More and more pathogens become resistant to antibiotics which causes a serious health crisis. Infections that could be easily treated in the past become lethal threats again. This situation requires new ways of treating and curing infections. The goal of this project is to find an alternative infection treatment that is not based on antibiotics but that harnesses the natural interactions between microbes. Pathogens that want to infect our body must first deal with the natural bacteria that already live there (commensals). Only if these bacteria allow the pathogen to grow in our body we can become sick. Accordingly, if we find commensals that inhibit pathogens we could use them as a natural treatment against infections. Developing a method to find such anti-pathogenic commensals is the central idea of this project. For that purpose, we image and identify microbes inside the gut of the host C. elegans. We will then use this data to identify microbes that out-compete pathogens – these are microbes that the pathogen cannot locally live together. We complement measurements within the worm gut with in vitro measurements outside the worms, where we will measure complete interaction networks for microbial communities. We are in particular interested in how the structure of those networks decides if a pathogen can persist in the community or not. Finally, we will add microbes - that we identified to repel pathogens - to the sick host to see if diseases can be treated this way. This method would allow for a completely new and highly individualized way to treat infections with only minor side effects.