A Bacterial Extracellular Matrix Protein Forms a Supramolecular Metallogel

The microbial extracellular matrix (ECM) is a complex network of self-secreted biopolymers that unite cells in biofilms, providing them with structural integrity and protection from antimicrobials. Made of fibrillar proteins interspersed with polysaccharides, ECM material is often implicitly held responsible for biofilms' hydrogel-like mechanical signature; the exact molecular mechanisms of the ECM assembly and their link to hydrogel formation, however, remain enigmatic. Here, we discover that in the presence of zinc ions, TasA, the major ECM protein of Bacillus subtilis, forms hydrogels that recapitulate the mechanical properties of whole biofilms. Electron and atomic force microscopies as well as small-angle X-ray scattering showed that cross-linking with zinc ions induced a transition in the TasA architecture from one-dimensional fibers to two-dimensional nanosheets. Electron paramagnetic resonance and X-ray diffraction data link this dramatic morphological shift to molecular changes in the coordination environment of zinc ions and protein structure. This study provides the first example and a novel molecular mechanism of supramolecular metallogelation by a native bacterial protein. Bridging between molecular processes, mesoscopic structure, and macroscopic material properties, it not only advances the fundamental understanding of ECM assembly in situ but also establishes artificial ECM-based (metallo-)hydrogels as matrix mimics for bacterial infection studies.