Influence of Surface Modification and Artificial Aging on Streptococcus mutans Biofilm Formation and Oral Cell Responses to Orthodontic PMMA: An In Vitro Study

Background/Objectives: Photoreactive zwitterionic PMBPAz coatings have previously been used to modify dental PMMA, but their integrated performance on clinically polished orthodontic PMMA under a specified composite-aging challenge remains insufficiently characterized. This in vitro study compared untreated, polished, and PMBPAz-MPC-coated PMMA before and after laboratory aging. Materials and Methods: Heat-polymerized PMMA specimens were tested for early Streptococcus mutans adhesion, 24 h biofilm indices, surface chemistry and wettability, protein adsorption, mechanical properties, and human gingival fibroblast responses. A separate four-time-point MPC-PMMA series evaluated artificial saliva immersion alone, while the composite-aged groups also underwent thermocycling and simulated toothbrushing. Results: Compared with polished PMMA, MPC-PMMA showed lower 2 h adhesion by 0.73 log10 CFU/cm2 (81%) before aging and 0.62 log10 CFU/cm2 (76%) after composite aging; the corresponding decreases in crystal violet biomass were 44.6% and 37.9%. In the saliva-only series, coating-associated XPS phosphorus decreased from 3.8 to 3.3 at.%, and the contact angle increased from 18.6° to 21.7° at T28; following composite aging, the corresponding values were 2.9 at.% and 24.3°. Extract-exposed fibroblasts showed group-dependent changes in LDH and inflammatory mediators. Conclusions: Under the specified in vitro conditions, PMBPAz-MPC treatment of polished orthodontic PMMA was associated with reduced early S. mutans colonization, including after composite aging. The findings do not establish clinical efficacy or long-term biological safety.