Photodynamic Therapy

Antimicrobial photodynamic therapy (aPDT) is an emerging strategy to treat antibiotic resistant micro- organisms. This technique is selective, painless, less invasive, and can be focused on multiple targets, thereby ensuring photodestruction of target cells without producing photoresistant cells. The antimicrobial effect of this strategy relies on an oxidative burst that occurs when photoreactive molecules are exposed to certain wavelengths of light. A nontoxic dye or photoreactive molecule called a photosensitizer, light of suitable wavelength, and molecular oxygen are the three major compo nents involved in the photodynamic inactivation of microbial cells. Generation of reactive oxygen species (ROS) plays a key role in aPDT. The excited photosensitizer, after illumination with a light of suitable wavelength, enters into an unstable triplet state resulting in ROS generation, which occurs through two different photoprocesses. In a type I photoreaction, electron transfer from the triplet state photosensitizer to the surrounding oxygen occurs and results in the formation of super-oxide, peroxide, and hydroxyl radicals. In a type II photoreaction, energy is transferred directly from the triplet state photosensitizer to molecular oxygen, thereby forming singlet oxygen which is highly reactive. ROS can act on a diverse range of cellular targets, causing death of a broad range of pathogens by damaging almost all biomolecules including lipids, protein, and nucleic acids