Effect of Flow-Driven Shear Stress Gradient on the Motility of Pseudomonas aeruginosa

Presenter: Sumit Saha, Mechanical Engineering

Authors: S. Saha, B. Behkam

Abstract: Flow-driven shear stress significantly influences bacteria near-surface motility; however, the role of transverse shear gradients remains unclear. Here, we studied Pseudomonas aeruginosa motility in seven distinctly different shear gradients (0–0.0554 Pa·s/m) in a microfluidic device by 3D tracking. The responses of wild-type P. aeruginosa (WT) and its swimming (ΔmotAB) and surface  ( ΔpilA) motility mutants towards shear gradients were quantified using the shear taxis index, STI = (∂τ/∂y)/(∂τ/∂y)ₘₐₓ, where ∂τ/∂y is the viscous shear gradient along bacterial trajectory and (∂τ/∂y)ₘₐₓ is the maximum gradient within the interrogation window. All strains migrated from high to low shear stress regions. As the shear gradient increased, the mean STI increased from zero to a peak value at an optimal gradient, then declined. WT displayed peak response at 0.0138 Pa/m, but ΔmotAB and ΔpilA showed their strongest response at a shallower gradient of 0.0069 Pa/m. In all cases, responses were negligible at 0 and 0.0554 Pa/m. Peak responses for WT, ΔmotAB, and ΔpilA were respectively 24, 30, and 23 times higher than their corresponding no-flow controls. Near channel walls, shear gradients impose asymmetric forces on bacteria that generate torque, guiding bacteria toward lower-shear regions.