The human microbiome plays essential roles in modulating human health and disease. Based on preliminary data as well as previously published work, I hypothesise that 1) the oral cavity acts as a reservoir of microbes which continuously seed the gut and that 2) specific aberrations of the microbiota along the gastrointestinal tract (GIT) are linked to the development of colorectal cancer (CRC) and oral cancer (OC). My aim is to test these hypotheses through an integratation of metagenomic and metatranscriptomic data generated from microbial consortia in the colons/recta as well as oral cavities of patients with CRC or OC, and compare these to healthy individuals. Successful completion of the project objectives would address the questions: which microbial strains survive passage from the oral cavity in to the gut? Does the mouth act as a reservoir for strains driving CRC development? And can a panel of multi-factorial biomarkers outperform current CRC and OC diagnostic standards?