Lichen Polyphenolic Compounds for the Eradication of Biofilms.

Lichens, due to their symbiotic nature (association between fungi and algae), constitute a chemical factory of original compounds. Polyphenolic compounds (depsides and depsidones) are the main constituents of lichens and are exclusively biosynthesized by these organisms. A panel of 11 polyphenols was evaluated for their anti-biofilm activity against biofilms on the maturation phase (anti-maturation) (MMIC) as well as on preformed 24-h-old biofilm (anti-biofilm) (MBIC) using the XTT assay. Minimum inhibitory concentrations of compounds (MICs) against planktonic yeast were also determined using a broth microdilution method. While none of the tested compounds were active against planktonic cells (IC > 100 µg/ml), three depsides slowed the biofilm maturation (MMIC ≤12.5 µg/ml after 48 h of contact with cells). Evernic acid was able to both slow the maturation and reduce the already formed biofilms with MBIC ≤12.5 µg/ml after 48 h of contact with the biofilm. This compound shows a weak toxicity against HeLa cells (22%) at the minimal active concentration and no hemolytic activity at 100 µg/ml. Microscopic observations of evernic acid and optimization of its solubility were performed to further study this compound. This work confirmed the anti-biofilm potential of depsides, especially evernic acid, and allows to establish the structure-activity relationships to better explain the anti-biofilm potential of these compounds.