  Many organisms form colonies for a transient period of time to withstand
environmental pressure. Bacterial biofilms are a prototypical example of such
behavior. Despite significant interest across disciplines, physical mechanisms
governing the formation and dissolution of bacterial colonies are still poorly
understood. Starting from a kinetic description of motile and interacting cells
we derive a hydrodynamic equation for their density on a surface. We use it to
describe formation of multiple colonies with sizes consistent with experimental
data and to discuss their dissolution.
