Bacterial Spores Respond to Humidity Similarly to Hydrogels

Bacterial spores have outstanding properties from the materials science perspective,

which allow them to survive extreme environmental conditions. Recent work by Harrellson

et al. [Nature 619, 500-505 (2023)] studied the mechanical properties of Bacillus Subtilis

spores, and unique evolution of these properties with the change of humidity. The

experimental measurements were interpreted within the theory of poroelasticity, assuming

that the spores behave as water-filled porous solids, subjected to hydration forces. Here

we revisit their experimental data using literature data on vapor sorption on spores and

ideas from polymer physics. We demonstrate that upon the change of humidity the spores

behave like rubber with respect to their swelling, elasticity and relaxation times. This

picture is consistent with the knowledge about materials comprising the bacterial cell

walls -- cross-linked peptidoclycan. Our results provide an interpretation of the mechanics

of bacterial spores and can help in developing novel synthetic materials mimicking the

mechanical properties of the spores.