Antibacterial activity of phellodendron bark against Streptococcus mutans.

Streptococcus mutans is a major cause of tooth decay due to its promotion of biofilm formation and acid production. Several plant extracts have been reported to have multiple biological activities such as anti-inflammation and antibacterial effects. This study investigated the antibacterial activity of three plant extracts, phellodendron bark (PB), yucca, and black ginger, and found that PB had a stronger effect than the other extracts. Then, the minimum inhibitory concentration (MIC) of PB against 100 S. mutans strains was investigated. The MIC range of PB was 9.8-312.5 µg/mL. PB suppressed the growth kinetics of S. mutans in a dose-dependent manner, even at sub-MICs of PB. Then, we investigated the effect of PB on S. mutans virulence. The PB suppressed biofilm formation at high concentrations, although PB did not affect the expression of glucosyltransferase genes. Additionally, PB suppressed the decrease in pH from adding an excess of glucose. The expression of genes responsible for acid production was increased by the addition of excess glucose without PB, whereas their expression levels were not increased in the presence of 1× and 2× MIC of PB. Although PB showed a bacteriostatic effect on planktonic S. mutans cells, it was found that more than 2× MIC of PB showed a partial bactericidal effect on biofilm cells. In conclusion, PB not only showed antibacterial activity against S. mutans but also decreased the cariogenic activity in S. mutans.

Antibacterial activity of disodium succinoyl glycyrrhetinate, a derivative of glycyrrhetinic acid against Streptococcus mutans.

Streptococcus mutans is a cariogenic bacterium that localizes in the oral cavity. Glycyrrhetinic acid (GRA) is a major component of licorice extract. GRA and several derivatives, including disodium succinoyl glycyrrhetinate (GR-SU), are known to have anti-inflammatory effects in humans. In this study, the antimicrobial effect of GRA and its derivatives against the S. mutans UA159 strain were investigated. Minimum inhibitory concentrations (MICs) of GRA and GR-SU showed antibacterial activity against the S. mutans strain, whereas other tested derivatives did not. Because GR-SU is more soluble than GRA, GR-SU was used for further experiments. The antibacterial activity of GR-SU against 100 S. mutans strains was evaluated and it was found that all strains are susceptible to GR-SU, with MIC values below 256 µg/mL. A cell viability assay showed that GR-SU has a bacteriostatic effect on S. mutans cells. As to growth kinetics, sub-MICs of GR-SU inhibited growth. The effect of GR-SU on S. mutans virulence was then investigated. GR-SU at sub-MICs suppresses biofilm formation. Additionally, GR-SU greatly suppresses the pH drop caused by the addition of glucose and glucose-induced expression of the genes responsible for acid production (ldh and pykF) and tolerance (aguD and atpD). Additionally, expression of enolase, which is responsible for the carbohydrate phosphotransferase system, was not increased in the presence of GR-SU, indicating that GR-SU suppresses incorporation of sugars into S. mutans. In conclusion, GR-SU has antibacterial activity against S. mutans and also decreases S. mutans virulence.