Discovery and development of novel antibacterials from natural products

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) is a major human pathogen associated with a variety of moderate to severe infections. The multi-drug resistant nature of this pathogen makes treatment difficult. The main aim of this study was to screen a library of marine samples for novel compounds with antibacterial activity. A high-throughput assay was developed to identify active extracts, followed by determination of minimum inhibitory concentrations (MICs) for hits. The toxicity of marine samples was measured against different cells lines. Crude extracts were purified using column chromatography (HPLC), with bioassay-guided fractionation. This led to the identification of seven molecules with antibacterial activity, including two novel series of amine natural products, and several previously reported compounds. Nine samples (of 1434 screened) were identified as promising antibacterial leads, all of which showed limited toxicity to mammalian cells. The next step was to develop synthetic pathways to these novel natural products and derivatives, and examine the antibacterial activity of the synthetic compounds. Synthetic molecules were broadly active against multiple bacteria including MRSA, E. coli, P. aeruginosa and virulent M. tuberculosis. Several of these synthetic compounds displayed a suitable therapeutic index for their target pathogens. Combinations of synthetic compounds and known antibiotics showed synergistic effects. Several fluorescent antibacterial analogues were shown to associate with the cells in fluorescence microscopy experiments. These data suggest that the chosen marine extracts are good leads for new antibacterial drugs. The final part of the thesis aimed to identify bacteria with antimicrobial activities from contaminated terrestrial sites. 38 bacteria were isolated with broad-spectrum antimicrobial activity. These include strains from genera Flavobacterium, Caulobacter and Ensifer, not previously known to produce bioactive molecules.