## 4 Discussion  
In our earlier work, the photocytotoxic effect of Pc+ was assessed against planktonic and biofilm forms of microorganisms, including reference and clinical strains [29]. An extensive drop in cell survival of the planktonic form of _P. aeruginosa_ NCTC 6749 was observed in the photocytotoxicity study with Pc+ against strains isolated from severe and difficult-to-treat infections. In this study, clinical planktonic and sessile strains of _P. aeruginosa_ isolated from chronic wounds and lower respiratory tract infections were also subjected to Pc+ treatment. It is worth noting that the antimicrobial susceptibility patterns and biofilm forms of the evaluated strains isolated from different infection sites were different. Among the strains with strong biofilm biomass production there were both multi-drug resistant as well as antibiotic sensitive strains. However, when analyzing antimicrobial resistance on the ability to form biofilms, it can be concluded that the strong-biofilm producer strains are those which most frequently include microorganisms resistant to more than four antibiotics. In our previous study, Pc+ revealed excellent photocytotoxicity against planktonic cells of both Gram-positive and Gram-negative bacteria. The photoactivity  
Figure 5: SEM image of a biofilm of _Pseudomonas aeruginosa_: (**A**) treated with Pc+ in the non-irradiated test containing a photosensitizer concentration of 1.0 \(\times\) 10\({}^{-4}\) M—dark toxicity (L– Ps+), and (**B**) treated with Pc+ at a concentration of 1.0 \(\times\) 10\({}^{-4}\) M and irradiated by light dose of 3.6 J/cm2—photocytotoxicity (L+ Ps+). Arrows indicate changes in the cellular morphology, including alterations in cellular size and structures, thus suggesting cellular rupture. (SEM \(\times\) 10,000; scale bar—1 \(\upmu\)m). of Pc+ toward _P. aeruginosa_ NCTC 6749 was only slightly lower than that of _E. coli_ and Gram-positive bacteria [28]. For this reason, further examination with the use of clinical strains of _P. aeruginosa_ was included in the current study. In the in vitro antimicrobial photocytotoxicity study against _P. aeruginosa_, cationic phthalocyanine successfully reduced planktonic bacterial cells, with reduction factor (R1) values much higher than 4 log for nearly all tested strains (except strains no. 10 and 22). It is worth noting that for all tested strains the 3-log criterion for bactericidal activity for antibiotics was achieved. Bactericidal activity of antibacterial agent in vitro is usually defined as a reduction of the number of viable cells by at least 3 log. It is worth noting that for all tested strains the parameter was exceeded. Moreover, the differences in the sensitivity to antibiotics and the degree of biofilm production by the analyzed strains did not affect their sensitivity to the photocytotoxic effect of the tested phthalocyanine. The potential of various Pcs for antimicrobial photodynamic therapy was confirmed in various studies [41; 42; 43]. [PERSON] et al. presented high photoinactivation of Gram-positive _S. mitis_ and Gram-negative _E. coli_ bacteria by cationic zinc(II) phthalocyanines [41]. In another study, a cationic zinc(II) phthalocyanine derivative was active against both Gram-positive and Gram-negative bacteria. _E. coli_ and _S. aureus_ strains were inactivated with 675 nm light in the presence of 1 mM photosensitizer under 5 min irradiation with a reduction of about 5 log [42]. Other authors studied zinc(II) and ruthenium phthalocyanine derivatives in multi-cationic dendrimers against different microorganisms. Zinc(II) phthalocyanine derivatives at a concentration of 1 mM and light dose of 60 J \(\times\) cm\({}^{-2}\) inactivated _S. aureus_ strains with a reduction factor of 6 log, whereas ruthenium phthalocyanines appeared less harmful against _S. aureus_, leading to a bactericidal effect only at maximum concentration and light dosage (1 mM and 60 J \(\times\) cm\({}^{-2}\)). Photoinactivation of _E.