Effects of Acidic Exposure on Surface Properties and Bacterial Adhesion of Three Resin-Based CAD/CAM Materials

CAD/CAM reinforced resin composites are increasingly used in restorative dentistry because of their favorable mechanical and esthetic properties. This in vitro study evaluated the effect of acidic exposure on surface properties and Streptococcus mutans biofilm formation on three CAD/CAM resin composites: a short-fiber-reinforced composite (SFRC), a woven glass fiber-reinforced composite (TRINIA; TR), and a resin nanoceramic-reinforced composite (Lava Ultimate; LU). Twenty-four specimens from each material were prepared and assessed for surface roughness and wettability using an optical profilometer and contact angle goniometer before and after 24 h of storage in neutral (pH 7) or acidic (pH 4) buffer solutions (n = 12 per group). Surface morphology was further examined using scanning electron microscopy (SEM). Biofilm formation was quantified using S. mutans colony-forming unit (CFU) counts, followed by confocal laser scanning microscopy (CLSM) for biofilm architecture and viability. At baseline, TR exhibited the highest roughness values (0.744 to 0.785 µm, p < 0.001), whereas no significant differences in contact angle were observed among materials. Acidic exposure significantly increased surface roughness and reduced contact angle values in all examined materials (p < 0.001). Compared to storage in neutral conditions, bacterial adhesion was significantly reduced in TR and LU stored in acid (p < 0.001), but SFRC showed no significant change (p = 0.079). Among specimens exposed to acidic conditions, TR exhibited the highest bacterial adhesion (3.24 log₁₀ CFU/mm²) relative to SFRC and LU (p < 0.001). CLSM analysis demonstrated denser biofilm accumulation on TR and LU compared with SFRC. These findings demonstrate that acidic exposure significantly affects the surface properties and bacterial adhesion behavior of CAD/CAM resin composites in a material-dependent manner. SFRC showed no statistically significant difference in S. mutans CFU/mm² between neutral and acidic storage conditions despite measurable surface changes, highlighting its potential suitability for clinical applications associated with acidic oral environments.