Mechanosensing of shear by Pseudomonas aeruginosa leads to increased levels of the cyclic-di-GMP signal initiating biofilm development

Significance It is well established that mechanical inputs, such as strain and elasticity, can be sensed by eukaryotic cells and can impact phenotype and behavior. In contrast, very little is known about how prokaryotes may respond to mechanical inputs. Here, we show that bacteria can sense shear and can respond by initiating biofilms. This is an important advance in fundamental microbiology and mechanobiology. Biofilms are difficult to prevent using extant approaches. Our knowledge points the way to a hitherto-undeveloped type of antibiofilm surface that thwarts mechanosensing by not sustaining sufficiently high shear. This would prevent bacteria from sensing surface attachment, activating cyclic-di-GMP signaling, and forming a biofilm. Biofilms are communities of sessile microbes that are phenotypically distinct from their genetically identical, free-swimming counterparts. Biofilms initiate when bacteria attach to a solid surface. Attachment triggers intracellular signaling to change gene expression from the planktonic to the biofilm phenotype. For Pseudomonas aeruginosa, it has long been known that intracellular levels of the signal cyclic-di-GMP increase upon surface adhesion and that this is required to begin biofilm development. However, what cue is sensed to notify bacteria that they are attached to the surface has not been known. Here, we show that mechanical shear acts as a cue for surface adhesion and activates cyclic-di-GMP signaling. The magnitude of the shear force, and thereby the corresponding activation of cyclic-di-GMP signaling, can be adjusted both by varying the strength of the adhesion that binds bacteria to the surface and by varying the rate of fluid flow over surface-bound bacteria. We show that the envelope protein PilY1 and functional type IV pili are required mechanosensory elements. An analytic model that accounts for the feedback between mechanosensors, cyclic-di-GMP signaling, and production of adhesive polysaccharides describes our data well.