London School of Hygiene & Tropical Medicine

By: Ian Passmore and Brendan Wren

Vaccines are one of the most important tools in preventing bacterial diseases and in combating the increasing threat of antimicrobial resistance. For a vaccine to work effectively it needs to trigger an immune response to an important molecule (or collection of molecules), specific to a particular pathogen. Many bacteria are covered in sugar-based molecules known as glycans. Each species of bacteria has a unique glycan on its surface, which the immune system can use to identify it. Advanced knowledge of which glycans correspond to infectious bacteria can help the immune system prepare for future infections. Therefore, glycans are excellent targets for vaccines.

To date, the most successful vaccines against bacterial diseases are ‘glycoconjugates’, which have been used for over 30 years to prevent infections that cause pneumonia and meningitis. Glycoconjugate vaccines are made up of two parts: a glycan attached to a protein. The glycan part teaches the immune system which bacteria to prepare for and the protein part helps trigger the immune system to provide longer-lasting protection.

However, unlike other biomolecules, glycan biosynthesis is complicated. Glycans are built by combinations of enzymes acting in concert in a biosynthetic pathway that has been optimised by millions of rounds of natural selection. This means that glycan biosynthesis can be influenced by a variety of external factors. Designing a system to control the correct amounts of and conditions for each enzyme is extremely challenging. This presents a significant barrier for understanding and exploiting glycans.

Beating nature at its own game

Advances in the field of synthetic biology have offered solutions to these challenges. By combining rapid DNA assembly, massive parallel testing, and novel engineering approaches, glycan biology can now be studied in ways that were not previously possible. In our recent publication, we demonstrated how such synthetic biology principles can improve the yield of glycoconjugates produced in modified strains of bacteria. In this study, we took a glycan biosynthesis pathway from the human gut pathogen, Campylobacter jejuni, and reconstituted it in our engineered strains of E. coli. However, rather than building just one version of the pathway, we built many versions, each with a unique amount and combination of the glycan-building enzymes. Using a rapid screening method, we were able to identify the best-performing pathway variants- those that produced the most glycan. Remarkably, we identified pathways that outperformed glycan and glycoconjugate production observed in the native organism, effectively improving on years of evolution, and beating nature at its own game.

A faster and cheaper way of producing vaccines

Current methods for producing glycoconjugate vaccines are slow and laborious, requiring several complicated rounds of purification which can often alter the structure of the glycan. As a result, these vaccines, although incredibly effective, are very expensive and only realistically affordable for health systems in high-income countries. Engineered bacteria are an attractive alternative for production of glycoconjugates. Safe laboratory strain of E. coli can be genetically manipulated to produce all the components of a glycoconjugate vaccine in a single step.

Bacterial-engineered glycoconjugate vaccines can be produced for a fraction of the price of chemical conjugates. Cheaper vaccines are universally beneficial, but the benefits are greater in low-resource settings where the burden of disease is often higher. Simplified production processes also mean that these vaccines can be manufactured and distributed locally, rather than having to be produced and transported from higher income countries.

What’s next?

One of the key principles of the synthetic biology approach is to break a complex system down into its smallest possible units, which can then be quickly recombined in a new configuration. Much like how standardisation of parts led to rapid technological advances in the industrial revolution, the use of standardised modular molecular parts has led to similar breakthroughs in biotechnology. The use of synthetic biology will enhance preparedness for future pandemics and help push vaccines as a meaningful strategy to counteract anti-microbial resistance.