**1.1 History of photodynamic therapy**

The therapeutic properties of light were observed already in ancient Greece, Egypt, and India. However, they were not widely used for many centuries [1]. The history of modern photodynamic therapy (PDT) dates back to 1900, when Oscar Raab discovered the toxic properties of the dye acridine red on *Paramecium* spp. [2]. He and his supervisor, Hermann von Tappeiner, noticed a positive effect of illumination on the toxic activity of this dye. In his later work, von Tappeiner and his colleagues applied this approach to inactivation of bacteria [3] and to treatment of skin cancer [4]. In 1909, von Tappeiner introduced the term "Photodynamic Action" and showed that oxygen is essential for this procedure [5]. PDT has been studied and developed as an anticancer therapy for a long time and was approved by the Food and Drug Administration in the 1990s for various applications in this area of medicine [6–8]. The antimicrobial properties of this approach were unfairly forgotten for several decades. However, interest in antibacterial PDT has been rekindled and is continuously increasing because multidrug resistance of pathogenic microorganisms has become a serious threat to public health. Photodynamic antibacterial chemotherapy (PACT) has become a promising approach for combating bacterial infections, which are resistant to modern antibiotics.
