Essential Oil of Plectranthus cylindraceus Hochst. ex. Benth from Ethiopia: Chemical Composition and Antimicrobial Activity

Abstract

Traditionally Plectranthus cylindraceus Hochst. ex. Benth (Lamiaceae) is used as a disinfectant and for the treatment of various illnesses. Hydrodistillation of the fresh aerial part of P. cylindraceus collected in Ethiopia yielded 0.39 % (v/w) colourless oil with a strong but pleasant smell. Analysis of the oil by gas chromatography-mass spectrometry (GC-MS) afforded twenty-nine compounds which accounted for 95.26 % of the oil with two of the components yet to be identified. The oil was found to be rich in oxygenated monoterpenes (90.00 %), the major constituents being camphor (40.93 %) and 1,8-cineol (25.38 %). In vitro antimicrobial activity of the oil investigated by disc diffusion technique indicated that it was active against both Gram-positive and Gram-negative bacteria as well as some fungal strains. The activity of the oil was more prominent against several Gram-negative bacterial strains including Escherichia coli, Shigella spp and Vibro cholerae with a minimum inhibitory concentration (MIC) value of 25 μg/ml. Among the bacterial strains tested Shigella flexneri Type 6 was the most susceptible against which the oil displayed an MIC value of 10 μg/ml. The oil did also demonstrate a significant fungal inhibitory potency against Aspergillus niger and Candida albicans with an MIC value of 400 μg/ml. At similar concentrations, the oil and the standard antimicrobials used in this study exhibited comparable activities against the tested bacterial and fungal strains. Moreover, the mode of action of the oil was proved to be bactericidal and fungicidal. The study provides evidence for the broad-spectrum antimicrobial effect of P. cylindraceus essential oil, a possible justification for the traditional use of the plant in the treatment of skin, digestive, respiratory and inflammatory diseases which may be related to microbial infections.