A bacterial extracellular matrix protein forms a supramolecular metallogel

The microbial extracellular matrix (ECM) is a complex network of self-secreted biopolymers uniting the cells in biofilms, providing them with structural integrity, and contributing to their elevated resistance to antibiotic treatments. Recently, there is a growing realization that a regulated, bidirectional cross-talk of bacteria and ECM confers biofilms with tissue-like traits, however, the mechanisms of spatio-temporal self-organisation of ECM and its regulation are still poorly understood. In the model organism for biofilm formation Bacillus subtilis, TasA is the major protein component of the extracellular matrix. We recently showed that TasA, isolated in the form of stable and structured globules, assembles into elongated and ordered fibers via a donor-strand complementation mechanism. In this study, we discovered that in the presence of zinc metal ions, TasA is able to form hydrogels with > 97% water content. Electron- and atomic force-microscopies as well as small angle X-ray scattering measurements show that cross-linking with zinc ions induces a transition in TasA morphology from one-dimensional fibers to two-dimensional sheets. Electron paramagnetic resonance measurements then show that such a significant morphological shift is associated with molecular changes in the coordination environment of zinc ions, which lead to structural changes at the protein level. When assembling into macroscopic networks, TasA-Zn metallogels exhibit viscoelastic properties and a fast recovery following an excessive strain. These metallogels represent a novel class of bacterially-derived ECMs that form easily at room temperature without covalent crosslinking, and may be used as a natural matrix-mimics in biofilm models for infection studies.