Global pharmaceutical consumption is rising with growing and ageing human population and more intensive food production. Recent studies have revealed pharmaceutical residues in aquatic ecosystems and organisms. Among pharmaceuticals, antibiotics, which are natural, synthetic or semi-synthetic compounds, able to kill and inhibit growth or metabolic activity of microorganism, are one of the most widely used categories. Environmental concentrations of antibiotics are often low, but their continuous discharge in the environment, exposes aquatic organisms during their entire life cycle. There is currently little data on the non-therapeutic effects of antibiotics. The microbiota represents the trillions of microorganisms inhabiting a host. Gut microbiota contributes to the development of an immune response, to metabolic processes and it influences the host behaviour. Gut microbiota is very plastic and recent insights have suggested that most diseases could be linked in some way to an alteration of the gut microbiota (i.e. a dysbiosis). However, no studies so far have investigated the impact of antibiotic residues on wildlife’s gut microbiota composition and its effect on disease occurrence. Top predators of aquatic ecosystems such as mustelids are exposed to high levels of pollutants and are considered as keystone sentinel species for the study of pollutant effects on wildlife. Mustelids are also a reservoir species to zoonotic disease, as they are exposed to diverse types of viruses that can be passed from and to domestic animals and humans. The aims of this research action are to 1) determine the ‘natural’ variation of gut microbiota composition according to biological factors, 2) investigate a threshold effect level for which pharmaceuticals could lead to health effects, 3) test if the individuals with high concentrations of pharmaceuticals are more likely to develop infectious diseases. I will investigate these questions in the American mink Neovison vison.