**1.2 Photosensitizers and their mechanism of action**

PACT is based on the exposure of bacteria to photosensitive compounds—photosensitizers (PSs). When a PS located in the bacteria or on the bacterial surface is exposed to light (usually visible), it transfers from its low-energy ground state to an excited singlet state. Return of the PS to its ground state is accompanied by either emission of fluorescence or transition of the PS to a longer-living, higherenergy triplet state (PS\*) via intersystem crossing. The PS\* in turn reacts with surrounding molecules to form free radicals and hydrogen peroxide (Type I reaction) or transfers its energy to molecular oxygen to produce singlet oxygen and other highly reactive oxygen species (ROS; Type II reaction) [9, 10]. Type I and Type II reactions occur simultaneously, and the ratio at which they occur depends on both the PS type and the surrounding conditions. A detailed description of the photosensitization process can be found in the recent reviews of Castano et al. [11] and Cieplik [10]. ROSs formed in this process oxidize biomolecules, damage the cell membrane, and ultimately lead to cell death [12]. PACT usually proceeds predominantly through Type II processes. However, since Gram-negative bacteria are more susceptible to OH. radicals than to singlet oxygen, the Type I reaction may be more efficient against such microorganisms [13, 14].
