Determining the Metabolic Activity and Flow Boundary Conditions of Bacteria from Diffusion Measurements

When live bacteria are adhered onto a microcantilever, the nanomechanical displacements of the microcantilever increase noticeably, suggesting that bacteria fluctuations drive the microcantilever to a higher amplitude than thermal fluctuations. Here, we have performed similar measurements by adhering various bacteria onto the surface of 8-μm-diameter microbeads. After bacteria adhesion, we image the Brownian motion of these single microbeads suspended in liquid by high-resolution optical microscopy. We then calculate the mean square displacements and diffusion constants from the trajectories of the microbeads. Interestingly, microbeads with non-motile bacteria show an unexpected diffusion profile: in a given interval, microbeads with dead bacteria on them tend to diffuse farther compared to those with live bacteria. We will discuss possible sources of this unexpected behavior, including modifications to the no-slip boundary condition on the surface of bacteria.