Collective Escape of Chemotactic Swimmers through Microscopic Ratchets

We report on the emergence of spontaneously forming migrating bands of E.coli bacteria inside a microchannel containing microstructured ratchets. We show that a collection of bacteria is able to migrate against the funnel-shaped barriers by creating and maintaining a chemoattractant gradient. A transition between pure rectification and chemotaxis-driven collective motion is predicted from theoretical models, and is observed experimentally as the initial inoculation density is varied.

Figure 1

(a) Fluorescence micrograph of individual cells (labeled green) inside the microchannel. Cell traps, where the rectification bias is reversed, have a higher concentration of cells. (b) False-color image showing the number of cells in each chamber as a function of time. Note that three spontaneously forming migrating bands of bacteria are observed, launching at $T=0$, 2, and 4 h, and that the first one is scattered back after it reaches the end of the microchannel (between $T=4$ and $T=6\mathrm{h}$).

Figure 2

(a) Motion of individual bacteria during a single 7.5-s video recording. Red indicates motion toward the inoculation port (to the left of the figure), and a circle denotes the starting point of the recorded motion. (b) Average vector field of the cells’ motion. The red lines represent vectors pointing to the left, and black lines to the right.

Figure 3

Visualization of funnel rectification in terms of cross sections. Cells swimming toward funnels from the left encounter an effective cross section different from that encountered by cells swimming from the right.

Figure 4

In these experimental plots, the labeled number of cells refers to the cells initially placed into the chamber closest to the inoculation port. The circle shows the position at which a packet stopped migrating, according to visual inspection.