Recent studies suggest an association of periodontitis with an overall increased cancer risk but also with cancer development in specific anatomic regions, such as in the colon. The gut microbiome in general and specifically Fusobacterium nucleatum have been described to likely affect colorectal cancer (CRC) development. The present project intends to evaluate the possible contribution of the oral cavity as a reservoir for dissemination of F.nucleatum to the colon. Additionally, the gut microbiome and dietary intake will be assessed as possible factors influencing colonization of CRC sites with F. nucleatum and an association of F.nucleatum colonization in the colon with parameters in the saliva and serum will be tested. One-hundred-fifty patients diagnosed with CRC will be included; after cancer diagnosis and before starting cancer treatment a periodontal examination will be done and saliva and serum samples and at the time-point of tumor resection a biopsy of the CRC sites will be collected.