Advances in aPDT based on the combination of a porphyrinic formulation with potassium iodide: effectiveness on bacteria and fungi planktonic/biofilm forms and viruses

The increasing world-wide rate of antibiotic resistance as well as the capacity of microorganisms to form biofilms, have led to a higher incidence of mortal infections that require alternative methods for their control. Antimicrobial photodynamic therapy (aPDT) emerged as an effective solution against resistant strains. The present work aims to evaluate the aPDT efficiency of a photosensitizer (PS) based on a low-cost formulation constituted by five cationic porphyrins (FORM) and its potentiation effect by KI on a broad spectrum of microorganisms under white light (380-700 nm, 25 W/m2). The aPDT assays were performed 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, of the fungi Candida albicans and on a T4-like bacteriophage as a mammalian virus model. The results indicate that the FORM alone is an efficient PS to photoinactivate not only gram-negative and gram-positive bacteria, but also C. albicans, in planktonic and biofilm forms, and T4-like phage at low concentrations (< 5.0 µM). The presence of KI enhanced the photodynamic effect of this FORM for all microorganisms on the planktonic form, allowing the reduction of PS concentration and treatment time. The results also show that the combination FORM/KI is highly efficient in the elimination of already well-established biofilms of E. coli, S. aureus and C. albicans. This effect is probably associated with longer-lived iodine reactive species produced during the aPDT treatment.