Abstract: Objective：To evaluate the effects of a commercial bleaching agent containing 35% (mass fraction) hydrogen peroxide on the growth of Streptococcus mutans biofilm on enamel disc surface. Methods:A total of 20 enamel disks were made from human extracted teeth and the enamel surfaces were kept intact. The discs were autocalved and randomly divided into two groups: bleaching group and control group. Each group contained 10 discs. For bleaching group, the enamel discs were whitened by commercial 35% hydrogen peroxide according to the instruction (Beyond^TM Professional Dental Whitening Kit, Beyond Technology, TX,USA ); no treatment for control group. All the discs were kept in sterile human saliva for 3.5 hours, and then the mixture of brain heart infusion broth (BHI) medium and Streptococcus mutans were added. The discs and Streptococcus mutans were incubated together in BHI medium with 5% CO₂ (volume fraction), at 37 ℃. After 3, 7, 14, 21 and 28 d’s incubation, two discs of each group were taken out and the biofilms on the enamel surfaces were evaluated by using conventional bacteria counts and confocal laser scanning microscope (CLSM). The bacteria in the biofilm on one disc enamel surface were analyzed by plating on BHIS agar and the colony-forming units were counted. The biofilm on the other disc surface was stained using a twocolour fluorescent dye kit (Bacerial Viability Kit L-7012) for CLSM. Results:The vital bacteria counts of vital cells in the 3, 7, and 14 d’s biofilms of the bleaching group were significantly fewer than those of the control group. Especially in the 3 days’ biofilm on the whitened surface, the vital bacteria counts ［(3 595±2 903) μm² vs. (89 155±65 963) μm²，t=8.71，P=0.00］ and proportion of vital bacteria ［(26.0%±16.4%) vs.(92.2%±10.9%)，t=19.93，P=0.00］ were significantly fewer than those of the control. While, for the 21d’s biofilm, the vital bacteria counts and the percentage of the vital cells of the bleaching group were more than those of the control group significantly ［(66 262±23 772) μm² vs. (51 184±20 502) μm²，t=2.59，P=0.012］.Conclusion:The hydrogen peroxide-containing bleaching agent may inhibit the growth of Streptococcus mutans biofilm for about 3 weeks; but after 3 weeks, it seems that the bleached surface will increase the growth of biofilm. Whether the whitening therapy will increase caries susceptibility of the bleached surface needs further research.

Key words: Tooth bleaching, Dental enamel, Biofilms, Streptococcus mutans, Microscopy, confocal