Particle diffusion in a quasi-two-dimensional bacterial bath

We study the effect of bacterial motion on micron-scale beads in a freely s uspended soap film. Given the sizes of bacteria and beads, the geometry of the experiment is quasi-two-dimensional. Large positional fluctuations an o bserved for beads as large as 10 mu m in diameter, and the measured mean-sq uare displacements indicate superdiffusion in short times and normal diffus ion 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.