Bacteria’s tail spins make water droplets swirl

When bacteria band together, they can turn a fairly tame drop of water into a swirling vortex. The swirl is spun up by the bacteria’s tail-like flagella and the basic physics of how fluids flow, researchers report June 23 in the Proceedings of the National Academy of Sciences. The finding could enable scientists to control the collective motion of the microbes and possibly prevent the spread of infection in the human body, the researchers suggest.

In a droplet of water, bacteria start swimming in every direction. They collide with each other and then start circling the droplet in the same direction. But that synchronized swimming only lasts so long. The force from the flagella of bacteria at the outer edge of the droplet becomes so strong that the microbes on the inside can’t keep up: They ultimately have to go with the flow formed from the outer bacteria's flagella and start swimming in the opposite direction, a pattern that swirls into a vortex.