Figure 8.

Assembly of microcolonies driven by pili-mediated cell-to-cell interactions. (A) Assembly of 1200 cells on a substrate with weak pili-substrate interactions and strong pili–pili-interactions (see tables 3 and 4). After initializing cells homogeneously on the substrate (top left), colonies begin to form after a few minutes (top center). They grow by single cells colliding with the less motile colonies. After one hour, almost all cells are assembled into colonies (top right). (B) Initial mixture of normal cells (yellow) and ΔpilT mutants (red). These mutants have pili which cannot pull. (C), (D) Colonies formed after one hour for strong and weak pili-surface interactions. Stronger pili-surface interactions lead to smaller colonies. The corresponding initial state is given in (B). The inset depicts a close-up in of a typical colony and shows that the mutant cells accumulate a the surface of the colony.