The antimicrobial activity of South African lichens and lichen-derived usnic acid

Old Main Building, Groote Schuur Hospital, Anzio Road, Observatory 7925, South Africa

Nine lichen specimens were collected from various locations in the Western and Eastern Cape provinces of South Africa with the aim of isolating compounds exhibiting antimicrobial activity. The specimens were ground and extracted using acetone. The extracts were subsequently tested against Staphylococcus aureus, Klebsiella pneumoniae, Mycobacterium aurum and Candida albicans. The best activity obtained against S. aureus was from the extract of Telochista sp. with a minimum inhibitory concentration (MIC) of 50µg/ml, followed by Usnea sp. with and MIC of 62.5µg/ml. None of the extracts exhibited particularly good activity against K. pneumoniae, with Flavoparmelia sp. being the most active with an MIC of 1mg/ml. This extract also exhibited the best antifungal activity with an MIC of 31.25µg/ml against C. albicans. The Usnea sp. exhibited potent antimycobacterial activity with an MIC of 62.5µg/ml against M. aurum. HPLC analysis of the extracts revealed that the major constituent of most of the lichen extracts tested was usnic acid (C₁₈H₁₆O₇). This compound exhibited antimycobacterial activity against M. aurum with an MIC of 15.6µg/ml and inhibited the growth of S. aureus at 500µg/ml, while not affecting the growth of K. pneumoniae and C. albicans. Furthermore, usnic acid was tested for antimalarial activity against Plasmodium falciparum D10. Usnic acid inhibited 50% of plasmodial growth at a concentration of 21.6µg/ml.

Acknowledgements: South African National Research Foundation, University of Cape Town