ABSTRACT
In recent years, a non-invasive treatment strategy for the control and management of various infections caused by bacteria, fungi, and viruses has been introduced: photodynamic therapy (PDT). PDT is an appropriate light-dependent reaction involving photochemistry and photophysics, leading to the generation of cytotoxic free radicals or reactive oxygen species (ROS) such as singlet oxygen. Antimicrobial photodynamic therapy (aPDT) serves as an alternative therapeutic method for the treatment of microbial infections without causing the emergence of resistant strains. There are three major counterparts of PDT: a light source, a photosensitiser, and oxygen. The photosensitiser upon light illumination excites to a higher energy state from the ground state and interact with cell macromolecules. The excited state of photosensitiser interacts with cell in either type 1 or type 2 reaction mechanisms. Type 1 reaction is electron transfer leading to the generation of superoxide, hydrogen, and hydrogen peroxide radicals by reacting with molecular oxygen. Type 1 pathways exert biological damage to the cell membrane integrity. Type 2 is an energy transfer pathway involving the production of singlet oxygen. These highly reactive state free radical exhibit cell membrane and cell wall damage leading to the major microbial damage. aPDT acts as non-selective mechanism where the multiple biological targets are proteins, lipids, and DNA. Cellular localisation of photosensitiser and diffusion length of free radicals decides the oxidative targets of aPDT. aPDT effectively used to combat both planktonic cells and biofilms. In biofilms, matrix, cell surface, and cytoplasm are the major targets of aPDT. This chapter provides a detailed understating of mechanism of action of aPDT.
