Background and Aim:  With increasing demand for orthodontic treatment among adults and with changing oral flora and increased number of white spot lesions around orthodontic appliances in patients undergone orthodontic treatments and also with controversial reports about the role of different brackets types on adhesion of Streptococcus mutans, this study was aimed to compare the effect of different orthodontic bracket material and design (metal, ceramic, self -ligating) on growth and adherence of streptococcus mutans.
Materials and methods: This experimental in vitro study was accomplished on 60 premolar bracket samples. Three types of brackets (metallic, ceramic and self-ligating) were used for comparison of streptococcus mutans adhesion. Each of the brackets was placed into small size plates containing 15cc of sterile Nutrient broth with %20 sucrose. Then 5cc of bacterial suspension were added to each plate. Plates were incubated for 5 days at 37°C. Then, after removal of the nutrient broth liquid from plates, the plate was turned upside down and the number of colonies attached to the plate was counted by eye according to Colony forming unit (CFU). In order to confirm the results of the first phase of the study, the contaminated brackets were placed in the blood agar medium. The results of the second test confirmed the validity of the first step. Data were analyzed by ANOVA test.
Results: The results of the study showed that the maximum streptococcus mutans adhesion was belonged to self-ligating (51.25±15.01), followed by metal (21.8±7.1) brackets and there was lowest adhesion was to ceramic brackets (15.5±3.6) and the differences were statistically significant (p<0.001). The second stage of the study also confirmed the results of the first stage (p<0.01).
Conclusion: The growth and adhesion of Streptococcus mutans is different in the presence of various orthodontics bracket material and bracket designs.  In an in vitro setup, the self-ligating metallic brackets showed higher adhesion and growth of Streptococcus mutans than conventional twin metal brackets, followed by Ceramic brackets.