Passive Microrheology to Measure Microbial EPS Production in Situ

Bacteria in soil produce extracellular polymeric substances (EPS) that consist primarily of polysaccharides, but also contain a variety of proteins, lipids, and DNA to form biofilms. EPS is a hydrogel that protects bacteria and retains moisture in dry soil environments.  Our previous work has suggested that EPS production promotes greater moisture retention in soil, but we have no way of quantifying EPS production. Microbial function including EPS production can vary dramatically at the micrometer scale.  There is a need for in situ optical techniques to measure responses at the appropriate scale. Here we develop a passive microrheology technique to measure EPS production in situ in soil emulating microfluidic devices. First, carboxylated polystyrene microspheres (Nile Red FluoSpheres, 1 μm) were suspended in solutions of known viscosity and the solutions were loaded into replicated microfluidic devices. Next, the random motion of individual spheres was measured by inverted widefield fluorescence microscopy and the trajectories of individual microspheres was analyzed using the Multi-Particle Tracking plug-in for ImageJ. After calculating the mean-squared displacement during a series of time lags from each particle trajectory, an estimate of the solution viscosity can be found using the Stokes–Einstein–Sutherland equation. Here we report solution viscosities for solutions of 0-40% glycerol in water within a microfluidic device. We conclude that passive microrheology can determine solution properties at the appropriate spatial and temporal scale for measuring pore-scale changes in EPS production in situ. This research will lead to a greater understanding of the pore-scale function of soil-microbe systems, and may one day lead to biotechnologies that help support more sustainable agriculture production.