Collective force generation by groups of migrating bacteria

TL;DR

This study investigates how groups of bacteria generate and coordinate forces during collective migration, revealing force amplification mechanisms in twitching and gliding motility using traction force microscopy.

Contribution

First application of traction force microscopy to bacterial collective migration, demonstrating force amplification and coordination in Myxococcus xanthus groups during twitching and gliding.

Findings

Twitching generates local hotspots with forces around 50 pN, amplified to 100 pN in groups.

Gliding produces low traction individually, but is amplified fivefold in groups.

Forces during twitching and gliding are complementary and show collective amplification.

Abstract

From biofilm and colony formation in bacteria to wound healing and embryonic development in multicellular organisms, groups of living cells must often move collectively. While considerable study has probed the biophysical mechanisms of how eukaryotic cells generate forces during migration, little such study has been devoted to bacteria, in particular with regard to the question of how bacteria generate and coordinate forces during collective motion. This question is addressed here for the first time using traction force microscopy. We study two distinct motility mechanisms of Myxococcus xanthus, namely twitching and gliding. For twitching, powered by type-IV pilus retraction, we find that individual cells exert local traction in small hotspots with forces on the order of 50 pN. Twitching of bacterial groups also produces traction hotspots, however with amplified forces around 100 pN. Although twitching groups migrate slowly as a whole, traction fluctuates rapidly on timescales <1.5 min. Gliding, the second motility mechanism, is driven by lateral transport of substrate adhesions. When cells are isolated, gliding produces low average traction on the order of 1 Pa. However, traction is amplified in groups by a factor of ~5. Since advancing protrusions of gliding cells push on average in the direction of motion, we infer a long-range compressive load sharing among sub-leading cells. Together, these results show that the forces generated during twitching and gliding have complementary characters and both forces are collectively amplified in groups.