Due to an increase in microbial resistance, the need to find alternatives to conventional antibiotics chemotherapies has become important. There are currently few alternatives to conventional antibiotics. Of those alternatives, repurposing traditional medicinal plants is perhaps the most promising alternative. In particular, combination therapy using plant extracts with conventional antibiotics may provide potential alternatives to overcome bacterial resistance mechanisms and to repurpose conventional antibiotics. Despite first Australians’ use of plants as medicines, relatively few studies have reported on Australian native medicinal plants combined with conventional antibiotics. Syzygium australe and Syzygium luehmannii are native Australian medicinal plants known for their high antioxidant properties. The first aim of my study was therefore to study the food preservative properties of these plant extracts. The second aim of this thesis was to analyse the antibacterial activity of Australian native Syzygium spp. extracts against diarrhoea and food poisoning bacteria, and against bacterial triggers of some autoimmune inflammatory diseases. This was achieved using both disc diffusion and liquid dilution assays, MICs were calculated, and a comparison was made. The effect of the extracts was also tested in combination with conventional antibiotics to determine the class of interaction on two panels of bacteria. Phytochemical screening of extracts using gas chromatography-mass spectroscopy headspace analysis (GC-MS) was also undertaken to identify notable extract components. This analysis identified phenolic compounds, tannins, flavonoids, saponins and terpenoids in the extracts. It is likely that these phytochemicals may contribute to the potent antibacterial activity of these extracts. Overall, this study suggests that possible resistance modifying agents in the Syzygium spp. may induce interactive effects when the plant extracts are combined with conventional antibiotics. However, various enzymatic and metabolomic studies (e.g., HPLC-MS metabolomic fingerprinting) should be undertaken order to achieve an accurate insight into the synergistic mechanisms and other combinational effects. Isolation of these potential resistance modifying agents within the Syzygium spp. extracts may be highly beneficial in drug designing against the microbial triggers of some autoimmune diseases.