Secondary metabolites of different species of lichen were tested for their activities against a variety of microbial species. While gram-negative rods and fungi were not inhibited by these compounds, Staphylococcus aureus, Enterococcus faecalis, Enterococcus faecium, and some anaerobic species (Bacteroides and Clostridium species) were susceptible at the concentrations tested. Vulpinic acid generally was less active than usnic acid, regardless of its stereochemistry. The susceptibility to usnic acid was not impaired in clinical isolates of S. aureus resistant to methicillin and/or mupirocin.The intensive use of antibiotics has selected for antibiotic resistance factors and facilitated the spread of multiply resistant microorganisms. Some of the resistant strains such as Corynebacterium jeikeium or Mycobacterium avium belong to species with low pathogenicities; however, highly clinically relevant species such as Staphylococcus aureus (4, 5, 11), Enterococcus faecalis, and Enterococcus faecium (1) also acquired resistance to additional antibiotics. The resulting strains limit the spectrum of therapeutically active antibiotics drastically. Therefore, there is a continuous need for modifications to established anti-infective compounds and for the detection of new chemicals. The search for new chemical structures with antimicrobial activity traditionally requires testing of large batteries of different microbial products. A more selected way of finding new compounds could be the use of experiences of traditional medicine. As early as the 18th century B.C. in Egypt (16), lichen extracts had been used topically. Pharmacologic research in the beginning of the antibiotic era in the 1950s defined usnic acid and related products as the active compounds in lichens (2, 6-8, 12, 15). Preliminary reports revealed an activity mainly against gram-positive cocci. However, most of these experiments were carried out more than 40 years ago, so that the data obtained are not comparable to standardized test results. We therefore characterized the antibacterial activities of three acids obtained from lichen.The antimicrobial activities of the lichen substances (ϩ)-usnic acid (Fig. 1) (Sigma Chemical Co., St. Louis, Mo.) and (Ϫ)-usnic acid (Fig. 1) of Cladonia stellaris (Opiz) Tonzar & Viezda (Sigma Chemical Co.) and vulpinic acid (Fig. 2) of Letharia vulpina (L.) Hue (Sigma Chemical Co.) were tested against selected aerobic and anaerobic microorganisms by standardized methodology (13,14). After ruling out the intrinsic activity of tetrahydrofuran (Fluka Chemie AG, Buchs, Switzerland), tetrahydrofuran (0.2%, by volume) was used as the solvent mediator for the lichen compounds. Because of the limited solubilities of the tested compounds in water-based nutrient broths (cation-adjusted Mueller-Hinton broth for aerobic bacteria, Sabouraud broth for Candida species, and Schaedler broth for anaerobic bacteria) (9, 13, 14), the maximum achievable concentration in broth was 32 g/ml. Stock solutions were filter sterilized as described by Ghione ...