Figure 1: The mechanism of antibacterial PDT. Photosensitizers can be preferentially uptaken by bacteria, accumulating inside the bacteria and in the cytoplasm membranes, or in the vicinity. Upon absorption of a photon by the ground-state photosensitizer after light illumination, the reactive oxygen species (ROS) will be generated from two alternative pathways: type I mechanism and type II mechanism. The generated ROS then react rapidly with their environment depending on the localization of the excited photosensitizer: bacteria cell wall, lipid membranes, proteins and enzymes, and nucleic acids. The reaction of these important cellular components may result in necrosis or apoptosis of the bacteria at last.