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

Flow-driven shear stress is known to significantly influence bacteria near-surface motility; however, how shear stress gradients perpendicular to the flow influence bacterial motility remains unclear. Here, we studied Pseudomonas aeruginosa motility in the presence of seven distinctly different shear gradients (0–0.0554 Pa·s/m) in a microfluidic flow chamber. Wild-type P. aeruginosa (WT) and its swimming (ΔmotAB) and twitching (ΔpilA) motility mutants were tracked in 3D. The strength and direction of their response 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 possible gradient within the interrogation window. Surprisingly, all three 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. But each strain’s response was distinct. The WT strain displayed its strongest response at 0.0138 Pa.s/m. Unexpectedly, both ΔmotAB and ΔpilA mutants showed their strongest response at a shallower gradient of 0.0069 Pa.s/m. In all cases, the response weakened at lower and higher gradients, becoming negligible at 0 and 0.0554 Pa.s/m. Peak responses for WT, ΔmotAB, and ΔpilA were respectively 24, 30, and 23 times higher than their corresponding no-flow controls. In a shear gradient field, unequal shear forces acting on a bacterium generate a torque that can orient it toward the low stress region; however, the sensitivity of the response to the gradient slope and strain type suggests a combined biophysical and biological underpinning, which will be investigated in the future. The shear taxis behavior reported herein occurs at low shear rates (0.04–0.4 s⁻¹) and in the direction perpendicular to the flow. Thus, it is distinct from rheotaxis (bacteria upstream swimming), reported at higher shear rates (14–14,000 s⁻¹).