Abstract: Objective To investigate the effects of D-tryptophan (D-Trp) on the formation of Streptococcus mutans (S. mutans) biofilm and the dispersal of 24 h-old biofilm, and the drug susceptibility of S. mutans against chlorhexidine (CHX) under the role of D-Trp. Methods Optical density assay was used to evaluate the growth curve of S. mutans exposed to 5.0 mmol/L D-Trp for 28 h. The non-treated group was not added with D-Trp. After treatment with 1.0, 2.5 and 5.0 mmol/L D-Trp, crystal violet staining was used to observe the changes of S. mutans biofilm formation in treatment group and nontreatment group. Crystal violet staining and confocal laser scanning microscopy (CLSM) were applied to illustrate the effects of 1.0, 2.5 and 5.0 mmol/L D-Trp on the dispersal of 24 h-old S. mutans biofilm. Resazurin sodium was used to indicate the effect of 5.0 mmol/L D- Trp on the minimum inhibitory concentration (MIC) and the minimum biofilm inhibitory concentration (MBIC) of treatment groups and negative control group. Results The growth curves of planktonic S. mutans within 28 h was consistent in treatment group and the non-treated group, both attained exponential phase after 4 h and reached stationary phase at 22 h. Notably, when compared with non-treated group, the biomass of S. mutans biofilm was increased with time from 0 to 72 h after treatment with 1.0, 2.5 and 5.0 mmol/L D-Trp. And at the same time point, the biomass was significantly less in each subgroup of treatment group than that of non-treated group (P＜ 0.05). Crystal violet staining demonstrated that values of biomass（OD570） were less in treatment groups treated with 1.0, 2.5 and 5.0 mmol/L DTrp than those of non-treated group (P＜ 0.01). CLSM also showed that bacteria was adhered to the surface of media in treatment groups treated with 1.0, 2.5 and 5.0 mmol/L D-Trp. The values of biomass were lower in treatment groups than those of non-treated group (P<0.01). The MIC against S. mutans was 0.073 mg/L in both experimental group and negative control group. The values of MBIC were 0.293 mg/L and 2.344 mg/L in experimental group and negative control group, respectively. Under the action of 5.0 mmol/L D-Trp, the MBIC of S. mutans was reduced to 1/8. Conclusion Results indicate that D-Trp may inhibit the formation of S. mutans biofilm and promote the dispersal of biofilm already formed. DTrp may further help CHX exert its bactericidal activity to S. mutans.

Key words: Streptococcus mutans, biofilms, Chlorhexidine, microbial sensitivity tests, D- tryptophan, the minimum inhibitory concentration