Antibacterial Efficacy of N-Acetyl Cysteine Combined with Two Photodynamic Therapy Protocols Against Enterococcus faecalis in Infected Root Canals: An In Vitro Study

Aim: Enterococcus faecalis (E. faecalis) is a resilient endodontic pathogen associated with persistent root canal infections. The objective of this investigation was to assess and compare antibacterial efficacy of N-acetyl cysteine (NAC) alone and in combination with two photodynamic therapy (PDT) protocols utilizing methylene blue (MB) and indocyanine green (ICG) as photosensitizers. Methodology: Seventy-eight extracted human mandibular premolars were instrumented, sterilized, and inoculated with E. faecalis (MTCC 439). After 14 days of incubation, samples were randomly divided into 3 groups (n = 26 each): Group I – NAC alone; Group II – NAC + PDT with ICG (1 mg/mL, 810 nm diode laser); Group III – NAC + PDT with MB (20 µg/mL, 660 nm diode laser). Bacterial samples were collected before and after disinfection using sterile paper points, and quantified via colony-forming unit (CFU) analysis on Brain Heart Infusion (BHI) agar. Data were log10-transformed and analyzed using paired t-tests and one-way ANOVA with Tukey's post hoc test (P<0.05). Results: All groups demonstrated significant CFU reduction post-treatment (P<0.0001). Mean CFU reductions were: Group I – 0.84 ± 0.29, Group II – 1.52 ± 0.54, and Group III – 1.35 ± 0.47. Group II & III showed significantly greater reductions than Group I (P < 0.0001 and P=0.0003, respectively). No significant difference was observed between Group II & Group III (P=0.3554). Conclusion: NAC combined with either MB- or ICG-mediated PDT significantly improved disinfection of E. faecalis-infected root canals compared to NAC alone. This combination offers a promising adjunctive approach for managing persistent endodontic infections.