Optical Diffusometry of Bacteria-laden Single Microbeads

Microcantilever transducers have been used to show that metabolic activity of bacteria can be correlated with their random nanomechanical fluctuations. Here, we demonstrate an alternative approach for measuring these bacterial fluctuations using an optical technique. In the experiments, we adhere various bacteria onto the surface of 3-8 μm microbeads via electrostatic forces. After bacteria adhesion, we image the Brownian motion of single microbeads suspended in liquid by high-resolution optical microscopy. Mean square displacements and diffusion constants are calculated from trajectories of the microbeads. We observe that, when a few motile bacteria such as E. coli are adhered to a bead, its diffusion constant increases significantly. Furthermore, the dynamics of the bead changes from diffusive to ballistic, similar to that of a Janus particle. Conversely, a microbead coated with non-motile bacteria such as S. epidermidis shows diffusive motion but with a subtle increase in the diffusion constant compared to that of a bead with no bacteria. Our approach allows for the measurement of the metabolic activity of various bacteria and can possibly be developed into a rapid antibiotic susceptibility test.