Abstract: We investigate a new form of collective dynamics displayed
by Thiovulum majus, one of the fastest-swimming bacteria known. Cells
spontaneously organize on a surface into a visually striking
two-dimensional hexagonal lattice of rotating cells. As each
constituent cell rotates its flagella, it creates a tornado like flow
that pulls neighboring cells towards and around it. In the first part
of the talk, we describe the earliest stage of  crystallization, the
attraction of two bacteria into a hydrodynamically-bound dimer. In the
second part of the talk, we present the dynamics of bacterial
crystals, which are composed of 5--200 hydrodynamically bound cells.
As cells rotate against their neighbors, they exert forces on one
another, causing the crystal to rotate and cells to reorganize. We
show how these dynamics arise from hydrodynamic and steric
interactions between cells. We derive the equations of motion for a
crystal, show that this model explains several aspects of the observed
dynamics, and discuss the stability of these active crystals.