Development of a Photodynamic Therapy Protocol to control Skin Infections by Pseudomonas aeruginosa

Antibacterial resistance is one of the most significant global health threats, being, in 2019, directly responsible for 1.27 million global deaths and contributing to 4.85 million deaths. The complication of infection treatments leads to an urgent need for novel therapeutics strategies. Antimicrobial Photodynamic Therapy (aPDT) has demonstrated efficacy in eliminating various microorganisms and has become increasingly important as an effective strategy against multidrug-resistant strains. In this study, we assess the photodynamic efficiency of two different photosensitizers (PSs), Methylene Blue (MB) and a Phthalocyanine (ZnPc4+), {4, 40, 400, 4000-(29H, 31H-phthalocyanine-1,8,15,22-tetrayl-k4N29, N30, N31, N32) tetrakis [1- methylpyridiniumato 2-)]} zinc (4b) tetraiodide, for controlling skin infections by a clinical Pseudomonas aeruginosa strain. Both PSs were evaluated individually and in combination with Potassium Iodide (KI), a potentiator inorganic salt with photodynamic activity. Pseudomonas aeruginosa is a ubiquitous Gram-negative bacterium and one of the most life-threatening pathogens that are frequently implicated in skin infections. For this study, in vitro assays were performed with planktonic cells and biofilms, along with ex vivo assays with porcine skin. In the in vitro assays with MB and with ZnPc4+ in combination with KI, the detection limit of the method that is used to quantify the bacteria was reached. When the PSs were conducted individually, MB was effective against planktonic cells and biofilms, but ZnPc4+ was only effective against planktonic cells. Regarding our ex vivo studies, only the MB without KI was effective against Pseudomonas aeruginosa. The addition of KI on porcine skin did not potentiate aPDT with MB and ZnPc4+. Although the PSs alone and in combination with KI on porcine skin did not achieve an inactivation as high as that in the in vitro assays, promising results were obtained with MB after two cycles of treatment. This emphasizes the need for further studies to identify novel strategies to optimize the best ex vivo conditions, namely, under red light and using more treatment cycles.