The type IV pilus steering committee: how Pil-Chp controls directional motility

Many microbial species live on surfaces and employ various strategies for initiation of and survival within a surface-attached community. One such strategy implemented by many bacterial species is to move across surfaces using grappling hook-like appendages called type IV pili (TFP) which extend, attach to the surface, and retract to pull the cell body forward. In the bacterium Pseudomonas aeruginosa, TFP motility, or twitching, is controlled by the Pil-Chp system. P. aeruginosa uses this system to traverse surfaces and gather information about the local chemical and physical environment. The Pil-Chp system shares many similarities to the well-studied flagellar chemotaxis system (Che), which biases locomotion of swimming cells up or down gradients of chemical stimuli. However, many important differences have been described, while others await discovery. Some of these differences have even led to speculation that chemotaxis may not be a primary role for Pil-Chp. Thus, recent studies have focused on addressing whether P. aeruginosa uses chemotaxis to bias the direction of motility on a surface, and if so, what role does Pil-Chp play in this process? In this review, we focus on current progress in the field toward gaining insight into these questions.