Photodynamic Therapy (PDT) is based on the light-induced and photosensitizer-mediated overproduction of reactive oxygen species to kill harmful cells or tissues. The success of the photodynamic approach for treatment of cancer is based on the combination of multiple effects: PDT induces direct tumour cell death, damage to the microvasculature, and stimulation of a local inflammatory reaction. For direct cell killing, the mode of cell death triggered by PDT can be influenced by altering the treatment protocol and can lead to a desired ratio between necrosis and apoptosis most advantageous for complete tumour eradication. Today, world-wide approvals exist for the application of specific photoactive compounds against skin, bladder, brain, pancreatic and ophthalmic cancer. Combination therapies with chemotherapeutics widen the field of applications. The convincing benefits of the photodynamic principle – high effectiveness and negligible side-effects – have fired the development of its application against microorganisms, even if resistant against conventional treatment. Today, Photodynamic Inactivation (PDI) has been proven applicable against bacteria, fungi, viruses and parasites. PDI by this serves as very promising approach to treat local infections in humans.

The presentation gives an introduction into PDT, outlines the benefits of the photodynamic approach and discusses the relevance of the cell death mode for the outcome of the tumour treatment. Important physicochemical properties of the photoactive compounds for treatment of tumours and microorganisms are defined. Furthermore, PDI is suggested as central antimicrobial strategy in the One Health concept. Very recent developments regarding the application of PDI in food safety and plant protection are presented as well.

• PD Dr. Kristjan Plaetzer, Laboratory of Photodynamic Inactivation of Microorganisms, Department of Biosciences and Medical Biology, Paris Lodron University Salzburg