## 1 Introduction  
The current emergence and worldwide spread of multidrug-resistant bacteria are of great concern. Among bacteria that have developed multiple mechanisms of resistance against clinically relevant antibiotics, _Pseudomonas aeruginosa_ is particularly noteworthy. _P. aeruginosa_ is an opportunistic, nosocomial pathogen that causes infections of particular danger for immunocompromised and hematological patients, as well as patients with cystic fibrosis or burns [1; 2; 3; 4; 5]. This bacterium was found to be one of the most frequent pathogens associated with nosocomial pneumonia (including ventilator-associated pneumonia), septicemia, urinary tract infections, as well as surgical site and wound infections [6; 7; 8; 9; 10]. _P. aeruginosa_ is not only naturally resistant but can also acquire further resistance to multiple antibiotic classes, making it extremely difficult to treat. Moreover, _P. aeruginosa_ can survive and multiply in a nutrient-depleted environment, as well as produce many virulence factors. It is also known to produce robust biofilm, which complicates the treatment of severe infections appearing within the lower respiratory tract and chronic -wound infections [11; 12; 13]. Therefore, biofilm eradication appears to be one of the most important steps in the treatment of these infections [14; 15; 16]. Serious limitations of traditional antibiotic and chemotherapeutic approaches for the treatment of bacterial infections, mainly related to the emergence of multi-drug resistant bacteria, triggered a search for new therapeutic methods. One of these seems to be photodynamic therapy (PDT), also known as photodynamic inactivation (PDI) or photodynamic antimicrobial therapy (PACT), which has shown potential as a non-antibiotic modality treatment for various infections. PACT is the method by which a combination of a sensitizing phototherapeutic agent (photosensitizer), molecular oxygen and visible light destroys microbial cells through the generation of reactive oxygen species (ROS), including singlet oxygen [17; 18]. Singlet oxygen and other oxygen radicals cause lethal effects within a variety of bacterial cells [19; 20; 21; 22]. The mechanism of the photodynamic reaction starts with irradiation of a photosensitizer (Ps) with light of an appropriate wavelength. The energy transfer from the excited Ps to oxygen in its triplet state causes the formation of reactive oxygen species (ROS), including singlet oxygen (\({}^{1}\)O\({}_{2}\)). ROS react with cell organelles and disrupt their functions as a result of previously mentioned physicochemical processes in PACT. Two main oxidative mechanisms of photoinactivation (PI) are considered for the inactivation of the target cells. The type I pathway involves electron/hydrogen atoms--transfer reactions from a Ps in its triplet state with the participation of a substrate, which leads to the production of radical ions. The type II pathway relies on singlet oxygen (\({}^{1}\)O\({}_{2}\)) production in the reaction of a Ps with triplet state oxygen. Both processes lead to ROS, such as \({}^{1}\)O\({}_{2}\) and free radicals, which can irreversibly alter vital components of cells, finally leading to lethal oxidative damage [23; 24]. Very intensive worldwide studies aiming to develop an ideal photosensitizer for PACT are ongoing, with many multidisciplinary research groups having so far evaluated the potential of various agents against different microorganisms [25; 26]. The properties of phthalocyanines (PCs) have met most of the requirements for an ideal photosensitizer in PACT, being fully synthetic macrocyclic compounds belonging to the porphyrinoid family. Pcs, unlike porphyrins, consist of four isoinode heterocycles bound together by _meso_ nitrogen atoms in place of methine groups. Interestingly, they can be easily modified by both the introduction of different metal cations to the macrocyclic core as well as by modification with various functional groups to their periphery [27; 28].