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 bacteria to drag themselves through biofilms on environmental surfaces with a twitching motility. This twitching motility alternates with a type-IV pili-induced slingshotting motion.