Species of the genus Plectranthus, a member of the mint family (Lamiaceae), have been used in alternative medicines by third world countries dating back to the early Chinese empire. Plectranthus species have been used in the past for coughs and colds (P. ambiguus) and as a mouth-wash for loose and bleeding teeth (P. laxiflorus). The crushed leaves of P. madagascariensis are used by the Xhosa as an ointment for scabies. P. hadiensis is used orally as a cough mixture. Eight species from the genus were chosen to study the essential oil composition and antimicrobial activity. Antimicrobial assays incorporated four bacteria, two yeasts and one fungus. Disc diffusion assays on Bacillus cereus showed P. venteri to be one of the more active oils. Using gas chromatography-mass spectroscopy the essential oil composition was determined of the hydro-distilled oils. The major compounds identified in P. venteri were p-cymene (4.32%), linalool (0.77%) and limonene (1.04%). Other species exhibiting
antimicrobial activity include P. ciliatus, P. hadiensis and P. porphyranthus. Identified compounds in these oils included germacrene D, bicyclogermacrene, spathulenol, a-fenchone, cubenol, T-cadinol, ß-caryophyllene and cubenol.
Plectranthus ciliatus showed good results in the MIC/microplate assays with no growth occurring in any of the wells after 24 hours. Compounds identified in the oil included bicyclogermacrene (16.83%), spathulenol (15.52%) and germacrene D (9.37%). Comparatively the dark blue form P. zuluensis showed growth in all wells when tested against B. cereus and P. grandidentatus showed growth in all the wells when tested against Staphylococcus epidermidis. It is evident that the essential oils that hail from indigenous Plectranthus species show antimicrobial activity, providing scientific evidence for their use in traditional herbal preparations.
Principal supervisor: Prof AM Viljoen
Co-supervisor: Dr SF van Vuuren (WITS)