Boundary Conditions Modulate the Orientation of Myxococcus xanthus Ripples

Abstract

Myxococcus xanthus are a species of social bacteria. They have the unique ability to self-assemble into "ripples". These ripples look just like ripples on the surface of a fluid and are made of alternating bands of high and low cell densities of cells. This thesis investigates how different boundary conditions affect the orientation and behavior of myxobacteria ripples. We use dense colonies of myxobacteria and stencils cut from silicone mats to create different boundary conditions, then image at 10x magnification. We found that the overall orientation of ripples can be either radial around the center, or parallel throughout a shape. For smaller shapes, the ripples tend to be parallel, while larger shapes allow ripples to have symmetry and organize radially. At the boundaries, the ripples mostly are parallel to the direction of the boundary, since the boundary forces a node to be there. As we get farther from the border in large shapes, wavefronts can interact and start to form a radial pattern throughout the shape. The behavior of the wavefronts depends on the width of the shape at that location. In narrow regions, the ripples are oriented parallel to one another, while in wider regions, the ripples take on the radial pattern. We also investigate how myxobacteria ripples behave while crossing a boundary created by two substrates. We find that the orientation of the ripples change across the change in surface in a manner resembling refraction in Snell's law. However, we find that the ratio of the refractive indexes in our experiment are actually the reciprocal of Snell's law.