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Min, JiaNi (2013) Phytochemical characteristics of selected Chinese medicinal plants and investigation of the antibacterial and antioxidant activity. Masters thesis, London Metropolitan University.
In the past decade, the increasing number of antibiotic-resistant bacterial strains has become an emerging health problem. There is a lack of new antibiotics available to treat existing and emerging multidrug-resistant pathogens. Most new antibiotics are modifications of existing drugs. So, it is necessary to find new antibacterial agents, preferably with new modes of action. The plants in which phytochemical constituents exhibit antibacterial activities have received increased interest. A literature search was carried out by Web of Science to identify suitable candidates for screening for antibacterial activity. Desmodium styracifolium (Osb) Merr and Siegesbeckia orientalis. L, which have been used for their antibacterial properties for treating wounds, flu, fevers and burns of skin in traditional Chinese medicine, were chosen for this investigation.
In the minimum inhibitory concentration (MIC) assay, it was confirmed that some cold extracts of D. styracifolium and S. orientalis have inhibitory activity against Staphylococcus epidermidis, Bacillus subtilis and Staphylococcus aureus while the MIC values of these extracts against Escherichia coli were greater than 512 µg/ml. Compared to S. orientalis, only the cold acetone extract of D. styracifolium had inhibitory activity against B. subtilis and S. aureus (MIC= 256 µg/ml and 128 µg/ml). Therefore, S. orientalis was to be chosen to do further research for the active ingredients. Bioassay guided fractionation of the most active extracts to separate compounds was carried out using various chromatographic techniques including thin layer chromatography (TLC), vacuum liquid chromatography (VLC), size exclusion chromatography (SEC), solid phase extraction (SPE) and high performance chromatography (HPLC). The compounds which exhibit antibacterial activity were identified using mass spectrometry (MS) and nuclear magnetic resonance (NMR). The results showed that hexadecanoic acid may be found in S. orientalis (MIC against S. aureus = 64 µg/ml). The 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical assay was used for the estimation of efficiency of the antioxidant activity of the extracts. It was found that all extracts had antioxidant activity.
Bacterial DNA gyrase is a proven target for antibacterial agents. Gel-based DNA supercoiling was used to test if inhibitors of DNA gyrase were contained in the extracts of S. orientalis, D. styracifolium and Prunella vulgaris. It was found that the concentration of the cold extracts which inhibit DNA gyrase was greater than 300 µg/ml.
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