Antimicrobial photodynamic therapy on Streptococcus mutans is altered by glucose in the presence of methylene blue and red LED

Background: Dental caries are a multifactorial disease that progressively produces tooth destruction as a result of bacterial colonization of enamel surface, especially Streptococcus mutans. The objective of this work was to investigate the role of glucose in antimicrobial photodynamic therapy (aPDT) on S. mutans. Methods: S. mutans ATCC 25175 were cultured on microaerophilia at 37 degrees C for 48 h, and we tested aPDT in the presence of 50 mM glucose. Bacterial suspension was used to investigate aPDT with 100 mu M methylene blue (MB) under LED emitting radiation at lambda = 660 nm and parameters as following (P = 473 mW; I = 166.8 mW/cm(2), and doses of 5, 10 and 20 J/cm(2)). A seventy-two hours biofilm was grown on 96 flat buttoned well-plate and irradiation was performed from 10 to 80 J/cm2 at similar conditions. Results: There was no dark toxicity nor bacterial death regarding LED irradiation on suspension and on biofilm. Nevertheless, aPDT presented expressive bacterial inactivation following 1 and 2 min of irradiation on cell suspension. On the other hand, there was no inactivation in the presence of glucose under the same conditions. Biofilm was completely inactivated by MB-mediated aPDT after 6 min of irradiation. However, the presence of glucose delayed the complete inactivation of the biofilm. Conclusion: The presence of glucose in the suspension drastically delayed the effect of aPDT on S. mutans and this effect is more pronounced in bacterial suspension than on biofilm. (AU)