Antimicrobial and antioxidant activity of Evernia prunastri extracts and their isolates

Abstract

Lichens are symbiotic organisms formed by a fungus and one or more photosynthetic partners which are usually alga or cyanobacterium. Their diverse and scarcely studied metabolites facilitate adaptability to extreme living conditions. We investigated Evernia prunastri (L.) Ach., a widely distributed lichen, for its antimicrobial and antioxidant potential. E. prunastri was sequentially extracted by hexane (Hex), dichloromethane (DCM) and acetonitrile (ACN) that were screened for their antioxidant and antimicrobial (against Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and Candida albicans) activities. The Hex extract possessed the highest antioxidant capacity (87 mg ascorbic acid/g extract) corresponding to the highest content of phenols (73 mg gallic acid/g extract). The DCM and Hex extracts were both active against S. aureus (MICs of 4 and 21 µg/ml, respectively) but were less active against Gram-negative bacteria and yeast. The ACN extract exhibited activity on both S. aureus (MIC 14 µg/ml) and C. albicans (MIC 38 µg/ml) and was therefore further fractionated by silica gel column chromatography. The active compound of the most potent fraction was subsequently characterized by 1H and 13C-NMR spectroscopy and identified as evernic acid. Structural similarity analyses were performed between compounds from E. prunastri and known antibiotics from different classes. The structural similarity was not present. Antioxidant and antimicrobial activities of E. prunastri extracts originate from multiple chemical compounds; besides usnic acid, most notably evernic acid and derivatives thereof. Evernic acid and its derivatives represent possible candidates for a new class of antibiotics. Graphic abstract

Lichen-Derived Compounds and Extracts as Biologically Active Substances with Anticancer and Neuroprotective Properties

“…So far over 70 different metabolites have been identified and characterized in E. prunastri, and evernic acid is regarded as one of its major secondary metabolites [42]. In our study, evernic acid was cytotoxic to the A-172 cell line in a broad range of concentrations (10-100 µM); however, the cytotoxic effect was moderate -the viability dropped to ~80%, as compared to the vehicle-treated control.…”

Effects of the Dibenzofuran, Usnic Acid, on Inhibition of Ocular Biofilm Formation Due to Coagulase-Negative Staphylococci

“…Another possible mechanism of UA activity against S. aureus is binding to allosteric sites on the protein surface of FabI (enoyl reductase enzyme), which affects enzyme activity [ 48 ]. Usnic acid was found to be active against S. aureus , with a MIC of 21 μg/ml [ 50 ]. Minimum inhibitory concentration values of UA against different clinical isolates of S. aureus range from 8 to 50 μg/ml [ 33 , 34 , 43 , 46 , 47 ].…”

Lichen Depsides and Tridepsides: Progress in Pharmacological Approaches

“…Evernic acid acts as an antimicrobial agent, inhibiting the growth of Staphylococcus aureus , Escherichia coli , Pseudomonas aeruginosa , and Candida albicans (MIC values from 0.98 to 125 µg/mL) [ 84 ]. In deepening the mechanism of these effects, it was observed that the virulence of the Gram-negative opportunistic pathogen Pseudomonas aeruginosa was reduced by inhibiting quorum sensing on diverse Pseudomonas aeruginosa strains (54% of gfp expression of lasB-gfp and 50% of rhlA-gfp at a concentration of 116 µM).…”

“…It should be also noted that evernic acid is a moderate inhibitor of tumor promoter-induced Epstein–Barr virus activation (64.6% of an inhibitory effect at a concentration of 50 µM) [ 71 ]. Finally, regarding the toxicity of this compound, in silico prediction tests showed no mutagenic effects, no tumorigenic effects, no reproductive alterations, and no irritant effects [ 84 ].…”