Antibacterial activity of Euphorbia damarana extracts and the isolation of triterpenoids

Abstract

The Euphorbia L. genus has over 2000 species and is regarded as the largest genus of flowering plants. Euphorbia damarana is a small tree belonging to the Euphorbiceae family which grows in the eastern parts of Namibia. It is a slender grey succulent round shrub. It is commonly known as Damara milk-bush or melkbos by the local community. E. damarana is believed to be one of the most poisonous plants in Namibia producing a milky latex that is capable of killing humans and animals. Poisonous plants occur in every garden worldwide thus forming an important part of the indigenous flora of southern African plants. They consist of a mixture of chemical compounds of which either a single or a collective number of compounds can be toxic. There is often variation of toxicity in different parts of the plants. The stem extract of E. damarana yielded a rubbery material and the dried latex extract yielded a sticky yellow material. The hexane and methanol extracts of the stems of E. damarana and the dried latex extract were tested for antibacterial activity against Klebsiella pneumonia, Pseudomonas aeruginosa, Staphylococcus aureus, Bacillus subtilis, Proteus vulgaris, P. mirabilis, Salmonella typhimurium, Achromobacter xyloxidans and unknown bacteria isolated from the soil of Namibia where the plant grows. Good antibacterial activity was obtained with both extracts using the TLC bio-autography method, however with the microtitre dilution method only the stem extract showed good antibacterial activity. The disc diffusion method did not show any inhibition. Toxicity studies of the stem and dried latex extracts on Vero cells did not show significant results. The antibacterial activity of the stem extract was observed with the soil isolate 11 (0.312mg/ml), P. aeruginosa (1.25mg/ml), B. subtilis (1.25mg/ml), isolate 5 (2.5mg/ml) and isolate 10 (2.5mg/ml) bacteria. For dried latex extract antibacterial activity was observed with isolate 11 (5.00mg/ml), P. aeruginosa (5.00mg/ml) and B. subtilis (5.0mg/ml). Bioassay guided fractionation of the stem extract of the plant led to the isolation of five pure compounds. The chemical structures of the isolated pure compounds was elucidated using NMR spectrometry as lupenone, euphol, lupeol, 20-hydroxy-lupan-3-one and 3-oxo-7,24E-tirucalladien-26-oic acid. All the isolated compounds belong to the family of triterpenoids. E. damarana showed promising antibiotic potential in this study. Since this plant has many other antibacterial compounds as seen on TLC plates of the major fractions and has also shown significant activity against some bacteria in bioassays, further fractionation of the major fractions is recommended in order to obtain more pure compounds that could be tested against bacteria. Toxicity studies on the identified compounds and other compounds that are yet to be identified is in progress and will be reported in further studies. There is limited information published on the phytochemical analysis and properties of E. damarana and for this reason it is important to do further research on this plant as it has potential to be used as a natural ethnomedicinal product.