Emergence of large-scale mechanical spiral waves in bacterial living matter

Propagating spiral waves have been discovered in various chemical, biological and physical systems. Spiral waves in multicellular organisms are often associated with essential living functions. Although certain eukaryotic microorganisms have long been known to generate spiral waves, evidence of spiral wave pattern has been lacking in the bacterial world. Here we report the discovery of a unique form of propagating spiral waves in dense bacterial populations where cells engage in cyclic force-generating processes driven by a grappling-hook-like motile organelle called type-IV pilus motor. Specifically, we discovered that synchronization of pilus activity in the bacterial living matter leads to large-scale spatiotemporal regulation of tension force in the form of propagating spiral waves. Theoretical modelling reveals that the spiral tension waves result from non-reciprocity in cell-cell interactions. Our findings reveal a mechanism of large-scale force regulation in bacterial world and may shed light on the emergent mechanics of biofilms and microbiomes. Pilus-driven bacterial living matter also provides a mechanical active medium for studying electrical or chemical spiral waves in living systems. The occurrence of propagating spiral waves in multicellular organisms is associated with key biological functions. Now this type of wave has also been observed in dense bacterial populations, probably resulting from non-reciprocal cell-cell interactions.