31]. Keller et al. [
29] investigated the
in vitro abilities of eight probiotic
Lactobacillus strains (
L. plantarum 299v, 931,
L. rhamnosus GG, LB21,
L. paracasei F19,
L. reuteri ATCC PTA 5289, DSM 17938 and
L. acidophilus La5) to coaggregate and inhibit mutans streptococci. All lactobacilli displayed coaggregation activity and inhibited the growth of clinical mutans streptococci. The growth inhibition was strain-specific and dependent on both pH and cell density [
29]. An
in vitro experiment also indicated that lactic acid production in suspensions of plaque and probiotic lactobacilli was strain-dependent [
32]. The salivary pH was not changed by oral consumption of
L. salivarius WB21 or TI 2711 in this study. A previous clinical study using oil containing
L. salivarius WB21 indicated no changes in salivary pH after 2 weeks of treatment [
9]. The short-term consumption of
L. rhamnosus GG and
L. reuteri for 2 weeks had no effect on acid production by supragingival plaque [
33]. In contrast, a significant reduction in plaque acidogenicity was found with long-term (6 weeks) consumption of
L. brevis CD2 [
31]. Furthermore, clinical assessments indicated that the effects of probiotics varied depending on host susceptibility. In fact, the levels of mutans streptococci in several participants were increased after oral consumption of
L. salivarius strains in the present study. Further studies of the characteristics of probiotic strains and the host responses are required to determine their appropriate applications, doses and treatment durations.
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Competing interests
The authors declare that they have no competing interest.
Authors’ contributions
SN designed the study, analyzed the data, and wrote the manuscript. NT collected salivary samples and carried out the bacterial and salivary experiments. YM and HT commented on the data and helped to write the manuscript. All authors read and approved the final manuscript.