A wall of funnels concentrates swimming bacteria

Abstract

Randomly moving but self-propelled agents, such as Escherichia coli bacteria, are expected to fill a volume homogeneously. However, we show that when a population of bacteria is exposed to a microfabricated wall of funnel-shaped openings, the random motion of bacteria through the openings is rectified by tracking (trapping) of the swimming bacteria along the funnel wall. This leads to a buildup of the concentration of swimming cells on the narrow opening side of the funnel wall but no concentration of nonswimming cells. Similarly, we show that a series of such funnel walls functions as a multistage pump and can increase the concentration of motile bacteria exponentially with the number of walls. The funnel wall can be arranged along arbitrary shapes and cause the bacteria to form well-defined patterns. The funnel effect may also have implications on the transport and distribution of motile microorganisms in irregular confined environments, such as porous media, wet soil, or biological tissue, or act as a selection pressure in evolution experiments.

FIG. 1.

Microstructures with funnel walls. (A) Schematic drawing of the interaction of bacteria with the funnel opening. Bacteria on the left side may (trace 1) or may not (trace 2) get through the gap, depending on the angle of attack. On the right, all bacteria colliding with the wall are diverted away from the gap (traces 3 and 4). (B) Scanning electron micrograph of the device. (C) Distribution of incoming and outgoing angles for bacteria colliding with a wall. Data were taken for 70 events.

FIG. 2.

Distribution of bacteria in a structure with a funnel wall. (A) Uniform distribution after injection. (B) Steady-state distribution after 80 min. (C) Ratios of densities in the left and right compartments versus time. The blue circles are experimental data, and the dashed red line is a fit of equation A2 from the Appendix.

FIG. 3.

Distribution of bacteria in a structure with a flat wall. (A) Steady-state distribution after 80 min. (B) A(t) versus time.

FIG. 4.

Serial wall chip with eight compartments inoculated with motile (green) bacteria and nonmotile (red) bacteria. (A) After 80 min, motile bacteria were concentrated on the far right and nonmotile ones remained homogeneously dispersed. (B) Densities of motile (green circles) and nonmotile (red circles) bacteria in the compartments (i). The dashed gray line represents an exponential fit to the densities of the motile cells.

FIG. 5.

Practical applications of structures with a funnel wall. (A) A series of funnel arrays can function as an effective pump to remove motile organisms from one reservoir and concentrate them in another one. (B) Spontaneous aggregation of motile bacteria inside a corral formed in the shape of letters.