  We introduce and analyze a model for osmotically spreading biofilm colonies
at solid-air interfaces that includes wetting phenomena, i.e. surface forces.
The model combines a hydrodynamic description for biologically passive liquid
suspensions with bioactive processes. We show that wetting effects are
responsible for a transition between persistent and arrested spreading and
provide experimental evidence for the existence of this transition for Bacillus
subtilis biofilms growing on agar substrates. In the case of arrested
spreading, the biofilm is non-invasive albeit being biologically active.
However, a small reduction in the surface tension of the biofilm is sufficient
to induce spreading.
