## 1 Introduction  
To date, the inappropriate and often excessive use of antibiotics has resulted in an increase in the prevalence of highly resistant pathogens. This phenomenon is considered a major threat extending to public health worldwide [1; 2]. Consequently, recent urgent calls to investigate new strategies for defeating the multi-antibiotic resistant microbial strains have emerged [3]. Photodynamic therapy (PDT) can serve as an effective alternative approach through its ability to inactivate resistant bacteria [4]. It can be described as a non-thermal photochemical reaction, where a non-toxic dye (photosensitizer--PS) is excited by light of an appropriate wavelength [5]. A long-lived triplet state of the PS is then produced; this can interact with molecular oxygen, leading to the generation of reactive oxygen species (ROS), including singlet oxygen (\({}^{1}\)O\({}_{2}\)), which may provoke cytotoxic effects coupled with the damage of critical biomolecules such as proteins [5]. Each of the three essential components(photosensitizer, light and oxygen) are quite harmless by themselves, but their combination in this manner gives rise to the generation of aggressively reactive and cytotoxic ROS, which exert damaging effects at both molecular and cellular levels [6]. PDT is characterized by a dual selectivity, which is based on (a) differences in bound photosensitizer concentrations between target and normal tissues, and (b) the degree of spatial confinement of the light applied onto the target area [7]. Various terms have been used to describe this treatment, and these include photo-activated chemotherapy (PACT), light-activated disinfection (LAD), photodynamic disinfection (PDD) and photoactivated disinfection (PAD) [8]. For antimicrobial purposes implemented in dental therapy procedures, and to avoid confusion with photodynamic therapy performed on tumours, antimicrobial photodynamic therapy (aPDT) is now regarded as the most suitable term [9]. Although more than a century has passed since the discovery of photodynamic therapy [10], this treatment is still not widely used. The main reason for this may be attributed to the multifactorial mechanisms of photomedicine. The mechanism of action of this modality correlates with several influencing factors, such as the photosensitizer applied, the light delivery system and the oxygenation level of the target tissue. A recent systematic review concluded that \"no safe recommendation on aPDT protocols can be extrapolated for clinical use at this point in time\" [11]. As such, the ideal protocol applied to maximize the benefits of aPDT remains largely unexplored. It has been suggested that tissue healing following photodynamic treatment is significantly improved. This can be explained by the fact that although the photosensitizer is topically administered to the target tissue, the delivered light scatters and diffuses beyond the area of interest. Consequently, a secondary therapeutic effect known as photobiomodulation (PBM) is exerted, which enhances the healing and repair of the surrounding tissues [12; 13]. PBM is considered to be an emerging therapy, with more than 50 years of intensive research supporting its deployment [14]. Indeed, in PBM, cells or tissues are basically exposed to light (lasers or LEDs) of low fluence, specifically within the 400-1000 nm wavelength range [15; 16]. The photon energy is transferred to specific chromophores and absorptive tissue molecules, such as cytochrome c oxidase in mitochondria and calcium channels in plasma membranes [17; 18]. PBM has been shown to increase adenosine triphosphate (ATP), nitric oxide (NO\({}^{\bullet}\)) and reactive oxygen species (ROS) production [19], promote DNA synthesis, accelerate tissue repair and induce stem cell proliferation and differentiation, through downstream effects [20]. As stated by [PERSON] and [PERSON], \"light therapy has many characteristics that can accurately be described as a Janus' effect\".