Biomineralization is a mineral precipitation process occurring in the presence of organic molecules and used by various organisms to serve a structural and/or a functional role. Many biomineralization processes occur in the presence of extracellular matrices that are composed of proteins and polysaccharides. Recently, there is growing evidence that bacterial biofilms induce CaCO3 mineralization and that this process may be related with their extracellular matrix (ECM). In this study we explore, in vitro, the effect of two bacterial ECM proteins, TasA and TapA, and an exopolysaccharide, EPS, on calcium carbonate crystallization. We have found that all the three biopolymers induce the formation of complex CaCO3 structures. The crystals formed in the presence of the EPS are very diverse in morphology and they are either calcite or vaterite in structure. However, more uniformly sized calcite crystals are formed in the presence of the proteins; these crystals are composed of single crystalline domains that assemble together into spherulites (in the presence of TapA) or dumbbell-like shapes (in the presence of TasA). Our results suggest the EPS affects the nucleation of calcium carbonate when it induces the formation of vaterite crystals and that unlike EPS, the proteins stabilize preformed calcite nuclei and induce their aggregation into complex calcite structures. Biomineralization processes induced by bacterial ECM macromolecules make biofilms more robust and difficult to remove when they form, for example, on pipes and filters in water desalination systems or on ship hulls. Understanding the formation conditions and mechanism of formation of calcium carbonate in the presence of bacterial biopolymers may lead to the design of suitable mineralization inhibitors.

Biofilms are aggregates of microbial cells that form on surfaces and at interfaces, and are encased in an extracellular matrix. In biofilms made by the soil bacterium Bacillus subtilis, the protein TapA mediates the assembly of the functional amyloid protein TasA into extracellular fibers, and it anchors these fibers to the cell surface. We used circular dichroism and NMR spectroscopy to show that, unlike the structured TasA, TapA is disordered. In addition, TapA is composed of two weakly interacting domains: a disordered C‐terminal domain and a more structured N‐terminal domain. These two domains also exhibited different structural changes in response to changes in external conditions, such as increased temperatures and the presence of lipid vesicles. Although the two TapA domains weakly interacted in solution, their cooperative interaction with lipid vesicles prevented disruption of the vesicles. These findings therefore suggest that the two‐domain composition of TapA is important in its interaction with single or multiple partners in the extracellular matrix in biofilms.

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

Photocatalysis has an exceptional capacity to eliminate a wide range of harmful microorganisms and is proven to be superior over commonly used disinfection methods. A visible light-induced photocatalyst, the BiOCl 0.2 Br 0.8 @gypsum hybrid composite, composed of microspheres (∼3 μm) molded with a gypsum composite as a honeycomb-shaped filter was proven to inactivate a large selection of bacteria including Salmonella typhi , Bacillus subtilis, and Listeria monocytogenes via remote photocatalysis. The chemical composition and morphology of the composite were unveiled with the help of scanning electron microscopy, transmission electron microscopy, N 2 sorption, Fourier transform infrared spectroscopy, diffuse reflectance spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. After 6 h under ambient conditions, our system declined the number of viable bacteria by fourfold. A similar effect was observed at a low temperature, where we rapidly and completely diminished L. monocytogenes inside a refrigerator within 24 h of visible light illumination.

The cover feature picture shows the sequence of the protein TapA along a colored ruler, depicting the degree of disorder: red being structured and blue being disordered residues. The bottom cord shows that the structure of TapA is environment‐dependent. The C‐terminal domain remains disordered, whereas the N‐terminal domain (red tangle) gains structure upon a temperature increase or the addition of liposomes (yellow balls in the fore‐ and background). More information can be found in the communication by L. Chai et al. on page 355 in Issue 3, 2019 (DOI: 10.1002/cbic.201800634).