Usnea barbata, Parmeliaceae
Usnea barbata is a tree lichen which hangs in greenish gray strands in the trees of northern hemisphere forests. Lichens grow on rocks and in trees. A lichen is not actually a plant, but two organisms, a fungus and an algae , which live together in a symbiotic relationship as one. The algae contains chlorophyll which harnesses sunlight to create sugars for the two organisms. The fungus provides stability and a structure for the algae to grow on. These two organisms have become interwoven and act like a new one which has its own characteristics. Examples include having distinct reproductive structures and differing biochemical pathways that produce unique secondary chemical compounds with antibiotic properties. These compounds are probably of much importance to the lichen, providing defense against bacteria, fungi, and other microorganisms. 
These antibiotic compounds are also valued by humans for their antibiotic and antifungal lichen acids, especially usnic acid, as well as barbatolic acid, lobaric acid, tartaric acid, and ascorbic acid (vitamin C). These lichen acids have strong antibiotic activity against gram positive bacteria, but little effect on gram negative bacteria. They have been used traditionally as treatment against the bacteria that cause strep throat, impetigo, pneumonia, trichomonas, and tuberculosis. It is thought that usnic acid acts to disrupt cellular metabolism in bacteria preventing energy production either by inhibiting ATP synthesis or by uncoupling oxidative phosphorylation. Human cells are different from bacterial cells and are not affected by usnic acid. There have also been several polysaccharides isolated from Usnea species that have shown anti-tumor activity in animal studies. Usnea has been used for its antifungal properties as a treatment against athlete’s foot and ringworm as well as for other infections including mastitis, urinary tract infections, colds and flus, bronchitis, and sinus infections. Large doses of usnic acid are not recommended as they have the potential to be toxic, causing gastro-intestinal irritation.
 Articus, K. (2004) Neuropogon and the phylogeny of Usnea s.l. (Parmeliaceae, lichenized Ascomycetes). Taxon 53(4): 925-934.
 Hobbs, Christopher (1986) Usnea: The Herbal Antibiotic and Other Medicinal Lichens, Botanica Press.
 Tilford, Gregory L. Edible and Medicinal plants of the West, ISBN 0-87842-359-1.