Application of a microfluidic T-sensor assay for the study of chemotaxis perpendicular to convective flow

Chemotaxis is the ability motile bacteria possess which enables them to detect and respond to specific chemical gradients in their environment. In many ecological conditions, this chemotactic response occurs in the presence of moving fluid. While there are numerous experimental assays for studying chemotaxis in static-fluid conditions, there are few assays capable of examining chemotaxis in well-characterized fluid flow conditions. The goal of this work is the development of the microfluidic assay, known as the T-sensor, for the study of chemotaxis perpendicular to fluid flow.; Prior to chemotaxis experiments, our ability to accurately measure and predict the chemical gradients at different experimental conditions (e.g. fluid velocities, axial distances) within the T-sensor was examined through the use of numerical simulations and dye tracer experiments. Through the use of a mathematical model we were able to demonstrate that the residence-time distribution would not impact the measurement of diffusion and chemotaxis within the T-sensor. Diffusion coefficients were estimated for Trypan Blue, but the values were 4 to 6 times greater than those found in the literature (2 × 10−6 cm2/s). This may be explained by the presence of density-driven convection although low Trypan Blue concentrations were used in the experiment specifically to avoid this. The value of the Trypan Blue diffusion coefficient also increased at greater axial distances from the junction. Due to these problems, it is evident that there is some aspect of fluid dynamics or mass transport that is not understood within this particular experimental system.; Chemotaxis experiments were conducted where both input streams to the T-sensor contained the same concentration of Escherichia coli HCB1 (∼108–109 cells/ml), but only one of the streams contained an attractant. A series of no-flow and flow (0.111 cm/s–0.0111 cm/s) control experiments were conducted where two chemotactic bands were observed in response to a gradient in α-methyl aspartate. The double-band response of E. coli HCB1 to other attractants (fucose, serine, and glutamate) was also observed in the T-sensor. A comparison of the chemotactic band behavior in the absence and presence of flow suggested that flow does not alter the chemotactic response.