Inhibitory activities of lichen-derived compounds against methicillin- and multidrug-resistant Staphylococcus aureus

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 mu g/mL (8.13 - 16.3 mu M) against a range of multidrug efflux pump expressing strains of S. aureus.