Data from: Multiscale mechanics of granular biofilms

Biofilms produce and maintain extracellular polymeric substances (EPS)
essential for their form and function. While biofilms are commonly
lamellar and frequently targets of removal, granular biofilms are
increasingly incorporated into water treatment strategies. In both cases,
the EPS (mainly consisting of proteins, polysaccharides, and extracellular
DNA) is largely responsible for their persistence. Unlike many granular
biofilms, which are formed in engineered industrial bioreactors, the “pink
berry” consortia is a naturally-occurring and robust granular biofilm of
photosynthetic bacteria, found only in intertidal pools of salt marshes
around Woods Hole, Massachusetts (USA). The pink berry biofilm’s unique
ecological niche has sparked over three decades of study, yet its
mechanical properties are completely unknown. Here, we characterized the
structural and mechanical landscape of pink berry granules to determine
the extent to which microscale heterogeneity influences macroscale
material properties. We performed microindentation measurements on intact
granules and nanoindentation measurements on thin sections. We report that
intact pink berry granules exhibited low reduced elastic moduli (E*pink
berry ≈ 0.5–10 kPa) and fast stress relaxation times (τ1/2 ≈ seconds),
consistent with previous investigations of soft and viscoelastic biofilms.
Nanomechanical measurements of thin pink berry sections revealed two
mechanically-distinct domains: a very soft extracellular polymeric
substance (EPS) matrix surrounding stiffer microcolonies of purple sulfur
bacteria (PSB). Light sheet fluorescence microscopy revealed the spatial
organization and distribution of cell-dense PSB microcolonies (34 vol. %)
within EPS matrix (66 vol. %), suggesting the nanomechanical behavior of
EPS dominates macroscale pink berry mechanics. Our multiscale experimental
approach combining mechanics and imaging may be broadly applicable to
investigations of complex soft materials, from synthetic hydrogel
composites to biologically heterogeneous spheroids, organoids, and
tissues.