Structural and Mechanistic Investigations of Antibiotic Production in Bacteria

There is an indisputable problem of antibiotic resistance and an urgent need for the discovery and development of new antibiotics that are cost effective to produce. In February 2017, the World Health Organisation (WHO) published its first ever list of antibiotic resistant priority pathogens that pose serious risks to human health. This list includes high priority targets such as the well-publicised methicillin resistant Staphylococcus aureus (MRSA) and there is clear recognition that urgent action and research into new antibiotics is required. However against this alarming headline, a drop in investment in research and development into new antibiotics has led to a dramatic fall in the number of new drugs being discovered and a reduction in knowledge and expertise that is capable of delivering them.

Natural products and their derivatives have, and will continue to be, an important source of these antibiotics that are critical for human and animal health. Polyketides are a family of natural products which include high value compounds including antibiotics. They are derived from a wide range of sources including bacteria and fungi. Over the last fifty years some understanding of the remarkably complex and diverse ways in which antibiotics are synthesised in Nature has been gained. Whilst bacteria and fungi might be viewed as simple organisms, they arguably outperform the world's best synthetic chemists in terms of their elegance and efficiency and it is this power we wish to harness. By fully understanding nature's biosynthetic machinery we can engineer pathways to deliver new bioactive compounds.

It turns out that many antibiotics are made by a series of chemical reactions catalysed by megaprotein assemblies that act as nano-scale factories inside the microbe. Simple organic molecules are activated and loaded at one end, joined together and then released as completed (usually elaborate) products at the other end. The nano-factories join the simple building blocks on an assembly line of individual modules, akin to a group of robots performing operations in vehicle manufacture. The chemical structure of each antibiotic is thus determined by the enzymes present at each stage of the assembly line, the blueprint being the biosynthetic gene cluster. We understand some rules for building these factories and can rearrange the order of modules to produce new compounds, but sometimes this just breaks the assembly line, or produces an unexpected compound. This reveals we don't truly understand how they have evolved to fit together and we do not know all of the chemical steps required.

Our aim is to investigate two different "factories" that produce anti-MRSA antibiotics. The first produces mupirocin, which is used commercially but is restricted to topical use and in nasal sprays because of its instability. The second is thiomarinol, structurally related to mupirocin, but has features that might lead to wider applications. A substantial number of important biosynthetic steps are not fully understood in both of these systems. Combining the expertise of chemists, biochemists, structural biologists and molecular modellers we will elucidate these steps which could lead to a more stable version of mupirocin. We will also use these systems to answer quite general questions so we can build new pathways to novel compounds in a rational way.