Bactericidal activity characterization of an expanded porphyrin on Gram-positive bacteria.

Porphyrines and their derivatives application, commonly associated with Photodynamic Therapy (PDT), is currently being applied to the treatment of diseases caused by bacteria, yeasts, viruses and parasites, as well as to sterilisation of blood and other products. We tested the bactericidal activity of an anionic expanded porphyrin on Gram-positive bacteria, including Enterococcus hirae and Staphylococcus aureus, under visible light eccitation (2W/m2). This activity is mediated by reactive oxygen species (ROS) produced by the molecule in presence of oxygen and light. Study of the kinetics of bacterial killing showed that 99% or more of S.aureus had been killed after 15 minutes of treatment with a 5 μM molecule dose and 80% of E.hirae had been killed after 1 hour of treatment with a 20 μM molecule dose. An incubation step in the dark didn’t improve the bactericidal activity. The interaction of the molecule with the bacterial cytoplasmic membrane and its ability to disrupt the membrane potential was analyzed. Cytoplasmic membrane depolarization was determined using the membrane potential-sensitive dye 3,3V-dipropylthiadicarbocyanine iodide (DiSC3(5)). Implication of ROS in cytoplasmic membrane depolarization was indirectly evaluated by the use of ROS scavengers (sodium benzoate, sodium thioglycollate). The data obtained from these experiments suggest that the primary mechanism of action of the molecule is bacterial cytoplasmic membrane lysis mediated by ROS.