Accessing Bioactive Natural Products from Cultured and Uncultured Microorganisms.

Abstract

Microbial natural products are an important source of novel medicines in an ongoing war against drug-resistant infections. Their unique chemical structures are capable of affecting new molecular targets that can help slow the ability of bacteria, viruses, fungi, and cancers to develop resistance mechanisms. To maximize the discovery of new medicines, we need to simultaneously explore the natural products of both cultured and currently uncultured microbes. This dissertation explores methodologies to facilitate access to bioactive medicines from both of these microbial sources.

To access the natural products of cultured microorganisms, we screened a library of extracts collected from microbial isolates for inhibitors of enzymatic targets involved in siderophore biosynthesis. Bioactivity-guided fractionation of the best hit led to the isolation of a novel class of antibiotics, the baulamycins. Analysis of the bioactivity of these natural products against the original enzymatic targets and several pathogenic microorganisms helped to elucidate their potency, broad-spectrum activity, and mode of inhibition. These new antibiotics from a cultured microbe serve as an important drug lead in the war against antibiotic resistance.

To access the natural products of uncultured microorganisms, we selected the clinically approved chemotherapeutic ET-743 as a model system. Researchers have long suspected that the medicine is produced by an uncultured microbial symbiont of a mangrove tunicate. We first sought to understand the biology of the uncultured microbe. We used metagenomic techniques to uncover its complete genome. Detailed analysis of the genome, its primary, and secondary metabolism provided a thorough look at its endosymbiotic lifestyle and the biosynthesis of ET-743.

In the last part of my dissertation, we utilized the newfound knowledge of the symbiont’s biology to take the first steps toward more efficient access of the drug. We biochemically analyzed a key enzyme involved in the production of the core of the anti-cancer medicine. This analysis supports our predicted role for the enzyme in ET-743 biosynthesis. It also supports the feasibility of reconstituting ET-743 and more potent or selective analogues in vitro. These collective methodologies can be applied to other systems, expanding our ability to harness the bioactive natural products of cultured and uncultured microorganisms.