Abstract: Objective: To observe the promoting effect of streptococcus mutans on demineralized enamel caused by carbonated beverages and the inhibition effect of magnolol on streptococcus mutans to reduce the demineralization of enamel blocks. Methods: Extracted premolars without roots for orthodontic purpose were selected. All the crowns were divided into mesial and distal enamel blocks that were prepared into 5 mm×5 mm×2 mm. These enamel blocks were soaked in sprite for 30 minutes×4 times, 2 hours at a time for 3 weeks to establish the demineralization model. The demineralized enamel blocks were placed in artificial saliva at 37℃ room temperature water bath to form the acquired pellicle. Then they were randomly divided into normal saline group (group A), streptococcus mutans suspension group (group B, OD of the bacterial suspension was 1), streptococcus mutans suspension + magnolol group (group C, the concentration of magnolol was 6.3mg/L). All enamel blocks were placed in anerobic box at 37℃ for 3 weeks. The microhardness value was tested by microhardness. The enamel surfaces were observed under scanning electron microscope. The fluorescence images were taken under the laser scanning confocal microscope. Results: After demineralization, the microhardness value of the three group were lower than that of before demineralization (P<0.05). The microhardness value of group A and group C was not different from that of before medication, however, higher than that of group B (P<0.05). Under scanning electron microscope, the enamel surfaces of group B were rough and the unequal size pore that formed by a great deal of fractured enamel rod, and a large number of bacteria attached. Enamel surfaces of group A showed some pores became shallow or disappeared. Enamel surfaces of group C pores significantly reduced, and a few bacteria attached. Under the laser scanning confocal microscope, the fluorescence area and intensity of group B were strong, the fluorescence area and intensity of group A and group C were lower than that of group B. Conclusion: Streptococcus mutans can accelerate demineralization of enamel caused by carbonated beverages. The magnolol can slow down the progress of caries by its inhibition effects on streptococcus mutans and reduction on demineralized enamel caused by carbonated beverages.

Key words: Magnolol, Streptococcus mutans, Demineralized enamel Inhibition effect