Smectic-like bundle formation of planktonic bacteria upon nutrient starvation

Bacteria aggregate through various intercellular interactions to build biofilms, but the effect of environmental changes thereupon remains largely unexplored. Here, by using an experimental device that overcomes past difficulties, we observed the collective response of Escherichia coli aggregates against dynamic changes in the growth condition. We discovered that nutrient starvation caused bacterial cells to arrange themselves into bundle-shaped clusters, developing a structure akin to that of smectic liquid crystal. The degree of the smectic-like bundle order was evaluated by a deep learning approach. Our experiments suggest that both the depletion attraction by extracellular polymeric substances and the growth arrest are essential for the bundle formation. Since these effects of nutrient starvation at the single-cell level are common to many bacterial species, bundle formation might also be common collective behavior that bacterial cells may exhibit under harsh environments.