Fast-swimming bacteria that live at the bottom of salt marshes spontaneously come together to form organized crystalline structures that move through water.

The bacterium Thiovulum majus is a large, round cell with hundreds of small flagella, or tail-like structures, that spin to propel the organism. Alexander Petroff of the Rockefeller University in New York and his colleagues observed the behaviour of individually spinning cells under a microscope and found that each created tornado-like flows in the liquid around them. These forces cause the cells to self-assemble into groups of between 10 and 1,000 cells, arranged in a two-dimensional crystalline hexagonal shape that rotates collectively.

Phys. Rev. Lett. 114, 158102 (2015)