Evaluation of pH at the Bacteria-Restorative Material Interface

Methods: Four fluoride-releasing restorative materials (glass-ionomer cement, resin-modified glass-ionomer cement and resin composites 1 and 2) were used. Each specimen was immersed in 50 mM potassium acetate buffer at pH 5.5 or 50 mM potassium phosphate buffer at pH 7.0 for 4 wks. An experimental apparatus with a well (4.0-mm in diameter and 2.0-mm in depth) was made of acrylic resin with restorative materials or acrylic resin (control) at the bottom. The well was packed with cells of Streptococcus mutans NCTC10449 (SM), and a miniature ion-sensitive field-effect transistor (Model PH-60T1, Nihon-Koden, Tokyo, Japan) was placed at the interface between SM cells and materials. The pH was monitored after the addition of 1% glucose for 90 min, and the fluoride accumulated in the cells was quantified using a fluoride ion electrode (Model-9409-BN, Orion, Cambridge, MA, USA).

Results: After 4 wks at pH 5.5, glass-ionomer cement, resin-modified glass-ionomer cement and resin composite 1 inhibited the pH fall, and the final pH values (4.98±0.17, 4.82±0.25 and 4.75±0.08) were significantly higher (p<0.05, Dunnett's test) than control (4.35±0.06). After 4 wks at pH 7.0, glass-ionomer cement and resin-modified glass-ionomer cement inhibited the pH fall, and the final pH values (5.05±0.37 and 4.77±0.25) were significantly higher (p<0.05) than control (4.31±0.01). Fluoride was detected in the SM cells attached to these materials, in the order of inhibition.

Conclusion: This method was useful to assess the pH at bacteria-restorative material interface. The inhibitory effects of pH fall were probably due to fluoride released from materials during glucose fermentation. There was no conflict of interest. Supported by Research-and-Education-Funding-for-Inter-University-Research-Project (2007-2011), MEXT, Japan.