In-vitro Activity of Usnic Acid Against Staphylococcus aureus and “MRSA”

Usnic acid is a common lichen metabolite reported to have antimicrobial activity. Objectives: This study aimed to investigate the potential use of usnic acid as a disinfectant against oral organisms and staphylococci in the light of increasing resistance to commonly-used antibiotics. Methods: Usnic acid was isolated from the lichen Cladonia arbuscula, purified to >99% and dissolved in DMSO. Disk-sensitivity and broth-dilution tests of sensitivity were performed using test strains of: Streptococcus mutans, Strept. sanguis, Lactobacillus casei, E. coli, Candida albicans, normal and methicillin-resistant (“MRSA”) strains of Staphylococcus aureus. Results: Only small zones of growth inhibition of the Gram-positive test strains were seen in the disk-sensitivity tests on blood agar. Sensitivity was more easily detected in broth cultures. Strept. sanguis, Staph. aureus and MRSA strains were the most sensitive of the strains tested with MIC and MBC values of 4µg/ mL. E. coli and Candida were resistant to usnic acid while the other test strains had much higher MBC values than their respective MIC value. Discussion: Increasing resistance to conventional antibiotics, especially among commensal organisms and hospital isolates of common pathogens is causing concern. Conclusion:Usnic acid may have useful properties, especially as a topical disinfectant, against otherwise resistant organisms such as MRSA.