Rigidity Governs Entrainment of Bacteria Cells in Biopolymer Scaffolds

Embedding cells in biopolymer networks is a promising route for designing responsive, dynamic materials for applications such as tissue engineering. The ability of cells to tune material properties relies on the physical properties of the biopolymers and their interactions with each other and the embedded cells. Here, we investigate how biopolymer stiffness and cross-linking influences cell entrainment within the network, and how the network is restructured by the embedded cells. Specifically, we design composites of Escherichia coli cells and cytoskeleton filaments, either semiflexible actin filaments or rigid microtubules; and characterize the effect of cell volume fraction and filament cross-linking on the structure and interactions of the components. We find that entropically driven depletion interactions between cells and filaments lead to emergent filament bundling at intermediate cell densities, which is more pronounced for actin compared to microtubules. Moreover, cross-linking of actin causes enhanced clustering of cells and colocalization of cells and filaments compared to the entangled case. Conversely, cross-linking of microtubules modestly suppresses cell-induced restructuring and colocalization compared to the entangled case. Our work highlights the ability of biopolymer stiffness and connectivity to selectively tune composite properties and cell entrainment for use in diverse applications from tissue engineering to biocement.