A high-throughput capillary assay for bacterial chemotaxis

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Zach Wissner-Gross (March 2, 2009)


A high-throughput capillary assay for bacterial chemotaxis

Russell Bainer, Heungwon Park, and Phillipe Cluzel

Journal of Microbiological Methods, 2003, 55, 315-319

Soft matter keywords

E. coli, chemotaxis, capillary


A methods paper, this article is relatively straightforward. The authors use a 96-well plate to perform a high-throughput assay for bacterial chemotaxis by setting up an array of gradients of L-aspartate (a chemoattractant). They establish these gradients using capillary tubes: first they dipped the tubes into motility medium either with or without the L-aspartate. They then sealed the tubes from above with wax, and finally they resuspended the tubes in 96-well plates inoculated with various concentrations of bacteria (E. coli). After 40 minutes, the capillaries were removed, along with any bacteria that had migrated into them. Cell density in the capillaries was determined by monitoring changes in the optical densities of the solutions in the capillaries over the course of several hours (Figure 1).

The authors' results matched those found in previous low-throughput experiments. In short, they found the expected logarithmic relationship between cell concentration and the "lag time" before the number of cells that migrated into the capillaries began to saturate their environment. Capillaries containing 10 mM L-aspartate were found to attract 15 times as many bacteria as capillaries lacking the L-aspartate (Figure 2). But again, while this result was simply being verified in this paper, the focus here was on the throughput of the experiment.

Soft matter discussion

As I stated in the summary, this article is rather straightforward, serving as a methods paper rather than going into physical detail. However, one point that I found intriguing was how the authors might extend their research by creating different types of gradients while maintaining the high-throughput nature of their assay.

First, let's verify that the gradients are even maintained throughout the bacterial inoculation. From class, we know that the height <math>H</math> of a capillary water column will be


where <math>\theta_E</math> is the contact angle, <math>\kappa^{-1}</math> is the capillary length, and <math>R</math> is the radius of the column. Since <math>\kappa^{-1}</math> is on the order of a few millimeters, and in this experiment <math>R\approx0.5</math> mm

The authors cite previous work showing that thinner capillaries result in a greater ratio <math>R</math> between the number of cells that migrate toward the attractant and those that migrate into capillaries without the attractant. Let's briefly think about why this might be the case. First, a thinner capillary will result in a higher water column, as demonstrated in class: the height <math>H</math> of a capillary water column will be


where <math>\theta_E</math> is the contact angle, <math>\kappa^{-1}</math> is the capillary length, and <math>R</math> is the radius of the column.