Peptidoglycan Dominates Bacillus Subtilis Spore’s Water-Responsive Actuation

Water-responsive (WR) materials that swell and shrink in response to changes in relative humidity (RH) show a great potential as high-energy actuators for soft robotics and novel energy harvesting systems. Bacillus (B.) subtilis spore is a WR material that shows an extremely high actuation energy density of ~10 MJ m^-3, surpassing that of existing artificial muscles and actuators. However, the underlying mechanism of spores’ powerful actuation remains unclear. Here, we present that peptidoglycan (PG) within spores’ cortex layer dominates spores’ WR behaviors, and PG’s WR energy density reaches 59.9 MJ m^-3 that exceeds spores’ record high energy density by three-fold. We found that, when RH is increased from 10% to 90%, PG’s exhibit WR strain of ~52.4%, which is ~5 times larger than that of spores. To correlate PG’s WR properties to that of spores, we analyzed PG’s distribution in 85 spores by three-dimensional reconstructing spores’ serial cross-section images obtained by an FIB-SEM dual-beam system. PG’s spatial distribution, together with its WR energy density, suggest that PG dominates spores’ water-responsiveness. Our finding of PG’s extreme water-responsiveness provides a new record energy density for actuators, but also suggests opportunities to grow spores with higher actuation energy density by using genetic modification.