Multiscale Mechanics of Granular Biofilms

Many hydrated and soft materials, including biological tissues and assemblies, permit fluid flow while retaining form and function. However, fundamental investigations of these complex materials are challenged by multiscale structural heterogeneity. Here, we characterized the structural and mechanical landscape of “pink berry” granular biofilms to determine the extent to which microscale heterogeneity influences macroscale material properties. We performed mechanical indentation measurements on intact granules as well as nanoindentation measurements on thin sections. We employed advanced imaging to correlate the internal structure and spatial organization of granules with quantitative measurements of their multiscale mechanical properties. We found that pink berries behave like soft (≈1 kPa), strain-rate dependent materials with fast relaxation times (≈1 s), indicating rapid transport of fluids with nutrients and other substances through the granule. Additionally, microscale mechanical measurements revealed individual contributions from the cell microcolonies and the extracellular matrix of the composite, summing to the observed macroscopic mechanical properties by applying the rule of mixtures based on structural composition obtained from imaging with fluorescence light sheet microscopy (≈34% cell microcolonies). This multiscale approach combining imaging and mechanics may be broadly applicable to investigations of other complex soft materials, from synthetic hydrogel composites to biologically heterogeneous spheroids, organoids, and tissues.