Endospore pili: Flexible, stiff, and sticky nanofibers

Species belonging to the Bacillus cereus group form endospores (spores) whose surface is decorated with micrometers-long and nanometers-wide endospore appendages (Enas). The Enas have recently been shown to represent a completely novel class of Gram-positive pili. They exhibit remarkable structural properties making them extremely resilient to proteolytic digestion and solubilization. However, little is known about their functional and biophysical properties. In this work, we apply optical tweezers to manipulate and assess how wild-type and Ena-depleted mutant spores immobilize on a glass surface. Furthermore, we utilize optical tweezers to extend S-Ena fibers to measure their flexibility and tensile stiffness. Finally, by oscillating single spores, we examine how the exosporium and Enas affect spores' hydrodynamic properties. Our results show that S-Enas (μm-long pili) are not as effective as L-Enas in immobilizing spores to glass surfaces but are involved in forming spore-to-spore connections, holding the spores together in a gel-like state. The measurements also show that S-Enas are flexible but tensile stiff fibers, which support structural data suggesting that the quaternary structure is composed of subunits arranged in a complex to produce a bendable fiber (helical turns can tilt against each other) with limited axial fiber extensibility. Finally, the results show that the hydrodynamic drag is 1.5 times higher for wild-type spores expressing S- and L-Enas compared with mutant spores expressing only L-Enas or "bald spores" lacking Ena, and 2 times higher compared with spores of the exosporium-deficient strain. This study unveils novel findings on the biophysics of S- and L-Enas, their role in spore aggregation, binding of spores to glass, and their mechanical behavior upon exposure to drag forces.