Antimicrobial Photodynamic Therapy: Basic Principles

PDT of infectious diseases of microbial origin is based on the property of selected photosensitising agents, especially those characterized by the presence of positively charged functional groups, to readily bind with the outer wall of bacterial, fungal and protozoan cells; once activated by irradiation with visible light, the photosensitisers generate reactive oxygen species which induce the irreversible modification of specific constituents of the wall. The consequent enhanced permeability of the wall allows sufficiently large doses of the photosensitiser to penetrate to the cytoplasmic membrane. The photoprocess then involves a number of endocellular activities (e.g., enzymic catalysis, metabolic pathways) whose integrity is critical for cell survival. The genetic material is generally involved in the late stage of the photoprocess, which minimizes the probability of inducing mutagenic processes. A suitable choice of the irradiation protocol usually leads to an extensive killing of microbial pathogens with no or minimal damage to host tissues. PDT differs from other usually adopted therapeutic modalities for the broad spectrum of action, the efficacy against antibiotic resistant cells, and the lack of selection of photoresistant strains.