Swimming bacteria remove resistance to flow

Water flows best when it’s chock-full of synchronized-swimming bacteria.

By coaxing billions of E. coli to work together, French researchers got a small sample of a bacteria-laden solution to have no resistance to flow, or zero viscosity. Such effortless motion is usually reserved for superfluids like liquid helium that are kept at frigid temperatures.

“The results are pretty compelling,” says Raymond Goldstein, a complex systems physicist at the University of Cambridge. The study, in the July 10 Physical Review Letters, demonstrates how the motion of microscopic organisms can drive the large-scale behavior of liquids.

Physicists Héctor Matías López and Harold Auradou at Paris-Sud University and colleagues dipped a hanging cylindrical probe into a small cup filled with a solution of water, E. coli and enough nutrients for the bacteria to swim but not divide. Then the physicists slowly rotated the cup and measured the torque exerted by the solution on the probe.

A viscous fluid like honey would tug on and spin the probe. But when infused with a strain of very active E. coli, the water solution exerted no torque on the probe, indicating zero viscosity. In some trials, the viscosity actually became negative: The solution in the counterclockwise-rotating cup exerted a clockwise torque on the probe.

Before the cup spins, the bacteria swim about randomly, Auradou says. But theoretical studies suggest that once the liquid starts to flow, the E. coli coordinate their motion. The rod-shaped bacteria push water in front and behind as they swim. Liquid fills in from the sides, nudging neighboring bacteria closer together and causing them to align and swim in concert. The bacteria’s collective pushing increases the speed at which adjacent layers of water can rush past each other, giving the solution a more efficient, less viscous flow. Bacteria may help scientists analyze tiny volumes of liquid by ensuring that samples don’t get stuck in micro-sized passageways.