The anatomy, ethnobotany, antimicrobial activity and essential oil composition of southern African species of Teucrium (Lamiaceae)

Three southern African endemic species of the genus Teucrium, T. africanum, T. kraussii and T. trifidum (Lamiaceae) are used in traditional medicine to treat digestive and respiratory ailments. These three species are poorly studied with regard to their anatomy, essential oil composition and the scientific validation of their traditional medicinal uses. A summary of ethnobotanical information is presented verbatim. Micro-morphological and anatomical studies of stems, leaves and fruits were done on specimens from three populations of each species using light microscopy and on specimens from single populations using scanning electron microscopy. Materials from single populations were used to obtain the essential oils and to prepare water, methanol and dichloromethane–methanol (1:1) extracts of the different plant parts. These were used for antimicrobial assays to determine the minimum inhibitory concentrations (MIC) against the bacteria; Bacillus cereus, Escherichia coli, Klebsiella pneumoniae, Moraxella catarrhalis, Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus pyogenes. The composition of the oils was determined by gas chromatography-mass spectrometry (GC-MS). The fruits are taxonomically important in their surface sculpturing and indumentum to discriminate among the three species. The surface sculpturing is more prominently reticulate in T. africanum and T. trifidum than in T. kraussii. The trichomes on the stem, leaf and fruit surfaces are peltate, capitate and non-glandular, which are common in the family and the genus. Sphaerocrystals, which are known to be taxonomically informative, were present in all three species. The volatile constituents of T. africanum and T. trifidum are very similar with α-cubebene, β-cubebene as main compounds in both species, and β-caryophyllene, δ-cadinene and several others as minor constituents. Teucrium africanum showed antibacterial activity against E. coli, with an MIC value of 0.125 mg/ml (dichloromethane: methanol extract) and against S. pyogenes, with an MIC value of 0.16 mg/ml (essential oil). These data at least provisionally validate the traditional uses of Teucrium species to treat digestive and respiratory ailments.