Investigating the Biophysical Mechanism Behind Mechanotaxis in Myxococcus xanthus

Abstract

When Myxococcus xanthus colonies are grown on hydrogels which have been compressed, the colony grows anisotropically, elongated perpendicular to the direction of compression. This phenomenon, known as mechanotaxis, was first reported in 1942 and since has been experimentally confirmed at the colony scale, but its underlying biophysical mechanism remains unclear. This thesis seeks to understand the cell--substrate interactions that give rise to mechanotaxis. We develop an improved hydrogel compression device which allows for controlled uniaxial compression while minimizing various confounding factors. Through a series of experiments using this device, we show that colony anisotropy arises from a motility driven single-cell reorientation effect significantly modulated by gel hydration. In sufficiently dry and strongly compressed substrates, we then show that mechanotaxis can occur in cells with either pilus- or gliding-based motility, indicating that the phenomenon is not exclusive to gliding motility as previously reported. Furthermore, we reproduce mechanotaxis on both agarose and PAA hydrogels, implying that the underlying mechanism depends on compression-induced anisotropies common to both substrates. Birefringence and bulk stiffness measurements suggest that anisotropic elasticity is a likely driver of the phenomenon. Finally, we discuss possible mechanisms of cell-substrate interaction that may explain our experimental findings. Together, these results provide a deeper understanding of the biophysical mechanism behind mechanotaxis in surface-motile bacteria on compressed substrates.