After years of preparatory research, the time is now ripe to understand the full picture of how our immune system exactly functions.
Suzan Rooijakkers
Suzan Rooijakkers

"Our immune system is super efficient." Suzan Rooijakkers, microbiologist at the University Medical Center Utrecht is enthusiastic about the subject. "When bacteria enter our body, our defence systems react within a few minutes. And 99% of the time it works wonderfully well. But how does it happen exactly?" Rooijakkers has received an ERC Starting Grant that will enable her to unravel this process at the molecular level with a research team over the next few years. This will also allow us to gain a better understanding of why it sometimes goes wrong and bacteria make us sick. "After years of preparatory research, the time is now ripe to understand the full picture of how our immune system exactly functions." This knowledge will be particularly useful now that more and more bacteria are resistant to antibiotics.
Text: Youetta Visser

Gram-negative bacteria

Rooijakkers' research focuses on so-called Gram-negative bacteria. These bacteria are normally eliminated by our complement system – a protein family in our blood, which is activated when bacteria invade our bodies. The complement proteins set off a chain reaction on the bacterial cells and open up a pore, or hole, in the cell membrane, which kills the cell. Rooijakkers explains: "During the chain reaction, so-called 'convertase' enzymes are produced, which first convert C3 molecules in order to cover the bacteria with complement proteins. The C3 convertase enzymes then change into C5 convertase enzymes, which are needed to open up the pores. In recent years, we have unravelled the structure of C3 convertase enzymes. Now it's time to investigate at the molecular level how C3 enzymes convert into C5 convertase enzymes and ultimately kill the bacteria." As yet, no one knows what C5 enzymes look like. In her application to the ERC committee, Rooijakkers was forced to make sketches she had devised herself in order to provide the committee members with an image. The committee understood well enough to see that insight into the functioning of our immune system is extremely valuable, especially at a time when many bacteria are resistant to antibiotics.

Imitating the surface of a bacterium

A large number of scientific advances have now made this research possible. "We know a lot more about bacteria than before," says Rooijakkers. "What is more, structural biology is providing new insights. It is now also possible to stain bacteria with fluorescent dyes so that you can follow molecules through a microscope." Significant hurdles have also been overcome in Utrecht. "My VIDI-funded research and work by two of my doctoral candidates, Daphne Stapels and Evelien Berends, have provided us with the necessary knowledge and instruments. Together with Prof. Piet Gros, an expert in the crystal structures of complement proteins, we have deciphered the structure of C3 convertases. We have also managed to recreate the surface of a bacterium and to isolate proteins from blood plasma and stabilize them. We now have an extremely pure system that we can use to take measurements and see how the conversion into C5 enzymes works. This is all possible in vitro, in test tubes; we don't do any animal testing."

"My ERC-funded team is made up of a biochemist from the USA, a bacteria expert and several doctoral candidates. We consult together every week so that all the researchers know what the others are working on."  The research focuses on two types of bacteria in particular: E. coli and Neisseria. E. coli causes a lot of clinical problems and serves as a structural model for other Gram-negative bacteria. Neisseria infections cause meningitis.

New techniques

There is a reason why this research is being carried out at the University of Utrecht. "We have a strong focus on molecular systems here. It's fantastic that I can work alongside Piet Gros, who is an expert in the complement system at amino acid level. I can continue to build on his work. Together we are developing techniques and tools that no one else uses outside the Netherlands."

Rooijakkers is a born researcher, continuously striving to know more. "I'm fascinated by the interaction between the immune system and bacteria. I'd love to pick it apart completely at an in-depth level. The funny thing is that back in primary school I took a psychological test, which recommended that I choose a scientific profession. That surprised me back then, because I came from a very small town and didn't know any scientists. But it turned out to be bang on."