  We present a joint experimental and computational study of the effect of
bacterial motion on micron-scale colloids contained in a two-dimensional
suspension of Bacillus subtilis. With respect to previous work using E. coli,
here we introduce a novel experimental set-up that allows us to realise a
two-dimensional bacterial suspension insensitive to either evaporation or fluid
flow. By analysing the mean square displacements of both bacteria and colloids,
we confirm the existence of a crossover from super-diffusive behaviour at short
time scales to normal diffusion at longer times. We also study the same
two-dimensional system by means of numerical simulations, using a suspension of
self-propelled dumbbells or the Vicsek model, which has been previously used to
study the dynamics of active particles. Our numerical results obtained with
both models are in broad agreement with the experimental trends, but only the
dumbbell simulations can match the experimental data quantitatively. The level
of agreement we find suggest that steric interactions due to collisions are
important players in determining collective motion of the bacterial bath, and
should complement hydrodynamic interactions in experiments.
