## 5 Conclusions  
Chronic wounds and lower respiratory tract infections constitute a complex and interdisciplinary health and social problem. Treatment of these infections is particularly problematic due to the presence of bacteria in the form of a biofilm. Cationic zinc(II) phthalocyanine with 2-(4-N-methylmorpholin-4-ium-4-yl)ethoxy groups subjected to the photocytotoxicity assessment against planktonic and biofilm forms of clinical strains of _P. aeruginosa_ demonstrated promising photocytotoxicity. The studied cationic phthalocyanine at a concentration of \(1.0\times 10^{-4}\) M, when activated by light, revealed a significant reduction factor of _P. aeruginosa_ bacteria, ranging from nearly 4 to 6 log when compared to the untreated control group. In the biofilm inactivation study, after single dose irradiation a decrease in the number of bacteria ranging from 1.3 to 4.2 log was observed. _P. aeruginosa_ strains used in this study exhibited various antibiotic resistance patterns and varying degrees of biofilm formation. Nevertheless, the phototoxic effect of Pc+ was not influenced by the degree of biofilm production or antibiotic resistance mechanisms present in the cells. The result is significant regarding the increasing resistance of bacteria to many drugs, especially beta-lactam antibiotics used in routine antibiotic therapy. An interesting increase of photoinactivation efficiency against _P. aeruginosa_ biofilms was achieved by the introduction of the second and the third irradiation steps. The second irradiation of the biofilm significantly improved the bacterial reduction factor from 3.4 to 5.5 log, whereas the boosted cell death response was observed after the third irradiation, when a bacterial reduction factor from 4.6 to 6.4 log was noted. Noteworthy is the fact that repeated exposure of short durations of time can be advantageous in the case of this type of biofilm, and the obtained result could be considered for the basis of a prospective therapy. To sum up, the herein researched phthalocyanine derivative could be considered as a potential photosensitizer against multidrug-resistant _P. aeruginosa_ of potential applicability in the treatment of chronic wound infection.  
**Author Contributions:** Conceptualization, J.D.; methodology, J.D., W.S. and T.K.; investigation, W.S., T.K., M.R. and D.K.; resources, W.S., M.R. and J.D.; writing--original draft preparation, W.S. and M.R.; writing--review and editing, T.G. and J.D.; supervision, J.D. and T.G.; project administration, J.D. and W.S.; funding acquisition, M.R. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The authors acknowledge the financial support for Miniatura project No. 2021/05/X/NZ5/01600 from the National Science Centre, Poland.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** The authors would like to acknowledge Poznan University of Medical Sciences Core Facility for providing access to the JEOL ISM-6380 LA scanning electron microscope.  
**Conflicts of Interest:** The authors declare that they have no conflict of interest.