Effect of Radix Glycyrrhizae Against Streptococcus Mutans in Vitro

Objectives: to investigate the antibacterial activity of decoction of radix glycyrrhizae (self-made) and glycyrrhizin acid (GA, C₄₂H₆₂O₁₆, purity 99%, purchased) against S. mutans (ATCC 25175) in vitro. Methods: The decoction of radix glycyrrhizae was prepared by boiling 20g radix glycyrrhizae in 200ml distilled water for three hours and filtered. Filter residues were put in another 200ml distilled water and boiled for one hour and filtered. Collected all the filtrate and adjusted the volume to 100ml and the final concentration of the decoction was 200mg/ml. The minimal inhibitory concentrations (MIC) of decoction and GA against S. mutans were studied using doubling dilution. The effects of decoction or GA on growth and acidogenic profile of S. mutans were investigated by detecting the Abs of bacteria suspension and the pH value of medium at definite time intervals during culture. Results: The MIC determined for decoction and GA were 25mg/ml and 3.125mg/ml, respectively. 12.5mg/ml and 6.25mg/ml decoction reduced the multiplication of bacteria significantly during log phase compared with control and the former had stronger effect (Fig1). Concordant results were seen in 1.565mg/ml and 0.783mg/ml GA (Fig3). Fig2 and Fig4 showed the pH of control group dropped below 5.0 from 6.7 at 10h after incubated. At the same time the pH were still above 6.0 in groups exposed to decoction and GA. The pH of control further dropped below 4.5 at 18h after incubated and then kept. The pHs of other groups were dropping slowly along with the incubation time and finally approached control group. The incubation time when the pH dropped below 4.5 for each group was as follows: control, 18h; 12.5mg/ml decoction, 28h; 6.25mg/ml decoction, 18h; 1.565mg/ml GA, 32h and 0.783mg/ml GA, 18h. Conclusion: The decoction of radix glycyrrhizae and pure GA all have antibacterial activity against S. mutans in vitro.