## 3 Results  
Comparison of the Anti-Bacterial Effect of 2.5% NaOCl with and without Er:YAG Laser Treatment on E. faecalis Biofilm in Tooth Root Canals  
In the first series of experiments, we wanted to know if Er:YAG laser treatment following 2.5% NaOCl irrigation provides a better anti-bacterial effect than 2.5% NaOCl irrigation alone. To this end, _E. faecalis_ biofilms were allowed to be formed in the tooth root canal prior to treatment for 28 days. Thereafter, the root canals were washed with 2.5% NaOCl solution for 60 s with or without Er:YAG laser treatment. In addition, the last group of infected root cannels were treated in the serial order of 17% EDTA solution, NaOCl solution and Er:YAG laser exposure. Whereas irrigation with NaOCl caused a 98.0 \(\pm\) 0.6% reduction in the bacterial count (\(p<0.01\); Figure 2), the combined treatment with Er:YAG laser caused an even stronger reduction of 99.77 \(\pm\) 0.14% (\(p<0.01\) in comparison to control; Figure 2). Exposing the root canals to 17% EDTA prior to NaOCl and Er:YAG laser treatment, caused even a further reduction in CFU to 99.95 \(\pm\) 0.04% (with \(p<0.01\)in comparison to control; Figure 2). These data demonstrate the importance of adding an Er:YAG laser-Endo tip to the root treatment with NaOCl and EDTA. Comparison of the Anti-Bacterial Effect of EDTA Solution with and without Er:YAG Laser Treatment on E. faecalis Biofilm in Tooth Root Canals  
Next, we compared the effect of adding Er:YAG laser treatment to root canals irrigated with 17% EDTA. The average value of colonies in the group treated with EDTA irrigation (blue) decreased by 69.0 \(\pm\) 9.1% when compared to the control group (black) that was only infected by the bacteria without treatment (_p_ < 0.01; Figure 3). In the group treated with an Er:YAG laser-Endo tip and a 17% EDTA irrigation (green) and the group treated with an Er:YAG laser-Endo tip and 17% EDTA following a 2.5% NaOCl irrigation (red), the CFU was reduced by 93.4 \(\pm\) 1.61% and 99.95 \(\pm\) 0.04%, respectively, when compared to untreated infected root samples (_p_ < 0.01; Figure 3). Figure 2: Comparison of the anti-bacterial effect of Er:YAG laser–Endo tip alone or combined with 2.5% NaOCl. The number of _E. faecalis_ colonies (\(\times 10^{4}\) CFU/mL) observed 24 h after treatment of the infected teeth using the following protocols: Group 1: The CFU of _E. faecalis_ in the tooth root canal without treatment (Control; black). Group 2: The CFU of _E. faecalis_ after 2.5% NaOCl irrigation (blue). Group 4: The CFU of _E. faecalis_ after treatment with the Er:YAG laser-Endo tip combined with 2.5% NaOCl irrigation (green). Group 8: The CFU of _E. faecalis_ after treatment with the Er:YAG laser-Endo tip combined with 17% EDTA irrigation and 2.5% NaOCl irrigation (red). N = 8 in each group. *_p_ < 0.01 for treated samples compared to control. \(p\) < 0.02 for Er:YAG with NaOCl versus NaOCl alone. No significant difference (_p_ = 0.28) between Er:YAG with NaOCl and Er:YAG with NaOCl and EDTA. Comparison of the Anti-Bacterial Effect of Er:YAG Laser-Endo Tip Combined with NaOCl and/or EDTA Irrigation of Infected Teeth on E. faecalis Biofilm in Tooth Root Canals  
Here, we wanted to compare the effect of Er:YAG laser-Endo tip treatment alone and in combination with various irrigation combinations of NaOCl and EDTA. The average number of _E. faecalis_ colonies in the group treated with an Er:YAG laser-Endo tip and saline irrigation decreased by 76.01 \(\pm\) 5.71% compared to the control untreated infected samples (\(p\) > 0.01; Figure 4). In the samples treated with an Er:YAG laser-Endo tip and a 17% EDTA irrigation step, the CFU was reduced by 93.44 \(\pm\) 1.61% in comparison to that of the control group (\(p\) > 0.01; Figure 4), this treatment was more efficient than the Er:YAG laser-Endo tip and saline irrigation group, but this was not statistically significant (\(p\) = 0.12; Figure 4).