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
Highlights
Antibiotic resistance is one of the biggest threats to global health and development today that is not counterbalanced by the development of new therapeutic agents: no new classes of antibiotics have been developed since 1987 (WHO 2020; Durand et al 2019; Hutchings et al 2019)
The inhibitory activity of different extracts of E. prunastri as well as of the fractions of the ACN extract was evaluated against clinical strains of four human pathogens, i.e., Grampositive bacterium Staphylococcus aureus, Gram-negative bacteria Pseudomonas aeruginosa and Escherichia coli, and the fungus Candida albicans
Usnic acid has been reported to be effective against antibiotic-resistant Gram-positive bacteria, e.g. vancomycinresistant enterococci and methicillin-resistant S. aureus (MRSA) (Araújo et al 2015)
Summary
Antibiotic resistance is one of the biggest threats to global health and development today that is not counterbalanced by the development of new therapeutic agents: no new classes of antibiotics have been developed since 1987 (WHO 2020; Durand et al 2019; Hutchings et al 2019). New resistance mechanisms are emerging and spreading globally, threatening our ability to treat common infectious diseases (WHO 2020). Considering that the most clinically relevant classes of antibiotics are derived from natural products (Hutchings et al 2019), we identified lichens as a potential source of new antimicrobial compounds. Lichens are organisms consisting of a fungus and a photosynthetic partner (cyanobacteria or algae) in a symbiotic relationship (Calcott et al 2018). This unique symbiosis generates diverse and proprietary biochemical compositions of lichen compounds which offer a vast potential for the discovery of novel classes of antimicrobial substances (Mitrovic et al 2011; Zambare and Christopher 2012). Depsides, depsidones and usnic acid derivatives are so far uniquely found in lichens (Stocker-Wörgötter et al 2013)
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