INAMULLAH HAKEEM SAID

• Plant natural product is becoming the target of anti-infective research, and many groups have identified that phenolic compounds are possessing broad-spectrum antimicrobial activity. However, the scientific challenge in drug discovery is the isolation and identification of pure biologically active compounds. Nevertheless, the science of metabolomics offers the possibility to measure full secondary plant metabolomes with a limited experimental effort to allow identification of metabolome differences using statistical analysis of LC-MS data. In this study, a series of uncharacterized Rhododendron plant extracts have been assayed and compared by principal component analysis (PCA) & multivariate statistical analysis (PLS-DA), thereby revealing identification of a single bioactive cannabinoid compound, which could be validated using activity-guided fractionation. Moreover, using LC-MS in combination with statistical analysis (i.e., PCA and PLSDA), the Rhododendron species were found to be a rich source of polyphenols derivatives. Where, a total of sixty-nine hydroxycinnamic acid derivatives in the leaves of ninety-eight Rhododendron species were identified. Next, through a combination of phytochemical profiles with antimicrobial susceptibility and cytotoxicity, complemented by phylogenetic analyses, potentially antimicrobial active but non-cytotoxic compounds were identified. Furthermore, it is known that the human diet consists of a large part of plant-derived products (like vegetables, fruits, tea, and coffee), which contain relatively high levels of essential polyphenols. Gut microbiota often transforms them before absorption. This transformation produces a series of chemically diverse gut floral metabolites with high bioactivity. It was much desirable to obtain quantitative information on the uptake of dietary polyphenols into gut microorganisms and to understand quantitatively the time scale of bacterial polyphenol uptake which is absent from the literature. Hence a novel mass spectrometry-based assay was introduced to determine intracellular concentrations of specific dietary polyphenols in selected gut microbes (Escherichia coli and Bifidobacterium bifidum). Additionally, this approach was extended to study the simultaneous uptake of phenolics from a model mixture of dietary phenolics and to assess the uptake of several phenolics into bacterial model (E. coli) from real food matrices with green tea and coffee as examples. The thesis also addresses a brief determination of intracellular accumulation of antibiotics in gram-negative bacteria (E. coli and P. aeruginosa) using the phenolic uptake LC-MS based method, which may shed light on transport and efflux of antibiotics in and out of bacterial cells. Finally, the developed analytical methods applied to the isolation and identification of antibacterial compounds from marine algae (Phaeodactylum tricornutum), where isolated EPA (Eicosapentaenoic acid) displayed an inhibitory effect against Bacillus subtilis and different potential pathogenic Vibrio species.