Usnea, commonly known as "old man's beard," is a type of lichen traditionally used in herbal medicine for its antimicrobial properties. There is some scientific evidence to support its use against Staphylococcus aureus (the bacterium responsible for many staph infections). Laboratory (in vitro) studies have shown that Usnea extracts, particularly those containing usnic acid (its main active compound), exhibit antibacterial activity against various strains of Staphylococcus, including methicillin-resistant Staphylococcus aureus (MRSA). For example, research published in Phytotherapy Research (2002) and the Journal of Ethnopharmacology (2007) demonstrated that usnic acid inhibits the growth of S. aureus in petri dish assays.

However, these findings are primarily limited to test-tube experiments. There are very few, if any, well-designed clinical trials evaluating the effectiveness of Usnea in treating staph infections in humans. Additionally, concerns about the toxicity of usnic acid, particularly when ingested, have limited its clinical application. Most use is topical and based on preliminary evidence.

In summary, while there is a scientific basis for Usnea’s antimicrobial activity against Staphylococcus species, the current evidence is mostly preclinical (rated 2/5), with limited human data to support its regular use for staph infections. Caution is advised due to potential toxicity and lack of standardized dosing.