The study of the antimicrobial properties of selected plants growing in the Free State

English: The Free State province has a rich diversity of indigenous woody plant species that exhibit antimicrobial activity towards both plant and human pathogens. Most of the province (about 72%) is located within the Grassland Biome, only about 21% is in the Nama-Karoo Biome, 6.64% in the Savanna Biome and with the Forest Biome making up only 0.05% of the province. The location of the province in what is mostly the Grassland Biome has resulted in extensive animal husbandry and intensive crop production. Grazing by animals, especially cattle and sheep, continues to have detrimental effect on the local pristine vegetation. On the other hand, intensive crop production, though done with utmost care in most cases, is also resulting in environmental degradation through water pollution by herbicides, both antimicrobials and insecticides. Chances are that some of these pesticides might end up in the food consumers buy, and may lead to health problems. There is great awareness in the Free State province for the need to conserve the environment and the fact that conservation can only be done through the knowledge of the vegetation ecology of the province. Towards this end, a lot of studies on the vegetation ecology of the different areas of the province have been attempted by a number of authors. Despite the existence of information on the vegetation ecology, the Free State still had to be mapped for plants with biological activities, in our case, antimicrobial activity. This exercise was successfully done. Antimicrobial screening of the crude extracts was done as a preliminary screen to determine which plant species exhibited this activity. Bench top assays were used due to their reliability, speed and cost effectiveness. The screen gave valuable information on the antimicrobial properties of these selected plants. By referring to their phytochemical compositions m literature, we were able to conclude why they are used in ethnopharmacology, as well as determine the active principles. Antibiotic plant species with ethnopharmacological value can also be used as agrochemieals against plant pathogenic miroorganisms with success. The extracts of selected plant species in our investigations were tested against both plant and human pathogens with encouraging results. The general absence of antifungal activity towards plant pathogenic fungi was attributed to lower doses of the extracts used, the crude nature of the extracts as well as the the probable presence of growth promoters for the fungi or the general absence of active antifungal substances by nature. The crude extracts of seven plant species out of a total of twenty six exhibited the most outstanding antibacterial activity as evidenced by broad spectrums of activity and larger inhibition zones in comparison with the rest of the plant species. The most resistant bacterial pathogen towards the crude extracts was Erwinia carotovora pv. carotovora while the least resistant was Moraxella catarrhalis. From the seven most active plant species, it was decided to do further investigations on Euclea crispa subsp. crispa. This was due to the fact that it is widely used in ethnopharmacology in areas like Kwa-Zulu Natal and also that it is one of the most common plant species in the province as well as in the country as a whole. The crude and semi-purified (liquid-liquid separation) extracts of E. crispa subsp. crispa showed antibacterial activity towards human pathogenic bacteria to varying degrees. The ethyl acetate fraction was the most active however, since only three human pathogenic bacteria were not inhibited by it. Klebsiella pneumoniae was the most resistant human pathogenic bacterium. With regards to plant pathogenic bacteria, generally, only the ethyl acetate fraction and (the crude extract) showed activity against all the bacteria. Antifungal screening of the semipurified extracts with human pathogenic fungi also showed that most of the antifungal activity was located in the ethyl acetate fraction. No antifungal activity was observed with the hexane fraction. Due to the more complex nature of the crude extract, further phytochemical studies were carried out only with the ethyl acetate fraction, the most active semi-purified fraction. Investigations into the general phytochemical profile of the ethyl acetate fraction of E. crispa subsp. crispa revealed that the plant extract contained essential oils, bitter principles, saponins, alkaloids and phenolic compounds; the first three being terpenoid derivatives. Bitter principles are in some cases reported as being mixtures of different classes. A terpenoid that tested positive for bitter principles and for saponins as well as possessing a steroid-like structure was isolated from the ethyl acetate fraction. This compound was active against M catarrhalis. Also isolated from the ethyl acetate fraction and showing antimicrobial activity were five flavonoids identified as hyperoside, quercitrin, catechin, epicatechin and gallocatechin. The most active flavonoids were epicatechin and hyperoside which inhibited the growth of Streptococcus pneumoniae and Haemophilus injluenzae respectively. The activities of the isolated flavonoids towards human pathogenic bacteria were lower than that that of the more complex ethyl acetate fraction. The conclusion arrived at was that these compounds function in synergy in the latter more complex fraction. Since most of the plant extracts are used in the form of complex mixtures in ethnopharmacology, a suggestion was made to use the ethyl acetate fraction in its complex form. It was further acknowledged that the use of complex mixtures instead of pure compounds does not eliminate the need to determine the chemical composition of the complex mixtures. This is necessary to determine the dosages and toxicity of the constituents of the extracts. In conclusion, it can be said that valuable information on the antimicrobial properties of selected plant species growing in the Free State Province was obtained during the course of our investigations. Also of value is the information on the distribution of these species in relation to biomes, geology and soil types. The availability of literature on the vegetation ecology of different parts of the province was also of immense help to us during this investigation.