Quorum sensing peptides are small protein-like compounds, which are used by certain bacteria to communicate with each other. However, this phenomenon is not limited to the bacteria themselves: recent research demonstrates that these compounds interact with human cells as well. As such, these particular compounds potentially can explain the role of bacteria in/on our body in health or disease. We previously found for the first time that a certain quorum sensing peptide, i.e. PapRIV, clearly influences the behavior of microglia cells in vitro. These cells are crucial for the functionality of the brain, with changes in their behavior contributing to different mental disorders. In this research proposal, we will further investigate the effects of this peptide on the microglia cells in vivo, as well as the influence on the behavior in mice. The results of this study will help in our understanding of how bacteria influence our health, opening novel diagnostic and therapeutic possibilities.