The increasing global rate of antimicrobial resistance have led to a higher incidence of mortal infections that require alternative methods for their control. Antimicrobial photodynamic therapy (aPDT) is gaining attention as an effective approach against resistant microbial strains. The present work aims to evaluate the photodynamic efficiency of a photosensitizer (PS) based on a low-cost formulation constituted by five cationic porphyrins (FORM) and its combined effect with potassium iodide (KI) on a broad spectrum of microorganisms. To this purpose, the aPDT assays were conducted with different concentrations of FORM (0.1 to 5.0 μM) and 100 mM of KI on planktonic and biofilm forms of Gram-positive (methicillin resistant Staphylococcus aureus – MRSA) and Gram-negative (Escherichia coli resistant to chloramphenicol and ampicillin) bacteria and of the yeast Candida albicans. These studies were performed under white light irradiation (25 W.m^-2) and the results obtained indicate that FORM alone, at low concentrations (< 5.0 μM), had an efficient photodynamic action towards the planktonic forms of the Gram-negative and Gram-positive bacteria and also against the yeast C. albicans. Moreover, the application of KI enhanced the photodynamic effect of FORM towards all tested microorganisms, allowing the reduction of PS concentration and treatment time. The results also showed that the combination FORM+KI was highly efficient in the elimination of the already well-established biofilms of E. coli, S. aureus, and C. albicans. This effect was probably associated with the longer-lived iodine reactive species produced during the aPDT treatment.

Acknowledgments: Thanks are due to the University of Aveiro and FCT/ MEC for the financial support to LAQV-REQUIMTE (UIDB/50006/2020) and CESAM (UID/AMB/50017/2019 and UIDB/50017/ 2020 + UIDP/50017/2020) research units, and to the FCT projects (PREVINE-FCT-PTDC/ASP-PES/29576/2017) through national founds (OE) and, where applicable, co-financed by the FEDER-Operational Thematic Pro-gram for Competitiveness and Internationalization−COMPETE 2020, within the PT2020 Partner-ship Agreement, and to the Portuguese NMR Network. Thanks are also due to the SANAQUA project MAR02.05.01-FEAMP-0004 and I9+Proalga project MAR2020-16-01-03-FMP-0011. The doctoral grants to C.V., M.B. and M.Q.M. are funded by FCT (SFRH/BD/150358/2019, SFRH/BD/121645/2016- and SFRH/ BD/112317/2015, respectively).