# Water surface tension modulates the swarming mechanics of Bacillus subtilis

## TL;DR

This study shows how water surface tension influences the swarming behavior and tendril formation in Bacillus subtilis colonies.

## Contribution

The novel finding is that water surface tension regulates swarming dynamics and tendril formation in B. subtilis through a feedback loop involving surfactin and quorum sensing.

## Key findings

- Low surfactin production increases water surface tension, restricting colony expansion and triggering quorum sensing.
- Breaking the surface tension barrier leads to bacterial streams forming tendrils.
- High surfactin levels reduce surface tension, enabling rapid bacterial migration on a thin water layer.

## Abstract

Many Bacillus subtilis strains swarm, often forming colonies with tendrils on agar medium. It is known that B. subtilis swarming requires flagella and a biosurfactant, surfactin. In this study, we find that water surface tension plays a role in swarming dynamics. B. subtilis colonies were found to contain water, and when a low amount of surfactin is produced, the water surface tension of the colony restricts expansion, causing bacterial density to rise. The increased density induces a quorum sensing response that leads to heightened production of surfactin, which then weakens water surface tension to allow colony expansion. When the barrier formed by water surface tension is breached at a specific location, a stream of bacteria swarms out of the colony to form a tendril. If a B. subtilis strain produces surfactin at levels that can substantially weaken the overall water surface tension of the colony, water floods the agar surface in a thin layer, within which bacteria swarm and migrate rapidly. This study sheds light on the role of water surface tension in regulating B. subtilis swarming, and provides insight into the mechanisms underlying swarming initiation and tendril formation.