Integrated control of surface adaptation by the bacterial flagellum

Many bacteria can alternate between motile and sessile lifestyles, and wide-ranging sets of environmental stimuli regulate the transition from a free-swimming to a surface-attached state. A transenvelope machine called the flagellum, known primarily for its role in promoting cellular motility, stimulates the motile-sessile transition by detecting contact with solid substrates. Recent work has revealed a striking level of sophistication within the regulatory circuits that link flagellar function to surface colonization. I describe the current paradigm whereby the flagellum promotes the sessile state by increasing production of the second-messenger bis-(3'-5')-cyclic diguanosine monophosphate (c-di-GMP). I then highlight studies that have identified multiple routes by which the flagellum activates c-di-GMP production, calling the concept of a linear surface recognition pathway into the question. I conclude by proposing a role for the flagellum as a signaling hub that integrates environmental stimuli to coordinate a surface colonization program that occurs across a range of spatial and temporal scales.