Dental acid etchant as a sensitizing agent in photodynamic therapy to reduce S. mutans in dentinal carious lesions

The study aims to assess the utility of dental acid etchant containing 37% phosphoric acid and methylene blue dye (DAE) as a sensitizing agent for photodynamic therapy (PDT) to reduce Streptococci mutans in dentinal caries. Forty-five permanent third molars were sectioned and the coronal dentin exposed. A cariogenic challenge was performed using brain-heart infusion (BHI) supplemented with 0.5% yeast extract, 1% glucose, 1% sucrose, and S. mutans ATCC 25175 standardized to 0.5 McFarland turbidity. Specimens were incubated in anaerobic jars at 37 °C for 15 days. During this period, BHI broth was renewed every 24 h. After 15 days, specimens were randomly divided into three groups (n = 15): DAE, application of dental acid etchant containing 37% phosphoric acid and methylene blue dye for 15 s; LLL, application of low-level laser (wavelength 660 nm, energy 4 J/cm², power 5 W) for 15 s; and PDT, application of DAE for 15 s followed by LLL irradiation (660 nm, 4 J/cm², 5 W). Carious tissue from each specimen was collected before and after the applications. Five decimal dilutions were performed and the resulting solution was seeded in mitis-salivarius-bacitracin agar. Plates were incubated in anaerobic jars at 37 °C for 48 h. Analysis of variance (ANOVA) with post hoc Tukey's test was used to compare total S. mutans counts. Significant reductions in S. mutans were observed after DAE application (40.70%, p < 0.0001), LLL (12.35%, p = 0.0036), and PDT (55.22%, p < 0.0001). Dental acid etchant containing 37% phosphoric acid and methylene blue dye can be used as a photosensitizing agent for PDT to reduce S. mutans burden in dentinal caries.