On the mechanism of Candida tropicalis biofilm reduction by the combined action of naturally-occurring anthraquinones and blue light

The photoprocesses involved in the photo-induced Candida tropicalis biofilm reduction by two natural anthraquinones (AQs), rubiadin (1) and rubiadin-1-methyl ether (2), were examined. Production of singlet oxygen (O-1(2)) and of superoxide radical anion (O-2(center dot-)) was studied. Although it was not possible to detect the triplet state absorption of any AQs in biofilms, observation of O-1(2) phosphorescence incubated with deuterated Phosphate Buffer Solution, indicated that this species is actually formed in biofilms. 2 was accumulated in the biofilm to a greater extent than 1 and produced measurable amounts of O-2(center dot-) after 3h incubation in biofilms. The effect of reactive oxygen species scavengers on the photo-induced biofilm reduction showed that Tiron (a specific O-2(center dot-) scavenger) is most effective than sodium azide (a specific O-1(2) quencher). This suggests that O-2(center dot-) formed by electron transfer quenching of the AQs excited states, is the main photosensitizing mechanism involved in the photoinduced antibiofilm activity, whereas O-1(2) participation seems of lesser importance.