Antimicrobial activity of hydroxyl radicals generated by hydrogen peroxide photolysis against Streptococcus mutans biofilm

Prevention of dental caries with maximum conservation of intact tooth substance remains a challenge in dentistry. The present study aimed to evaluate the antimicrobial effect of H₂O₂ photolysis on Streptococcus mutans biofilm, which may be a novel antimicrobial chemotherapy for treating caries. S. mutans biofilm was grown on disk-shaped hydroxyapatite specimens. After 1–24 h of incubation, growth was assessed by confocal laser scanning microscopy and viable bacterial counting. Resistance to antibiotics (amoxicillin and erythromycin) was evaluated by comparing bactericidal effects on the biofilm with those on planktonic bacteria. To evaluate the effect of the antimicrobial technique, the biofilm was immersed in 3% H₂O₂ and was irradiated with an LED at 365 nm for 1 min. Viable bacterial counts in the biofilm were determined by colony counting. The thickness and surface coverage of S. mutans biofilm increased with time, whereas viable bacterial counts plateaued after 6 h. When 12- and 24-h-old biofilms were treated with the minimum concentration of antibiotics that killed viable planktonic bacteria with 3 log reduction, their viable counts were not significantly decreased, suggesting the biofilm acquired antibiotic resistance by increasing its thickness. By contrast, hydroxyl radicals generated by photolysis of 3% H₂O₂ effectively killed S. mutans in 24-h-old biofilm, with greater than 5 log reduction. The technique based on H₂O₂ photolysis is a potentially powerful adjunctive antimicrobial chemotherapy for caries treatment.