## 4 Discussion  
The literature seems to be agreeing with the use of PDT as an adjunctive tool to the chemo-mechanical endodontic techniques, particularly in persistent endodontic infection conditions [13, 15]. [PERSON] et al. showed a significant reduction of diversity and number of bacteria in an in-vivo study on patients with secondary or persistent endodontic infections after PDT with TB [21]. The same results were demonstrated by [PERSON] et al., as it was found that the chemo-mechanical preparation combined with PDT with TB reduced the number of species of bacteria and no colony forming units were found in 11 out of 21 root canals with secondary endodontic infections [22]. [PERSON] et al. carried out a study to evaluate the effectiveness of PDT and NaCl irrigation against _E. faecalis_ with the same methodology as our study but a different PS type (MB). The study demonstrated relatively similar results, as the lowest survival rate was observed in the group of combined PDT with NaOCl. However, the survival rates were relatively lower in the 5.25% NaOCl group, PDT group, and combined PDT with NaOCl group when compared to the survival rates of our study groups [23]. Moreover, conflicting results were demonstrated in the literature. In a study on infected extracted teeth with _E. faecalis_, the greatest culture-negative samples were noted with the 3% NaOCl-irrigation-alone protocol in the secondary infection cases [24]. In fact, the heterogeneity of reported data and the absence of a standardized protocol of PDT were noted in a recent systemic review [15]. Many authors demonstrated the positive effect of PDT on _E. faecalis_ reduction on extracted human teeth when it was accompanied by various protocols [25, 26, 27, 28, 29]. Some authors demonstrated that the effect  
Figure 3: Box and whisker plot of mean bacterial load (\(\log_{10}\) CFUs/mm\({}^{2}\)) in each experimental group. of PDT in combination with various antimicrobial irrigants (NaOCl and EDTA) was more pronounced against _E. faecalis_, while the effect of irrigants alone was greater against _Streptococcus mutans_[25]. Others demonstrated that repeated PDT cycles enhanced the positive effect of PDT from 45% of _E. faecalis_ reduction to 95% and emphasized that the most effective antibacterial protocol is still the conventional chemo-mechanical debridement [30]. Others suggested the additional use of other irrigants such as EDTA or citric acid in combination with PDT and common antibacterial irrigants [29]. The heterogeneity of outcomes may be due to the presence of several sensitive factors in the PDT protocol that may affect the outcomes, including the type of PS, its concentration, its pre-irradiation time, the laser's wavelength, the irradiance time, the type of irrigant, its exposure time, the bacterial biofilms, and the utilized microbiological technique to evaluate the colony counts [14; 15; 23]. The selection of an appropriate PS should be based on the kind of the targeted bacterial species [13], whereas the antimicrobial capacity of PDT depends on the PS's abilities to interact with the bacterial membrane, to penetrate, to act inside the cell, and to form ROS after irradiance around the bacterial cell. Some authors found that Gram-negative bacteria, which are predominant in primary root canal infections, showed resistance against PDT because of the different outer membrane structure of Gram-negative bacteria, and because of the hydrophobic and charge effects of PSs [13]. In contrast, PDT showed clear effectiveness against Gram-positive bacteria--which play a role in secondary persistent root infections--due to the presence of relatively porous layer outside the cytoplasmic membrane of the bacteria that permits the diffusion of PS in sensitive sites of bacterial cells [12; 31]. The most commonly employed PSs in the endodontic field are TB and MB [15]. The antimicrobial effect of TB against Gram-positive and Gram-negative bacteria has been validated with more effectiveness against Gram-positive bacteria [31].