Colorectal cancer (CRC) is the third most common type of cancer and has a high mortality rate. Increasing evidence indicates that the microbiota influences both cancer initiation, progression and therapy response. Although the underlying molecular mechanisms are incompletely understood, they involve cross-interactions between microbiota, immune and tumor components. We recently discovered that genotoxic E.coli strains exacerbate CRC through epithelial adhesion mechanisms, and that also fungi, including Candida albicans can drive CRC development. We will investigate mechanistically how these microbes and their metabolites modulate CRC, and will isolate and characterize new CRC-asssociated microbes, with a specific focus on the intratumoral microbiota. We will perform functional studies in gnotobiotic transgenic mouse models of early CRC, and we will use intramucosal tumor organoid injection technology to model advanced and metastatic disease. Although immune checkpoint inhibition therapy (ICI) is not efficacious in most prevalent CRC subtypes, we obtained preliminary data showing that specific microbiota modulation can improve ICR efficacy in CRC mouse models. We will identify new ICI-modulating microbes and their metabolites from human CRC patients undergoing ICI therapy, and will unravel underlying ICI-promoting mechanisms. We hereby take an innovative approach to unravel complex host-microbe interactions in CRC development and therapy.