Adhesion, Biofilm Formation and Plaque-Related Potential of Streptococcus mutans on Orthodontic Materials: An In Vitro Comparative Study

Background: Orthodontic appliances introduce new surfaces into the oral cavity that can modulate biofilm formation and potentially increase the risk of white spot lesions. Material-dependent differences in surface roughness, wettability and geometry may influence early colonization by Streptococcus mutans, a key cariogenic pathogen. Objectives: To compare early adhesion and biofilm formation of Streptococcus mutans on five commonly used orthodontic materials: stainless-steel (SS) and nickel–titanium (NiTi) archwires, metallic and ceramic brackets, polymethyl methacrylate (PMMA) acrylic resin. Materials and Methods: Standardized specimens were prepared, polished when applicable, sterilized, and conditioned in artificial saliva. The tested materials included SS and NiTi archwires (3M Unitek, Monrovia, CA, USA), metallic and ceramic brackets (Ormco, Orange, CA, USA), and PMMA acrylic resin (GC Corporation, Tokyo, Japan). Early adhesion (CFU), biofilm biomass (crystal violet), and metabolic activity (XTT) were quantified after incubation with S. mutans. Surface roughness (Ra) and contact angle were measured, and correlations with microbiological endpoints were assessed. Results: A clear material-dependent gradient was observed. Stainless steel showed the lowest early adhesion and biofilm formation (5.20 ± 0.28 log10 CFU·cm−2; CV OD590 = 0.60 ± 0.14), followed by NiTi, metallic brackets, and ceramic brackets, while PMMA exhibited the highest bacterial load and biofilm biomass (6.09 ± 0.32 log10 CFU·cm−2; CV OD590 = 1.10 ± 0.17). Overall differences between materials were statistically significant (p < 0.0001). Surface roughness and contact angle positively correlated with bacterial colonization. Conclusions: Early S. mutans colonization is strongly influenced by orthodontic material properties, with smoother and less hydrophobic surfaces showing reduced biofilm formation. PMMA and bracket structures may pose higher cariogenic risk during treatment. These findings support the development of surface-engineered or biofilm-resilient orthodontic materials.