pH Response and Tooth Surface Solubility at the Tooth/Bacteria Interface

Evaluating the physiochemical processes at the tooth surface/bacteria interface is important for elucidating the etiology of dental caries. This study aimed to compare the mineral solubility and protein degradation of coronal enamel (CE) and root dentin (RD), and investigate the involvement of dissolved components in bacteria-induced pH changes using a model of tooth/bacteria interface. An experimental apparatus forming a well was made of polymethyl methacrylate, and a bovine tooth (CE or RD) specimen was fixed at the bottom of the well. A miniature pH electrode was placed on the tooth, and Streptococcus mutans NCTC 10449 cells, grown in 0.5% glucose-containing complex medium, were packed into the well. The pH at the tooth/S. mutans interface was monitored continuously for 120 min after the addition of 0.5% glucose at 37°C. S. mutans cells were recovered from the wells, and the amounts of lactate and calcium were measured using a portable lactate meter and a fluorescent dye, respectively. Proteolytic activity was also evaluated fluorometrically. The pH of the RD/S. mutans interface was significantly higher than that of the CE/S. mutans interface (30 min: 6.37 ± 0.12 vs. 6.18 ± 0.11, 60 min: 6.08 ± 0.14 vs. 5.66 ± 0.27, 90 min: 5.49 ± 0.24 vs. 5.14 ± 0.22, p < 0.05). Greater amounts of calcium were dissolved from RD (3.19 ± 0.74 µg/mL) than from CE (1.84 ± 0.68 µg/mL; p < 0.05), while similar amounts of lactate were produced. Proteolytic activity was not detected at any of the interfaces. These results indicate that RD is more soluble to bacteria-induced acidification than CE. This method can contribute to the evaluation and development of caries-preventive materials.