Persistent vs. arrested spreading of biofilms at solid-gas interfaces - the role of surface forces

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.