C. ROGGO¹, J. R. VAN DER MEER<sup>1

1</sup>Department of Fundamental Microbiology, University of Lausanne

Chemotaxis is a behavior by motile bacteria to sense the environment and swim in the direction of or away from chemical compounds. In a uniform environment bacteria swim randomly to explore the maximum space but when they are in presence of a gradient of attractant, they bias their swimming direction toward the highest concentration of attractant. Chemotaxis is rapid, and could thus be exploitable for developing biosensors with quick response. In addition it is conserved among motile bacteria and some species show chemotaxis toward toxic compounds.
Here we pursue the design of microfluidic chips in which a gradient of attractant can be generated which enables measurement of bacterial chemotaxis. We demonstrate two different design principles. In the first, three parallel flow channels are produced, separated by 700 nm-shallow filters, which allow the diffusion of small chemical molecules but prevent the passage of the bacterial cells. The chemical gradient forms perpendicular to the flow and cells are flown in the inner channel. The accumulation of cells on the side with the highest attractant concentration is observed by microscopy. We show the working principle both on Escherichia coli chemotaxis to serine and aspartate, as well as on Cupriavidus necator and the herbicide 2,4–dichlorophenoxyacetic acid (2,4-D).
To observe the chemotactic response of cells to a gradient even more directly, we fabricated a second device with valves permitting rapid opening and closing of channels. This allows to create a stable gradient and to bring in a package of cells, upon which individual trajectories can be followed. We show the functioning of the chip and how individual E. coli cells react to serine within minutes after exposure to the gradient.