Local chemotactic response of $Escherichia$ $coli$ in fluid and near surfaces

TL;DR

This study develops a microfluidic device to accurately measure the chemotactic response of E. coli to chemical gradients both in fluid and near surfaces, revealing differences in behavior.

Contribution

The paper introduces an optimized microfluidic setup for precise, individual-level analysis of bacterial chemotaxis in controlled chemical gradients.

Findings

Chemotactic velocity in fluid is proportional to the gradient divided by concentration.

On surfaces, chemotactic flux is significantly inhibited.

The susceptibility function describes bacterial response in fluid.

Abstract

Bacteria can adjust their swimming behaviour in response to chemical variations, a phenomenon known as chemotaxis. This process is characterised by a drift velocity that depends non-linearly on the concentration of chemical species and its "local" gradient. To study this process more effectively, we optimised a 3-channel microfluidic device to generate a stable, linear concentration profile of chemoattractants. This setup allows us to monitor the response of $Escherichia$ $coli$ to casamino acids or $\alpha$-methyl-DL-aspartic acid at the individual level. By analysing the movement of a population of individuals both in fluid and on surfaces, we achieve faster, more accurate quantification of the population's chemotactic response. In the fluid, the chemotactic response is described by the equation $v_c=\chi(c) \nabla c$, with $\chi(c) = \chi_0 /[(1 + c/c_-)(1 + c/c_+)]$ the chemotactic susceptibility. For $c_- \ll c \ll c_+$, i.e. when bacteria perform chemotaxis, the bacterial chemotactic velocity is proportional to the concentration gradient divided by the concentration and $v_c \propto \nabla c/c = \nabla (\log c)$. However, on surfaces, the chemotactic flux is inhibited.