We humans live in a close relationship with different microorganisms and bacteria are the most common cell type in the body, an adult human carries 1-2 kg of bacteria mainly on the skin and in the intestines. This so-called normal flora contributes to a normal and healthy life. At the same time, outside bacteria, but also members of the normal flora, can cause disease. For more than 30 years, we have discovered, purified and studied bacterial proteins that interact in different ways with the human host in which the bacteria live. Many of these proteins have been isolated from streptococci, bacteria that can appear in the throat and on the skin without causing any symptoms of disease, but which also cause mild (tonsillitis, rose fever) and life-threatening infections (sepsis and deep tissue infections). In this project, we want to investigate how some of the streptococcal proteins we have discovered may contribute to severe infections. These include a group of antibody-binding surface proteins (members of the so-called M-protein family) and a protein that the bacteria deliver to the environment (protein SIC). We also want to study an enzyme (IdeS) that cleaves human IgG antibodies with a unique degree of specificity, an activity that allows the bacteria to defend themselves against our immune system. The fact that IdeS only cleaves IgG antibodies and no other protein, on the other hand, has opened an exciting opportunity to treat patients with disease-causing IgG antibodies with this bacterial enzyme.