Isolation and biological characterisation of plant-derived chemicals for antibacterial purposes in food animals and humans
New diseases requiring medical intervention are continually emerging whilst known diseases remain a constant threat due to continual mutation in their genome. This recurrent metamorphosis makes treatment more difficult due to the constant change. An example of which would be the emergence of many antibiotic resistant strains of the methicillin resistant Staphylococcus aureus which is a worldwide epidemic. Pathogens such Listeria monocytogenes, Escherichia coli and Staphylococcus aureus have been the source of a variety of human and animal diseases resulting in economic losses. The ease with which these pathogens adapt to antibacterial remedies means that many antibacterial remedies are rendered ineffective months after their first introduction. The discovery of penicillin marked the beginning of decisive treatment for bacterial infections. However, the extensive use of antibiotics in modern agricultural food production has been the subject of increasing public concern, particularly when it has been linked to the continuous emergence of resistant bacteria; which can transfer resistant genes from animals to humans via the food chain. Recently the use of antibiotic growth promoters for the prevention of animal disease and growth promotion was banned in the EU as a part of measures geared toward preventing emergence of resistance to antibiotics especially those used in human medicine. This EU-wide ban has created a gap and led to a new urgency in the search for safe replacements due to the importance of antibacterial growth promoters in food/animal farming. Examination of trends in antibiotic discovery shows that the majority of the antibiotics in clinical use until now are from natural origin, a trend which is not about to change quickly; as other sources of antibiotics, (synthetic) have failed to produce significant new drugs to keep up with the rate of emergence of resistant bacterial strains. As a result, there has been a renewed interest in the discovery of antibiotics from natural sources.The aim of this study was to search and isolate plant-derived antimicrobial compounds which could be useful for disease control in animals and could have the potential for further pharmaceutical development in both human and veterinary applications. In this study, plant specimens were collected (based on the recommendations of trado-medical practitioners from Cameroon) and extractions were carried out. The resultant extracts were tested for antibacterial properties against microorganisms (of known public health importance) including Listeria monocytogenes NCTC11994 and Escherichia coli K12 NCTC 10538. Bioassay- guided fractionation procedures which involved the development of column chromatographic procedures and high performance liquid chromatography programs were used to isolate and purify bioactive components of extracts with antibacterial activity. Isolated compounds were analysed using nuclear magnetic resonance spectroscopy to help elucidate their respective chemical orientations. Secondary lines of evidence in structure elucidation included mass spectrometry to associate an exact mass with the structure. Structure activity relationship studies were also carried on the bioactive compounds. Toxicity profile studies were carried out and the identified compounds modes of action in combating Gram-positive microorganisms were investigated. In total, four compounds were isolated from Aframomum melegueta three of which are labdane diterpenes; two of these diterpenes, G3 and G5 were significantly (p <0.05) more potent than either ampicillin or gentamicin when combating Grampositive bacteria. The fourth compound (plasticizer) showed activity only against Gram negative E.coli and no activity against the Gram-positive organisms. The mode of action of G3 is penicillin-like and the toxicity profile on Caco-2 cell lines indicated that it is non-toxic against intestinal epithelial cells, a result which favours oral administration. This thesis resulted in the isolation and identification of two antibacterial compounds with therapeutic potentials which will be further assessed and developed for possible therapeutic application.
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