Particle Diffusion in a Quasi-Two-Dimensional Bacterial Bath

We study the effect of bacterial motion on micron-scale beads in a freely suspended soap film. Given the sizes of bacteria and beads, the geometry of the experiment is quasi-two-dimensional. Large positional fluctuations are observed for beads as large as $10\mu \mathrm{m}$ in diameter, and the measured mean-square displacements indicate superdiffusion in short times and normal diffusion in long times. Though the phenomenon is similar to Brownian motions of small particles, its physical origin is different and can be attributed to the collective dynamics of bacteria.