The human gastro-intestinal tract is home to trillions of microorganisms. These microbes are collectively referred to as the gut microbiome (GM). The majority of these microbes reside in the colon. Bacteria are the main constituent of this complex community, but also archaea, unicellular eukaryotes and viruses (especially viruses having bacteria as their hosts, so-called bacteriophages) are important members of the GM. During the last 2 decades it has become increasingly clear, that GM imbalances (dysbiosis) are an important part of disease etiology of a long list of diseases ranging from obesity and type 2 diabetes over autoimmune diseases like asthma and type 1 diabetes to colon cancer.  

The GM is key to human metabolism, as it provides energy and bioactives from otherwise indigestible parts of the diet such as fibers and indigestible carbohydrates. Interestingly, it has also during recent years been found, that the GM play pivotal roles in the efficacy of some orally delivered drugs. In some cases a prodrug is activated by gut microbial activity (examples include the antimicrobial compound prontosil and the anti-inflammatory drug sulfasalazine);