Bacterial colonies growing on soft agar often display complex branched structures and collective motions involving many cells. I will present a model which consists of reaction-diffusion equations coupled to a hydrodynamic equation for the velocity field of the bacteria-water mixture, which captures such complex behaviors. I will show numerical results which illustrate how complex collective motions within a colony may destabilize its boundary and lead to the formation of branches. I will also present phase diagrams, which classify colony morphology as a function of the initial amount of nutrients on the plate and of the wetness of the agar. This work is joint with Thierry Passot (Observatoire de la Cote d'Azur, Nice, France).