Experimental Verification of the Behavioral Foundation of Bacterial Transport Parameters Using Microfluidics

We present novel microfluidic experiments to quantify population-scale transport parameters (chemotactic sensitivity chi(o) and random motility mu) of a population of bacteria. Previously, transport parameters have been derived theoretically from single-cell swimming behavior using probabilistic models, yet the mechanistic foundations of this upscaling process have not been verified experimentally. We designed a microfluidic capillary assay to generate and accurately measure gradients of chemoattractant (alpha- methylaspartate) while simultaneously capturing the swimming trajectories of individual Escherichia coli bacteria using videomicroscopy and cell tracking. By measuring swimming speed and bias in the swimming direction of single cells for a range of chemoattractant concentrations and concentration gradients, we directly computed the chemotactic velocity VC and the associated chemotactic sensitivity chi(o). We then show how m can also be readily determined using microfluidics but that a population-scale microfluidic approach is experimentally more convenient than a single-cell analysis in this case. Measured values of both chi(o) [(12.4 +/- 2.0) x 10(-4) cm(2) s(-1)] and mu[(3.3 +/- 0.8) x 10(-6) cm(2) s(-1)] are comparable to literature results. This microscale approach to bacterial chemotaxis lends experimental support to theoretical derivations of population- scale transport parameters from single-cell behavior. Furthermore, this study shows that microfluidic platforms can go beyond traditional chemotaxis assays and enable the quanti. cation of bacterial transport parameters.