**2. Antimicrobial photodynamic therapy (aPDT)**

The mechanisms of aPDT are basically the same of PDT for tumors, based on the combined action of three elements: a photosensitizer (PS), a light source at appropriate wavelength to excite the PS and molecular oxygen (O2) in the target tissue.

The photodynamic process inactivating microorganisms occurs through the action of reactive oxygen species (ROS) that destroy vital constituents of fungi, bacteria, viruses and protozoa. In 1933 Jablonski published his article explaining the electronic states of a molecule and the transitions between them [18]. In this famous "Jablonski's diagram", we understand how a photosensitizer in the singlet ground state, moves to the excited singlet state after absorbing photons from a light source. And through the process named "intersystem crossing", a spin inversion occurs and then, this molecule goes to the excited triplet state, giving it conditions to transfer energy (type II mechanism) or electron (type I mechanism) to O2, generating ROS.

For antimicrobial purposes, the photodynamic action will take place within the cells or at the extracellular matrix of the microbial biofilm where the photosensitizer molecules are present, the main sites being the outer membrane or cell wall, membrane lipids and lipopolysaccharides. The singlet oxygen produced has a very small radius of action, less than 0.02 μm, so the damage produced by PDT will only be in the presence of the photosensitizer and under photoactivation. As a result, cell death is caused by cell wall or membrane lysis and/or inactivation of proteins or enzymes essential for microbial metabolism [19].
