The formation of biofilms provides structural and adaptive bacterial response to the environment. In Bacillus species, the biofilm extracellular matrix is composed of exopolysaccharides, hydrophobins, and several functional amyloid proteins. We report, using multiscale approaches such as solid‐state NMR (SSNMR), electron microscopy, X‐ray diffraction, dynamic light scattering, attenuated total reflection Fourier transform infrared (FTIR), and immune‐gold labeling, the molecular architecture of B. subtilis and pathogenic B. cereus functional amyloids. SSNMR data reveal that the major amyloid component TasA in its fibrillar amyloid form contain β‐sheet and α‐helical secondary structure, suggesting a nontypical amyloid architecture in B. subtilis. Proteinase K digestion experiments indicate the amyloid moiety is ~100 aa long, and subsequent SSNMR and FTIR signatures for B. subtilis and B. cereus TasA filaments highlight a conserved amyloid fold, albeit with substantial differences in structural polymorphism and secondary structure composition. Structural analysis and coassembly data on the accessory protein TapA in B. subtilis and its counterpart camelysin in B. cereus reveal a catalyzing effect between the functional amyloid proteins and a common structural architecture, suggesting a coassembly in the context of biofilm formation. Our findings highlight nontypical amyloid behavior of these bacterial functional amyloids, underlining structural variations between biofilms even in closely related bacterial species.—El Mammeri, N., Hierrezuelo, J., Tolchard, J., Cámara‐Almirón, J., Caro‐Astorga, J., Álvarez‐Mena, A., Dutour, A., Berbon, M., Shenoy, J., Morvan, E., Grélard, A., Kauffmann, B., Lecomte, S., de Vicente, A., Habenstein, B., Romero, D., Loquet, A. Molecular architecture of bacterial amyloids in Bacillus biofilms. FASEB J. 33, 12146‐12163 (2019). http://www.fasebj.org

The conformational strain diversity characterizing α-synuclein (α-syn) amyloid fibrils is thought to determine the different clinical presentations of neurodegenerative diseases underpinned by a synucleinopathy. Experimentally, various α-syn fibril polymorphs have been obtained from distinct fibrillization conditions by altering the medium constituents and were selected by amyloid monitoring using the probe thioflavin T (ThT). We report that, concurrent with classical ThT-positive products, fibrillization in saline also gives rise to polymorphs invisible to ThT (τ−). The generation of τ− fibril polymorphs is stochastic and can skew the apparent fibrillization kinetics revealed by ThT. Their emergence has thus been ignored so far or mistaken for fibrillization inhibitions/failures. They present a yet undescribed atomic organization and show an exacerbated propensity toward self-replication in cortical neurons, and in living mice, their injection into the substantia nigra pars compacta triggers a synucleinopathy that spreads toward the dorsal striatum, the nucleus accumbens, and the insular cortex.