  The bacterium Myxoccocus xanthus produces multicellular protective droplets
called fruiting bodies when starved. These structures form initially through
the active dewetting of cells into surface-bound droplets, where substantial
flows of the material are needed as the fruiting bodies grow and become round.
These dynamics are followed by a primitive developmental process in which the
fluid-like droplets of motile cells mature into mechanically-resilient mounds
of non-motile spores that can resist significant mechanical perturbation from
the external environment. To date, the mechanical properties of fruiting bodies
and the changes in cellular behavior that lead to maturation have not been
studied. We use atomic force microscopy to probe the rheology of droplets
throughout their development and find that relaxation occurs on two time
scales, $\sim$1~s and $\sim$100~s. We use a two-element Maxwell-Wiechert model
to quantify the viscoelastic relaxation and find that at early developmental
times, cellular motility is responsible for the flow of the material but that
this flow ceases when cells stop moving and become nonmotile spores. Later in
development there is a dramatic increase in the modulus of the droplet as cells
sporulate and the fruiting body matures, resulting in a mostly elastic
structure that can protect spores from harsh environmental insult.
