Microplastics (MP) provide a new habitat for biofilm-forming bacteria in aquatic systems. It was investigated if MP could become a vector for atypical bacterial assemblages, including potential pathogens. The MP-biofilms differed from those on seston and free-living bacterial assemblages, but less from those on control surfaces. Vibrio abundances were higher on MP than in the water, but highest on wood. MP are thus a distinct habitat for biofilm bacteria; however, their vector potential depends on the spatial, temporal, and environmental dynamics of the system.