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 (¹O₂) and of superoxide radical anion (O₂ ^•−) was studied. Although it was not possible to detect the triplet state absorption of any AQs in biofilms, observation of ¹O₂ 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 O₂ ^•− after 3h incubation in biofilms. The effect of reactive oxygen species scavengers on the photo-induced biofilm reduction showed that Tiron (a specific O₂ ^•− scavenger) is most effective than sodium azide (a specific ¹O₂ quencher). This suggests that O₂ ^•− formed by electron transfer quenching of the AQs excited states, is the main photosensitizing mechanism involved in the photoinduced antibiofilm activity, whereas ¹O₂ participation seems of lesser importance.