Aim: This study was conducted to assess the effect of different variables on antibacterial potential of photo-activated disinfection versus the effect of a chemical disinfection solution on carious dentin. Material and Methods: Seventy freshly extracted teeth were selected, All soft caries was removed by sterile excavator and specimens were weighted by five digit microbalance to be in range of 6-10 mg, Specimen divided into three groups: Group A1: disinfected with phot-activated disinfection (PAD) and divided into four subgroups(no=10 ) according to power& application time, Subgroup1(P1T1
: delivery power was 100mW, application time was 120 sec) Subgroup2 (P1T2: delivery power was 50mW application time was 30 sec) Subgroup3 (P2T1: delivery power was 100mW application time was 120 sec) Subgroup4 (P2T2: delivery power was 50mW,application time was 30 sec).Group A2: disinfected with chemical disinfectant (chlorohexidine scrub 2%).Group A3: disinfected with laser beam only & divided into two subgroups P1T1,P2T2. The specimens were plated &duplicated on blood agar then incubated in anaerobic gas pack jar at 37C for 7 days. Results: A statistically significant
difference was found among PAD, chlorohexidine, and laser groups in the percent decrease in the total bacterial count (P≤ 0.05). The greatest mean percent decrease was recorded in the PAD P1T1 followed by the cholorohexidine group, then the laser P1T1. Conclusion: The combination of toluidine blue O (TBO) and diode laser could be effective in reducing the bacterial viability. Chlorohexidine as chemical disinfectant was equally effective to photo-activated disinfectant with high delivery power and long contact time in reducing the total bacterial count.