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This project illustrates the antibacterial efficacy of crude aqueous infusions ofmedicinal plants against resistant pathogens including Methicillin-resistantStaphylococcus aureus (MRSA), Klebsiella pneumoniae, Acinetobacterbaumannii, Pseudomonas aeruginosa, and Escherichia coli. The plantinfusions of Rheum palmatum L., Arctium lappa L. and Paeonia suffructicosaAndr demonstrated the strongest anti-MRSA activity via multiple targets, withthe minimum inhibitory concentration (MIC) of 7.8 μg/mL. While strongantibacterial activity was exhibited by Sanguisorba officinalis L., Cyrtomiumfortunei J.Smith, Smilax china L. and Cornus officinalis Siebold & Zuccaqueous infusions against Gram negative resistant bacteria, with disruption ofthe bacteria membrane and interference with the intracellular structures bymultiple targets. A combination of plant infusions and antibiotics providedenhanced antibacterial activity of antibiotics, resulting in a significant decreasein antibiotic MIC, providing a potential path to restoring pathogen susceptibilityto antibiotics. Furthermore, both in vitro and in vivo toxicity tests have beenconducted on these plant infusions, indicating their safety profile. It can beconcluded from the results that multi-targeted action of aqueous crudemedicinal plant infusions contained synergistic phytochemicals effectivelyinhibits ESKAPE strains, which further suggested that medicinal plants couldpotentially be used to treat infectious diseases caused by resistant pathogensas a viable alternative to antibiotics, and could be an effective approach in thecontrol of antibiotic resistance in the near future.

Increased levels of multi-drug resistant bacteria worldwide pose a significant threat to human and animal health. The recommendations in the WHO's global action plan were to phase out and research alternatives to using antibiotics in animal farming. This study aims to identify natural plant supplements that can be used in animal feeds as sustainable alternatives to conventional antibiotics to promote animal health and performance. Plant extracts from traditional Chinese medicine were screened and their antibacterial activity was demonstrated against Campylobacter jejuni, Listeria monocytogenes, Escherichia coli and Salmonella enterica subsp. enterica serovar Enteritidis. Agrimonia pilosa Ledeb, Smilax glabra Roxb, Anemone chinensis Bunge, and Iris domestica (L.) Goldblatt and Mabb were found to exhibit strong broad-spectrum antibacterial activity against the above pathogens. A. pilosa Ledeb demonstrated antibacterial activity comparable to erythromycin and ampicillin. The synergistic antibacterial activity of combinations of these plant extracts and with antibiotics was also investigated against the above pathogens. Only one combination of extracts of A. chinensis Bunge and A. pilosa Ledeb indicated potential synergistic effects against both C. jejuni and E. coli. Combinations of A. chinensis Bunge and S. glabra Roxb, and A. pilosa Ledeb and S. glabra Roxb also indicated potential synergistic effects against E. coli. A. pilosa Ledeb, A. chinensis Bunge and S. glabra Roxb extracts each in combination with ampicillin and erythromycin indicated additive effects against C. jejuni, L. monocytogenes, E. coli, and S. enteritidis. In vitro assays were used to examine the bactericidal activity of A. pilosa Ledeb, S. glabra Roxb, A. chinensis Bunge, and I. domestica (L.) Goldblatt and Mabb over 24 hours in broth and in selective broth with chicken caecum content. The findings showed A. pilosa Ledeb, A. chinensis Bunge, S. glabra Roxb reduced caecum colonisation of E. coli, L. monocytogenes, and S. enteritidis in vitro within 0.5 hours. The results indicated a significant reduction of ≥99.9% of the above pathogens and rapid bactericidal activity comparable to ampicillin (P <0.001). The final part of this study investigated the effect of plant supplementation to broiler chicken diets on their performance, nutrient digestibility, and gastrointestinal tract microbiota. The results of the poultry trial highlighted the selective antibacterial activity of S. glabra Roxb, which decreased pathogenic bacteria, E. coli and Campylobacter spp. on day 21 (P <0.05) and 35 (P <0.01) and increased lactic acid bacteria relative abundance compared to the antibiotic group on day 14 (P <0.001) and 35 (P <0.01). In addition, S. glabra Roxb, and A. chinensis Bunge significantly increased weight gain compared to the negative control and the positive control receiving the recommended nutrient specification diet (P <0.001) and this was comparable to the positive amoxicillin control group. A. pilosa Ledeb, A. chinensis Bunge, and S. glabra Roxb significantly decreased feed conversion ratios compared to the negative control group and had comparable feed conversion ratios to the positive amoxicillin control group (P <0.001). The study contributes to addressing gaps in the research literature on the use of plant extracts as potential alternatives to antibiotics in poultry farming. It provides additional information on the antibacterial activity of many plant extracts against pathogens commonly found in the poultry gastrointestinal tract and new information about others including A. pilosa Ledeb, S. glabra Roxb, A. chinensis Bunge, and I. domestica (L.) Goldblatt and Mabb. The results of the poultry trial also indicate the potential of S. glabra Roxb and A. chinensis Bunge in reducing pathogens, enhancing the bird's beneficial microbiota, and improving bird performance. Overall, the findings of this study highlight the potential of using plant extracts as a supplement to poultry diets.

Different types of antibiotics and chemical preservatives have been widely used in the agricultural food industry for various purposes including improvement of productivity and profits in modern farming practice, control of animal diseases and extension of shelflife of food products. However, due to increasing public concerns on the potential harmful effect posed by antibiotic residues in foods and the development of antibiotic resistance as a result of extensive use of chemical antibiotics in agriculture, the European Commission has banned the growth promoting antimicrobials in food animal production since Jan 2006. Research on the potential application of naturally occurring compounds in the food industry has recently gained increasing attention. This study aim at evaluating the antimicrobial effects of naturally occurring compounds against pathogenic microorganisms relevant to food safety, public health, animal health and welfare. The overall objectives included the determination of the minimum inhibitory concentrations of the active compounds, investigation of the possible synergistic effects when combining different natural antimicrobial compounds and the investigation of antimicrobial modes of action of these active compounds. Antibacterial activities of naturally-occurring compounds were evaluated against Escherichia, coli (E. coli), Clostridium sporogenes and Mycobacterium avium subspecies paratuberculosis {Map). The most active compounds against these bacteriainclude 2,5-dihydroxybenzaldehyde, 2-hydroxy-5-methoxybenzaldehyde, apple Epolyphenols, green tea polyphenols, gallic acid, garlic powder, onion powder, onion oil,carvacrol, oregano oil, tra«5-cinnamaldehyde and cinnamon oil. These compounds were subjected to antimicrobial mode of action studies. Significant cellular responses e.g. leakage of phosphate and reduction of intracellular ATP wereobserved. Monolayer study, a biophysical approach, suggested that the compounds interacted with cell membrane lipids, which was in agreement with the major findings of this study obtained using the biological approaches. A partial synergistic effect against vM. smegmatis MC2155 was also observed when combining oregano oil and herbal extract Aw at sub-MIC concentrations.

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.