Objective    To study the effect of toluidine blue-mediated photodynamic therapy on biological membrane formation of dental plaques and its mechanism for the control of dental plaques and prevention of dental caries.     Methods    Biological membrane of dental plaques caused by 4 cariogenic oral bacteria was treated with photodynamic therapy. Concentration of the photosensitizer was 0.1 mg/mL and the light intensity was 94.5 J/cm

2

. Effect of photodynamic therapy on biological membrane formation of dental plaques was observed under a scan electron microscope  in 1

3 and 7 d after photodynamic therapy.     Results     In the control group

Streptococci were adhered to the acquired membrane

1 day. The number of different bacteria reached their peak in 3 d

with a grainy spike-like structure. A 3-dimensional structure of biological membrane was found in 7 d with massive matrix and irregular colloid substance. The number of bacteria was significantly decreased and the formation of biological membrane was inhibited in 1

3 and 7 d after photodynamic therapy.     Conclusion    Photodynamic therapy can inhibit the formation of biological membrane of dental plaques.