Under Pressure

Summary

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Klebsiella pneumoniae is a bacterium that can be found in the environment but also on humans. It usually causes no harm to a healthy person. However, in people with a weak immune system Klebsiella can lead to very severe disease and even death. These infections can be very difficult to treat, as Klebsiella can become very resistant to antibiotics. Because all these factors are often found in hospitals, where a lot of people have weakened immune systems and are generally more vulnerable, this bacterium is also known as a hospital bug. In order to prevent infections with Klebsiella via vaccinations or help the body to fight an ongoing infection, antibodies are considered as a promising tool. These very specific proteins of the human immune system guide the human immune reaction. One of the first systems in the human immune system to react to these bound antibodies is the complement system. The complement system labels bacteria for immune cells to destroy them, but it can also directly kill some bacteria by forming a hole in the bacterial cell wall. However, bacteria such as Klebsiella have developed ways to prevent the immune system from killing them. Those mechanisms are not always well understood. The goal of this PhD thesis was to study the different ways that bacteria have evolved to prevent being killed by the complement system.

In the first paper we looked at the surface of the bacterial cell wall. In Klebsiella (but also others) long sugar chains, the so-called O-antigen stick out from the surface and coat the whole bacterium. Bacteria can express a huge variety of different sugar chain patterns. We noticed that if Klebsiella had a specific sugar structure on their surface, they would better survive the attack of the immune system. We then looked more into the mechanistic details, and could show that the sugar structure prevents the complement system to form a hole in the bacterial cell wall via formation of the MAC pore.

In the second paper we looked at how the bacterium changes its surface composition when evolving to resist the antibiotic colistin. We found that in Klebsiella, mutations in a specific regulatory gene were responsible for resistance to the antibiotic. At the same time, that very mutation caused a change in the sugar chain coat covering the bacterium. This change made binding of antibodies possible and allowed the activation of the complement system. The complement system was then able to perforate the cell wall and kill the bacteria via membrane attack complexes.

In the third manuscript we looked more closely at the mechanism we found in the first paper. The hole that is formed in bacterial cell walls is the result of a line of reactions that happen one after the other. These reactions are very tightly regulated in the human body, because they can otherwise lead to diseases by being either not active enough to protect from infections, or by being too active and causing harm to human cells. We found that the sugar coat of Klebsiella would overactivate the complement at a critical step. This overactivation would disbalance the natural ratio of complement proteins needed to effectively form membrane attack complexes. This would then in turn prevent the complement system to form holes in the bacterial cell wall and the bacteria could survive. We could also show that in this case, to our surprise, inhibiting complement is something you usually don't want to do during an infection would increase killing of Klebsiella.

Lastly, in the fourth paper we looked at the mechanisms by which the different but equally dangerous bacterium Pseudomonas aeruginosa prevents being killed by the immune system. We used a random mutagenesis approach with small mobile genetic elements called transposons. This allowed us to look at the expression patterns of all non-essential genes that Pseudomonas expresses when exposed to the complement system. We found a couple of genes to be expressed more often in bacteria that were more resistant. One set of genes was particularly interesting, because it was not described before. Among other things, we found that one of these genes lead to the expression of a new undescribed protein. This protein prevented that Pseudomonas was being killed by the complement system even though complement formed a hole in the cell wall. We were unfortunately unable to further characterize this protein, but this work lays a solid foundation for further research.

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