Antimicrobial activity of volatile and non-volatile isolated compounds and extracts from the bark and leaves of Warburgia salutaris (Canellaceae) against skin and respiratory pathogens

Abstract

Warburgia salutaris is one of the most popular trees used in traditional medicine throughout tropical southeast Africa, because of its ability to treat a wide variety of illnesses caused by fungi, bacteria, viruses and insects. The aim of the study was to test the antimicrobial activity of W. salutaris and its isolated compounds against selected skin and respiratory pathogens. Furthermore, to investigate the main chemical compounds of this species and compare their antimicrobial activities with previously published studies. Crude extracts, essential oils and isolated compounds from the bark of W. salutaris, showed antimicrobial activity against skin pathogens (Pseudomonas aeruginosa ATCC 743971 and Staphylococcus aureus ATCC 25923) and respiratory tract pathogens (Klebsiella pneumoniae ATCC 13883 and Moraxella catarrhalis ATCC 23246). Bioassay guided fractionation of the dichloromethane extract (muthi market bark sample) and essential oil (from the bark of the cultivated tree) yielded six compounds, which included the two major essential oil components drimenol (1) and E-nerolidol (2) and the less volatile drimane sesquiterpenes from the muthi market bark: 12α-acetal-polygodial (3), polygodial (4), ugandensidial (5) and warburganal (6). Noteworthy activity against M. catarrhalis was observed by 2 with a minimum inhibitory concentration of 31 μg/ml and by 4 with an MIC of 25 μg/ml against K. pneumoniae. This is the first report of 1,2 and 3 isolated from W. salutaris and the identification of these compounds could explain the traditional use of this species as an inhalation therapy to treat respiratory ailments. Despite numerous studies, the chemical composition and biological activities of the leaves and bark of W. salutaris are not yet fully explored.