When observing the morphogenesis of Bacillus subtilis floating biofilms, we detect some kinds of bacteria self-assemblies at the liquid-air interface that further undergo a bioconvection process. Thus, the presence of the bacteria in the solution locally modifies the properties of the environment that in turn acts on the cells behavior. This interplay between the population of bacteria and the heterogeneities of the environment plays an important role in conditioning the biofilm development as well as its structure. Once it’s formed, the pellicle mechanically responds to any external load like a visco-elastic film. We measure an internal pushing force by using simple manipulations and a suitable force sensor. We conclude that this force help bacteria to shape the biofilm, improve its mechanical resistance and facilitate its invasion and self-repairing.