- Biofilms are aggregates of cells that form surface-associated communities. The cells in a biofilm are interconnected with an extracellular matrix, a network that is made mostly of polysaccharides, proteins and sometimes nucleic acids. Some extracellular matrix proteins form fibers, often termed functional amyloids or amyloid-like fibrils, to differentiate their constructive function from disease-related amyloid fibers. Recent studies of functional amyloid assembly have neglected their interaction with membranes, despite their native assembly in a cellular environment. Here, we use the protein TasA, a major matrix protein in biofilms of the soil bacterium Bacillus subtilis, as a model functional amyloid protein, and ask whether a bacterial functional amyloid interacts with membranes. Using biochemical, spectroscopic and microscopic tools, we show that TasA interacts distinctively with model bacterial membranes and that this interaction mutually influences the protein and the membranes’ morphology and structure. At the protein's level, TasA fibers of similar structure and morphology are formed in the absence of membranes and in the presence of the eukaryotic model membranes. However, in the presence of the bacterial model membranes, TasA forms disordered aggregates with a different β sheet signature. At the membrane's level, fluorescence microscopy and fluorescence anisotropy measurements indicate that the bacterial membranes deform more considerably than the eukaryotic membranes upon interaction with TasA. Our findings suggest that TasA penetrates bacterial model membranes more than eukaryotic membranes and that this, in turn, disrupts the membranes and alters the fiber formation pathway of TasA. Considering the important role of TasA in providing integrity to biofilms, our study of the TasA-membrane interactions may direct the design of anti-biofilm drugs to the protein-membrane interface.