Microplastics (MP) are already detected in all the environments becoming a global concern. They are quickly colonized by microorganisms forming a biofilm. The health risks associated to MP are still unknow but debated. One of the suggested risks it is the potential of biofilms on MP to harbor and disseminate pathogens. Some studies have already detected pathogens on MP. Besides, crowded biofilms are also a hotspot of horizontal gene transfer i.e. antibiotic resistance genes (ARGs). The transmission of waterborne pathogens and ARGs is a public health concern and one of the WHO urgencies. Wastewater treatment plants are one of the main sources of MPs, faecal bacteria and ARG in the environment, where they are in contact. Thus, faecal bacteria can get attached to the MP and reach the aquatic ecosystems in a more protected structure than as free bacteria. Bact&Plast aims to link these 3 different elements to evaluate the potential of microplastics to shelter and disseminate waterborne pathogens and ARGs. Bact&Plast combines an experimental approach using mesocosmos with environmental monitoring. First, we will evaluate the bacterial colonization of MP inquiring differences between plastic polymers and environmental conditions as the main driver of this process. To achieve this objective, we will use mesocosomos in controlled conditions and the evolution of the biofilm will be followed using massive sequencing to determine the bacterial diversity and functional diversity. A similar approach will be used to detect the potential of faecal bacteria to get attached on the biofilms. For this purpose, mesocosms will be performed with treated sewage diluted in freshwater and seawater. Faecal indicator bacteria (FIB), potential opportunistic pathogens and ARGs will be followed in the biofilm-MPs and water. Finally, the obtained data will be compared with MPs from environmental samples: sewage effluent, river water and sediments and sea water and sediments.