Photodynamic Therapy: A Viable Alternative Strategy to Control Microbial Invasions

Antimicrobial photodynamic therapy (aPDT) is a new-age therapeutic technique that by principle, focuses on the eradication of target cells by highly cytotoxic reactive oxygen species (ROS) generated through the activation of a chemical photosensitizer (PS) molecule with visible light of appropriate wavelength. The cytotoxic species can arise via two main mechanisms known as Type I and Type II photoreactions: the former leads to the generation of ROS and the latter to the formation of the singlet oxygen. These highly reactive oxidants can bring about instantaneous oxidation of a great array of biological molecules, causing havoc to the target cell. This technique provides significant advantages over conventional antimicrobial therapies in practice which are now facing the burning threat of growing complete resistance against them. To combat this world-wide health concern, new treatment strategies are the need of the time while ensuring no further rise of resistance against those alternative therapies, and aPDT appears to be highly promising in this aspect by fulfilling all the demands at the same time. It appears not only equally effective at killing both antibiotic-sensitive and multi-resistant bacterial strains, but also highly selective, non-invasive and rapid in action than other antimicrobial agents, and there have been no reports of resistance till date. The success of this phototherapy relies on several factors, including the target cell type, reaction conditions, and the type, molecular structure and cytolocalization of the PS; because its potency depends on the distribution, high reactivity and short lifetime of ROS as well as the PS itself in electronically excited states.