Fungal Anthraquinone Photoantimicrobials Challenge the Dogma of Cationic Photosensitizers

The photoantimicrobial potential of four mushroom species (i.e., Cortinarius cinnabarinus, C. sanguineus, C. rubrophyllus, and C. holoxanthus) was explored based on a light modified EUCAST protocol. The extracts were tested against Candida albicans, Escherichia coli, and Staphylococcus aureus under blue (λ = 428 nm and λ = 478 nm, H = 30 J cm-2) and green light (λ = 528 nm, H = 30 J cm-2) irradiation. Three extracts showed significant photoantimicrobial effects at concentrations below 25 µg/mL. Targeted isolation of the major pigments from C. sanguineus led to the identification of two new potent photoantimicrobials, one of them (i.e., dermocybin) being active against S. aureus and C. albicans under green light irradiation (PhotoMIC523 = 39.5 µM and 2.3 µM, respectively) and the other one (i.e., emodin) being active against E. coli in a low micromolar range (PhotoMIC428 = 11.1 µM). Intriguingly, dermocybin was not (photo)cytotoxic against three tested cell lines adding an additional level of selectivity. Since both photoantimicrobials are not charged, this discovery shifts the paradigm of cationic photosensitizers.