Dennis obtained his PhD in Food Microbiology from The Royal Veterinary and Agricultural University, Denmark in 2006, studying fermentation of cocoa. During his PhD he gained a strong interest in the emerging research field of the human gut microbiome, later with particular focus on the role of bacteriophages in gut microbial ecology and gut-related health, disease and how bacteriophages can be used to treat or prevent gut related disease. He was appointed Associate Professor in 2010 and Professor in 2017 at the Department of Food Science, University of Copenhagen and has to date published 231 peer-reviewed papers.