Extraction du solvant d'un hydrogel par des gouttes de bact\'eries B. subtilis

TL;DR

This study reveals that Bacillus subtilis bacteria produce surfactin, which causes water to flow out of the environment, leading to bacterial colony movement and swelling of drops, independent of bacterial growth.

Contribution

It demonstrates that B. subtilis can actively induce colony movement and swelling through surfactin-mediated osmotic effects, a novel displacement mode beyond motility.

Findings

B. subtilis drops increase in volume due to surfactin-induced osmotic flow.

Surfactin concentration needed is less than 1 mM, much lower than glucose.

B. subtilis can slide on tilted surfaces using surfactant effects.

Abstract

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.