Inhibitory Activities of Lichen-Derived Compounds against Methicillin- and Multidrug-Resistant Staphylococcus aureus

Abstract

The inhibitory effects of selected phenolic lichen substances were tested against a panel of methicillin- and multidrug-resistant Staphylococcus aureus. Depsidones with long alkyl chains on both of the aromatic rings were consistently active against the strains tested, comparable to or better than the level of clinically used antibacterial drugs. A similar level of activity was also observed for rhizocarpic acid. The previously described cytotoxic pentacyclic compound hybocarpone was by far the most active, exhibiting minimum inhibitory concentrations (MICs) of 4-8 microg/mL (8.13-16.3 microM) against a range of multidrug efflux pump expressing strains of S. aureus.

In vitroevaluation of the antibacterial activity of extracts from 34 species of North American lichens

“…Various studies have shown similar results (Kokubun et al, 2007;Pompilio et al, 2013). Extracts from nine of our tested lichen species showed relatively low MIC values (516 mg/ml) against the methicillin-resistant S. aureus and extracts from Rhizoplaca marginalis and Usnea hirta had the lowest values at 7.8 mg/ml.…”

“…Since Burkholder et al (1944) first reported on the antibiotic properties of lichens, a number of studies have investigated the antimicrobial activities of several lichen species against various Gram-positive, Gram-negative, and mycobacteria (Hobbs, 1986;Ingólfsdóttir et al, 1985;Kokubun et al, 2007;Lauterwein et al, 1995;Lawrey, 1989;Manojlovic et al, 2011). More specifically, studies have shown that lichen compounds either in crude extract or as purified compounds are not active against Gram-negative bacteria such as E. coli and P. aeruginosa (Francolini et al, 2004;Ingólfsdóttir, 2002;Lauterwein et al, 1995;Melgarejo et al, 2008;Paudel et al, 2012;Yilmaz et al, 2005).…”

“…Furthermore, microbial resistance has severely impacted a drug's long-term potential to return a profit (Leeb, 2004). These issues emphasize the increasing importance of investigating and developing new classes of antibiotics that are either opaque to pathogen resistance or express a new mode of action, hence increasing the potential lifetime of the antibacterial class (Kokubun et al, 2007).…”

“…Various lichen-derived compounds have been shown to have antimicrobial activities (Boustie et al, 2011;Podterob, 2008;Shrestha & St. Clair, 2013a;Shukla et al, 2010). Activity of lichen compounds has also been demonstrated against a limited number of drug-resistant pathogenic microbes including vancomycin-resistant enterococci and methicillin-resistant Staphylococcus aureus (Elo et al, 2007;Kokubun et al, 2007;Lauterwein et al, 1995). However, due to the diversity of lichen species and compounds, more aggressive efforts to identify and test lichen secondary metabolites for potential antibiotic properties seem to be logical.…”

Synthesis and antimycobacterial activity of (+)‐usnic acid conjugates

“…Usnic acid has antibiotic properties, both the natural enantiomers being active against Gram positive bacteria on susceptible and resistant strains . A more intense bacteriostatic rather than bactericidal action was reported on some metal complexes of usnic acid hydrazones …”