Unexpected Bipolar Flagellar Arrangements and Long-Range Flows Driven by Bacteria near Solid Boundaries

Experiments and mathematical modeling show that complex flows driven by unexpected flagellar arrangements are induced when peritrichously flagellated bacteria are confined in a thin layer of fluid, between asymmetric boundaries. The flagella apparently form a dynamic bipolar assembly rather than the single bundle characteristic of free swimming bacteria, and the resulting flow is observed to circulate around the cell body. It ranges over several cell diameters, in contrast to the small extent of the flows surrounding free swimmers. Results also suggest that flagellar bundles on bacteria that lie flat on a solid substrate have an effective rotation rate slower than “free” flagella. This discovery extends our knowledge of the dynamic geometry of bacteria and their flagella, and reveals new mechanisms for motility-associated molecular transport and intercellular communication.

Figure 1

Stationary flow field (mean over 20 s) surrounding a stuck cell and corresponding streamlines (green) computed from a regular grid of initial points. The arrow in the lower right corner represents $10\mu \mathrm{m}{\mathrm{s}}^{-1}$.

Figure 2

(a) perpendicular $U_{\perp }$ and parallel $U_{\parallel }$ components of velocity $\mathbf{U}$ as a function of the distance along the cell axis for a stuck cell (black & blue, respectively) and for a free swimming cell (red & green). (b) Same as for (a) except that data are for $V$ and distance is transverse to the cell axis. (c) velocity components.

Figure 3

Simulation streamlines in a plane (neglecting flows into the page) above an organism with two flagella close to an infinite wall. Inset: depiction of reduced model. Flagella are swept clockwise due to the asymmetry of the lower no-slip and upper (nominally) stress-free boundaries, and the resulting traction afforded the lower half of each flagellum.

Figure 4

Dynamic flowfield due to several stuck bacteria (both long and short) in a thin layer of fluid.