Antagonistic effects of combination photosensitization by hypericin, meso-tetrahydroxyphenylchlorin (mTHPC) and photofrin II® on Staphylococcus aureus

Photodynamic therapy involves the application of a photosensitizer activated by visible light to generate cytotoxic reactive oxygen. In addition to clinical investigations, in vitro studies concerning photodynamic potency of sensitizers as well as quantification of illumination procedures are necessary. In our investigation, the objective was to evaluate not only the effects of photosensitizer and light on Gram-positive Staphylococcus aureus, but also to investigate possible synergistic or antagonistic effects of these sensitizers. Therefore, we used hypericin, Photofrin II®, porfimer sodium and meso-tetrahydroxyphenylchlorin (mTHPC) alone, as well as in combination. Log-phase cells of S. aureus exhibited a marked sensitivity to white thermal light irradiation in the presence of Photofrin II® and mTHPC. However, hypericin caused a rather stimulated growth expressed in increased optical density (OD) and increase of total cell count (TCC) of the culture. Combination sensitization of S. aureus by Photofrin II® and mTHPC with hypericin likewise caused a stimulation of bacterial growth. No synergistic effects were obtained by combination of Photofrin II® and mTHPC; photoresponse of S. aureus was rather decreased by using combined porphyrins. In comparison, TCC and colony-forming units (CFU) were suppressed in the presence of mTHPC after an illumination procedure as well as in dark reactions. These effects were also obtained in the combination photosensitization by mTHPC and Photofrin II®. In the presence the of hypericin, photodynamic effects of mTHPC and Photofrin II® were inhibited. It was finally concluded that hypericin in our model is not a proper sensitizer for combination photosensitization due to antagonistic effects on photodynamic activity of mTHPC and Photofrin II®.