Anti-acidogenic Effects of Licorice Extracts on Oral Biofilm Bacteria

Objectives: It is well known that acids, produced by oral biofilm bacteria, demineralize tooth surfaces and promote the caries process. Drugs such as fluoride have been already reported to inhibit the acid-producing activity. However, if safer materials like plant extracts, already used as food additives or components of toothpaste, can inhibit the acid-producing activity, these materials may broaden our choice for the development of new methods to maintain/promote oral health. Therefore, we aimed to screen plant extracts and evaluate their anti-acidogenic effects on Streptococcus mutans.
Methods: Sixty plant extracts were tested. Streptococcus mutans NCTC 10449 (SM) were grown anaerobically in the presence of 0.1% each plant extract, and bacterial growth and culture pH were evaluated for screening. The extracts, which showed a significant inhibitory effect, were selected, and their effects on the glucose-induced pH fall (by pH electrode), acid-production rate (by pH stat) and metabolome profile (by capillary electrophoresis-mass spectrometer) of SM were further evaluated.
Results: Only licorice hot-water extract (Lico-ex) was selected by the screening process. Lico-ex inhibited the bacteria growth and pH fall of SM during growth significantly (p<0.05, Dunnett’s test). Lico-ex inhibited the glucose-induced pH fall (pH 5.6±0.2 vs 4.1±0.2, p<0.05, Dunnett’s test) and acid-production rate by 52.8±3.3% (p<0.05, paired t test). Metabolome analyses revealed that Lico-ex decreased the intracellular level of 3-phosphoglycerate (37.2±12.8%), 2-phosphoglycerate (38.3±10.0%) and phosphoenolpyruvate (25.9±7.5%) in the glycolysis, along with a significant decrease of lactate production (12.9±1.3%) (p<0.0025 after Bonferroni correction).
Conclusions: Lico-ex inhibited the growth and acid-production of SM. Moreover, metabolome analyses suggest that the Lico-ex inhibits the downstream of the glycolysis, catalyzed by glyceraldehyde 3-phosphate dehydrogenase and/or phosphoglycerate kinase, and lactate dehydrogenase. Since Lico-ex has been used as a food additive for a long time, it is expected that Lico-ex can contribute to our oral health, such as preventing dental caries.