Planta Med 2006; 72 - P_122
DOI: 10.1055/s-2006-949922

Lichens as a source of antibiotics against resistant bacteria

H Elo 1, J Matikainen 2, E Pelttari 1
  • 1Division of Pharmaceutical Biology, Faculty of Pharmacy, University of Helsinki, P. O. Box 56, 00014 University of Helsinki, Finland
  • 2Department of Chemistry, Faculty of Science, University of Helsinki, P. O. Box 55, 00014 University of Helsinki, Finland

Resistant bacteria such as vancomycin-resistant enterococci (VRE) and methicillin-resistant staphylococci, especially methicillin-resistant Staphylococcus aureus (MRSA), are at present great clinical problems. The antibiotic arsenal available against them is limited and the situation is worsening because new resistant mutations are emerging. Therefore, an intensive search for new active agents is ongoing.

We report that the lichen-derived old drug (+)-usnic acid and, especially, its sodium salt (sodium usniate) have potent antibacterial activity against VRE and MRSA. All MRSA strains tested were sensitive to sodium usniate (inhibitory zones of 21mm with 10µl of a 40mg/mL solution in DMSO on a paper disk of 6mm diameter). The activity of the free acid was somewhat lower, obviously because of lower solubility. In the case of VRE, sodium usniate gave inhibitory zone diameters of 32mm, indicating very high activity of great clinical interest, while usnic acid itself gave diameters of 18mm.

Considering clinical applications, it must borne in mind that usnic acid has in some cases caused severe toxic manifestations such as fulminant hepatitis, and can also cause allergic reactions. In spite of this, it may possibly constitute a last rescue in life-threatening cases where other therapies have failed.