Structure and Mechanics of Curli Fiber Materials


The presence of unwanted biofilms on surfaces is a great concern for industrial, naval, and healthcare fields, and many other settings. To better inhibit, remove, or harness the properties of biofilms, an explanation of how specific structural or chemical features of biofilm matrix components leads to enhanced adhesion and persistence is necessary. Top-down characterization of biofilm adhesive and mechanical behavior is needed to establish properties, however, neglecting molecular features prevents determining the physical causes behind these properties. While a particular biofilm matrix component, curli, is understood to be critical in biofilm adhesion, the exact properties of curli that facilitate this are unknown. This work seeks to use a bottom-up approach to describe the molecular features of the curli fiber and their potential implications in biofilm behavior. Individual curli subunits and their adsorption to polar and nonpolar surface types was investigated with regard to the energetic contribution of specific residues and role of flexibility in surface association. These results suggested that both the diverse chemical nature of amino acid side chains and the flexibility of certain regions within the CsgA are important in enabling adhesion to multiple types of inorganic surfaces. The structure of CsgA and CsgB subunits was further investigated by comparing a set of computationally predicted structures. These models were evaluated based on comparison to experimental information, expected residue alignment, and stability in Molecular Dynamics simulations. This study found the Robetta server to produce the most appropriate amyloid models of all prediction methods compared. CsgA and CsgB were further studied in their response to applied force and compared against well-known alpha-helical proteins, enabling inspection of the role of both sequence chemistry (CsgA vs. CsgB) and geometry (alpha vs. beta). While alpha-helices require more work per strain to unfold, beta-helices require more work per initial length and per hydrogen bond, marking the aspect ratio of helices as an important parameter in energy dissipation during unfolding. Together, these studies use molecular features to explain mechanical and adhesive behavior of curli materials, and can be extended to larger length scales for understanding curli's role in the biofilm matrix. This work has paved the way for investigations of curli mutants, assemblies, and nanocomposites including other biofilm materials. Highlighting the role of specific molecular features in important matrix components enables a mechanistic understanding of biofilm adhesion and opportunities extracting these properties for bioinspired materials.

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