Bacterial mechanosensing of substrate stiffness during biofilm initiation: a tale of two steps (and two sensors)

The attachment of bacteria onto a surface, consequent signaling, and the accumulation and growth of the surface-bound bacterial population are key initial steps in the formation of pathogenic biofilms – yet whether, and how, this is impacted by the mechanics of the surface is not known. We use ultrathin and thick hydrogels coated on glass coverslips to create stiff and soft composite materials, respectively, with the same surface chemistry. We find that the accumulation, motility, and signaling of the opportunistic human pathogen Pseudomonas aeruginosa are sensitive to material stiffness. Just after attachment, the cell-surface-exposed protein PilY1 acts as a mechanosensor to discriminate between substrates of different stiffness. This PilY1-mediated mechanoresponse gives rise to differences in cyclic-di-GMP signaling that are linked to surface motility and detachment. Later, the pilus retraction motor PilT also acts as a mechanosensor to discriminate between substrates of different stiffness, giving rise to differences in the growth of the surface-bound population. To our knowledge, this is the first demonstration of a two-stage mechanoresponse in early biofilm formation.