Some bacteria use type IV pili to move along surfaces within a biofilm. (Biofilms are to viscous for bacteria to swim through with flagella.) The bacterium extends and anchors its pili. As the pili retract the bacterium is pulled forward. This enables the bacterium to drag themselves through biofilms on environmental surfaces and alternates with a type-IV pili-induced slingshotting motion (see Fig. 6). The top illustration shows a bacterium dragging itself or "crawling" along a surface. Bacteria with polar pili are also able to pull themselves upright and "walk" along the surface as shown in the bottom illustration.