## 4 Discussion
coli_ with a reduction of more than 7 log using zinc(II) phthalocyanine derivatives and in the range of 2-5 log for ruthenium phthalocyanine derivatives with a light dose of 60 J \(\times\) cm\({}^{-2}\) has been noted [43]. A number of various Pcs, either alone or in combinations with different substances, have been tested for a potential photodynamic effect against _P. aeruginosa_, with differentiated antibacterial photocytotoxicities against _P. aeruginosa_[44; 45; 46; 47; 48; 49; 50; 51]. [PERSON] et al. presented the effectiveness of the silicon phthalocyanine derivative against Gram-positive bacteria, _S. mutans_ and _S. aureus_, with over 3 log reductions. However, no effect was observed on the _P. aeruginosa_ strain photoinactivated and irradiated under the same conditions (approx. 1 log reduction) [50]. In another study, two water-soluble cationic lutetium(III) acetate phthalocyanines (LuPc-5 and LuPc-6) were tested against _P. aeruginosa_ and _Candidida albicans_. Full photoinactivation of tested strains was achieved with LuPc-5 against _P. aeruginosa_ and _C. albicans_, whereas photoinactivation with the peripherally substituted LuPc-6 was less significant (2 log) [49]. [PERSON] and coworkers presented the photoinactivation efficiency of two zinc(II) phthalocyanine derivatives against Gram-positive bacterial strains. _S. aureus_ and _E. coli_ were inactivated with a reduction of 4 log in the concentration range of 5 nM to 0.4 mM, whereas the same reduction in the viability of _P. aeruginosa_ appeared in the range of 3.5 to 15.5 mM [47]. Many studies revealed that, although _P. aeruginosa_ could be successfully inactivated, higher doses of photosensizers and/or light are necessary. The sensitivity to photosensitization of Gram-positive and Gram-negative bacteria differs depending on their extracellular structures. The cell wall of Gram-negative bacteria is more complex than that of Gram-positive bacteria due to the presence of an outer membrane that covers the surface of the peptidoglycan [49; 50; 51; 52; 53]. The outer membrane is equipped with hydrophilic surfaces and lipophilic cores, which makes it a very effective barrier against lipophilic and hydrophilic molecules. Moreover, only amphipathic molecules, i.e., molecules with lipophilic backbones and polar/charged flanks, are capable of passing through this barrier [22; 54]. Despite this phenomenon, cationic photosensitizers replace an essential LPS-binding (LPS--lipopolysaccharide) of divalent Ca\({}^{2+}\) and Mg\({}^{2+}\) ions and thereby disrupt outer membrane stability, leading to the formation of channels in the outer membrane that can facilitate photosensitizer uptake [55]. The electrostatic interactions constitute an essential factor initiating cell-drug binding. Therefore, the outer membrane and cell membrane are the sites of action for cationic phototherapeutic agents, which explains the high photocytotoxic potential of the herein studied zinc(II) phthalocyanine against _P. aeruginosa_ strains. The fact that the tested Pc+ strongly absorbs red light means that the photodynamic process in tissues is effective, as red light is characterized by relatively deep tissue penetration. The huge threat of _P. aeruginosa_ to human health results from its innate resistance to many antimicrobials and its unique ability to form a biofilm. Biofilms often develop on the surface of wounds and is responsible for their chronicity and also complicates treatment. In a similar fashion, infections of the lower respiratory tract develop a biofilm, which makes it important to look for substances that act not only on the planktonic forms of bacteria, but, most importantly, on biofilms. Antibiotics and antiseptics used in the treatment of chronic wound infections are ineffective as antibiofilm agents because these agents have not been designed for killing of microorganisms within a biofilm but have only been designed to specifically focus on killing microorganisms in the planktonic state [57; 56; 37].