## 1 Introduction  
Root canal infections are initially treated by removing the infected tissues and disinfecting the root canal system with instrumentation and irrigation protocols. After that, clinicians perform an adequate root canal and apical sealing with the obturation and place an appropriate coronal sealing with a permanent restoration [1]. The root canal treatment usually saves the tooth and clears the infection; however, endodontic treatment can still fail [2]. Different factors have been identified as contributors to endodontic treatment failure, such as untreated or inadequate filling of the canals. However, pathogenic biofilms' persistence after the standard disinfection has been strongly attributed to endodontic failure and is known as one of the prime causes of unsuccess [3]. The presence of biofilms of putative pathogens trapped inside dentinal tubules, its ramifications and its relation with the recurrence of clinical symptoms and presence of periapical radiolucency is well reported in the literature [4]. The geometry of dentinal tubules with a narrow lumen (1-2 \(\upmu\)m) and considerable length (2-3 mm) challenge the disinfection. Previous reports have shown bacterial migration into dentinal tubules of the root at a depth of approximately 360-420 \(\upmu\)m [5]. In contrast, the irrigating solution, most commonly, sodium hypochlorite (NaOCI), has penetrability into dentinal tubules of approximately 130 \(\upmu\)m. Over and above, the quantification of penetration of the photosensitizer (PS) Toluidine Ortho Blue (TBO) into the dentin substrates via confocal Raman micro-spectroscopy noted the maximum depth of penetration of 45-60 \(\upmu\)m [6]. As illustrated in Figure 1, using confocal laser scanning microscopy, viable bacterial cells of _E. faecalis_ strain, labeled to show fluorescence in blue, penetrate deep into the dentinal tubules. The root canal system is very complex with an isthmus, lateral canals, accessory canals, and root canal ramifications [7]. Such complexity offers a challenge for optimal disinfection of the root canal system. These areas of isthmus and ramifications can harbor putative pathogens that may evade the antimicrobial activity of conventional disinfection protocols, including instrumentation and irrigation of the canals with auxiliary chemical substances. Previous investigations have shown that conventional root canal therapy is limited to 44% to 70% of disinfection [6; 8]. Moreover, studies, using culture-dependent approaches, have indicated bacterial growth after root canal disinfection completion [9]. Limited putative pathogens can survive in the root canal system with the lack of nutrients after the endodontic treatment [10]. _E. faecalis_ is a Gram-positive bacterium that is frequently recovered from persistent endodontic infections [11]. _E. faecalis_ is detected in 33% of persistent endodontic infections [12]. This pathogen can be found as a monoculture in root canals with failed treatment [13]. _E. faecalis_ has unique characteristics to survive, even after disinfection. This pathogen can penetrate the dentinal tubules, persist in a low-nutrient environment and pH, resist high salinity and temperatures, survive even with the presence of intracranial medications and irrigants, develop antibiotic resistance, and form biofilms in medicated canals [14]. Overall, the complexity of the root canal system and the current infection protocols offer a challenge for the success of root canal therapy. Therefore, dentist-scientists are constantly searching for additional disinfection approaches to improve root canal disinfection. Figure 1: Representative confocal microscopic image of _Enterococcus faecalis_ colonizing inside dentinal tubules (20\(\times\) magnification) may complicate the chemo-mechanical disinfection of root canal therapy. tion. Antimicrobial photodynamic therapy (aPDT) has been proposed as a supplemental approach to optimize root canal disinfection.