Peptidoglycan (PG), bacterial spores' major structural component in their cortex layers, was recently found to regulate the spore's water content and deform in response to relative humidity (RH) changes. Here, we report that the cortex PG dominates the Bacillus subtilis spores' water-content-dependent morphological and mechanical properties. When exposed to an environment having RH varied between 10% and 90%, the spores and their cortex PG reversibly expand and contract by 30.7% and 43.2% in volume, which indicates that the cortex PG contributes to 67.3% of a spore's volume change. The spores' and cortex PG's significant volumetric changes also lead to changes in their Young's moduli from 5.7 and 9.0 GPa at 10% RH to 0.62 and 1.2 GPa at 90% RH, respectively. Interestingly, these significant changes in the spores' and cortex PG's morphological and mechanical properties are only caused by a minute amount of the cortex PG's water exchange that occupies 28.0% of the cortex PG's volume. The cortex PG's capability in sensing and responding to environmental RH and effectively changing its structures and properties could provide insight into spores' high desiccation resistance and dormancy mechanisms.
Keywords: Bacillus subtilis spores; biomaterials; peptidoglycan; water-responsive materials.