The fermentation or anaerobic digestion of organic substrates like activated sludge from a wastewater treatment plant, or an organic waste from food production, involves the activity and interactions of a wide range of bacteria and archaea. Oftentimes, the availability of an inoculum determines what microbial cocktail is used in a bioprocess, but not its suitability. We work on determining ecological principles that let us predict the suitability of a microbial community to degrade a substrate. Currently, we explore how to engineer suitable communities by forcing community coalescence (e.g., through the mixing of different microbial communities) or to build new interactions in simplified synthetic communities. We use as model system fermentation or anaerobic digestion.