  We observe that small drops of a Bacillus subtilis suspension deposited on
agar strongly increase in volume while similar bacteria-void drops do not. By
measuring the bacterial concentration within the drop at different heights, we
show that the biomass increase due to the constant bacterial cell-division is
too small to explain the drop bloating. Rather, the increased volume is caused
by the presence of surfactin - a surfactant produced by the bacteria - which
induces a water flow out of the environment by an osmotic capillary effect. The
required concentration is very low (< 1 mM), four orders of magnitude smaller
than the concentration of, for example, glucose to produce a similar effect.
The ability of B. subtilis to extract water from its environment probably
contributes to collective migration modes like mass swarming. It also gives
rise to a new displacement mode independent of cellular motility: By combining
the osmotic and wetting effects of the surfactant, B. subtilis can actively
induce the sliding of the bacterial colony on substrates tilted by angles as
small as 0.1 degrees.
