In this work, we study the influence of substrate rigidity and flow shear rate on surface motility driven by the cellular type IV pili machinery, commonly called “twitching” motility, of the pathogen Pseudomonas aeruginosa.
First, we investigate the influence of polyacrylamide substrate stiffness on twitching motility. We show that the diffusive behavior of twitching motility is substrate-dependent, showing sub- and superdiffusive behavior on soft and stiff substrates, respectively. We show that the cause of the anomalous diffusive behavior is a product of the non-independence of the displacement increments, showing substrate-dependent long-range correlation. This long-range correlation had an impact on the persistence and speed of twitching bacteria.
Second, we investigate the influence of flow shear rate on twitching motility on glass substrates. We show that bacterial behavior is superdiffusive and that the diffusion coefficient is shear rate- dependent. The effect of shear rate on diffusivity is caused by an increase in displacement increments. It is not clear at this time whether the increase in displacement increments is passive in the direction of flow or the result of an active response to shear forces.
Finally, we investigated a potential effect of polyacrylamide spatial stiffness gradients on twitching motility, a phenomenon known as durotaxis. We show that under our approach and experimental condition, the twitching motility of Pseudomonas aeruginosa does not exhibit durotaxis.