Fungal Anthraquinone Photoantimicrobials Challenge the Dogma of Cationic Photosensitizers.

The photoantimicrobial potential of four mushroom species (i.e., Cortinarius cinnabarinus , C. holoxanthus , C. malicorius , and C. sanguineus ) was explored by studying the minimal inhibitory concentrations (MIC) via a light-modified broth microdilution assay based on the recommended protocols of the European Committee on Antimicrobial Susceptibility Testing (EUCAST). The extracts were tested against Candida albicans , Escherichia coli , and Staphylococcus aureus under blue (λ = 428 and 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 [PhotoMIC 530 = 39.5 μM (12.5 μg/mL) and 2.4 μM (0.75 μg/mL), respectively] and the other one (i.e., emodin) being in addition active against E. coli in a low micromolar range [PhotoMIC 428 = 11.1 μM (3 μg/mL)]. Intriguingly, dermocybin was not (photo)cytotoxic against the three tested cell lines, adding an additional level of selectivity. Since both photoantimicrobials are not charged, this discovery shifts the paradigm of cationic photosensitizers.