_Article_  
**Evaluation of Photodynamic Therapy Using a Diode Laser 635 nm as an Adjunct to Conventional Chemo-Mechanical Endodontic Procedures against**  
_Enterococcus faecalis_ Biofilm: Ex-Vivo Study  
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Entercoccus faecalisis a Gram-positive facultative anaerobe. In 24% to 77% of cases with secondary endodontic infections and peri-radicular lesions, _E. faecalis_ was isolated [3,4]. This may be due to its ability to penetrate deeply in the dentinal tubules, leading to difficulty in its elimination after mechanical instrumentation with common antimicrobial irrigation. Additionally, it has been reported that _E. faecalis_ can resist the antimicrobial effects of intracanal dressings with calcium hydroxide for 10 days [1,5]. Eradication of endodontic infection, elimination of microorganisms, and prevention of further infection are the main goals of endodontic treatment. With the available conventional treatment protocols, it is almost impossible to achieve these goals due to the presence of many factors, such as the complex anatomical structure of the endodontium, the biofilm lifestyle of bacteria, the increase of antibiotic-resistant strains, and the incapability of irrigants to penetrate the dentinal tubules at the apical portion of root canals [6, 7, 8]. In order to improve the antimicrobial capacities of conventional endodontic techniques, new approaches, instruments, and medications have been proposed. Photodynamic therapy (PDT) in the endodontic field has been proposed as an adjunctive modality to the conventional treatment protocols, especially for the management of refractory endodontic infections [7, 8, 9, 10]. The concept of PDT is the activation of a photosensitizing agent (PS) by light exposure in the presence of oxygen. After light irradiation, the PS transfers from its ground state into an excited state, leading to the production of highly reactive oxygen species (ROS). The resultant ROS may have the ability to target and destroy biomolecules present in the cell walls of bacteria [11,12]. In addition, PDT can stimulate bone formation in the peri-radicular area, resulting in acceleration of the healing process of bone [4]. One of the most remarkable advantages of PDT is its efficiency against multidrug-resistant strains without developing bacterial resistance or modifying bacterial sensitivity to antibiotic therapy with repeated applications of PDT [13,14]. There are many PSs that have been employed in PDT, such as phenothiazazines (including Methylene Blue (MB) and Toluidine Blue (TB)), phytotherapeutic agents, cyanine, hematoporphyrin derivatives, and phthalocyanines [13]. Recent systematic reviews have found that the outcome of PDT in removing endodontic biofilms from infected root canals is still not completely clear. This may be due to the diversity in the studies' methodology and PDT protocols [13, 14, 15]. Some authors have demonstrated better results of PDT through improving the evaluation methodology of the antibacterial effect [4]. Others attributed the reduced efficacy of PDT to the reduced penetration ability of PSs; therefore, they have suggested improving the pharmacological characteristics of PSs using semi-synthetic PSs (e.g., chlorophyll derivative [Zn(II)e6 Me]) or nanoparticles containing PS (e.g., rose Bengal-functionalized chitosan nanoparticle (CSRBnps)) [8,16]. The optimization of the antimicrobial effect of PDT might be achieved through the standardization of the PDT protocol. It is believed that studies with similar PS type and light source cannot be compared when the irradiation protocol, irradiation time, laser parameters, and PS concentration are different [13]. Therefore, carrying out further studies to achieve a standardized protocol of PDT is still recommended [13, 14, 15].