It's a pretty effect with an even prettier explanation: By putting the common gut bacterium Escherichia coli in just the right place, physicists have found they can make it swim in counterclockwise loop-the-loops. Other scientists had forced the bacterium to swim in clockwise circles. But the new effect has a particularly appealing explanation, and together the two findings might allow researchers to shuttle bacteria about in tiny "lab-on-a-chip" devices.
An E. coli bacterium is a tiny torpedo propelled by a corkscrew-shaped filament called a flagellum. The flagellum spins like a propeller, pushing water backward and thus the bacterium forward through the water. Normally, the bacterium moves in a straight line. But when it swims near a solid surface, like a manta ray skimming the seafloor, something different happens: It veers to the right—clockwise when viewed from above. Scientists provided a quantitative explanation of the effect in 2006.
The spinning flagellum not only pushes water backward but also swirls it so that it should flow sideways over the surface. But at the boundary of a liquid and a solid,water must obey a "no-slip boundary condition," which means that it's stuck to the solid and can't flow. So, instead of pushing the water one way across the surface, the spinning flagellum pushes itself sideways in the other direction. That causes the whole cell to yaw and its path to curve. And because all E. coli flagella curl and whirl in the same direction, the microbes always turn right.
Now, physicist Roberto Di Leonardo of the Italian National Research Council at the University of Rome "La Sapienza" and colleagues have observed the reverse effect. They imaged E. coli swimming in a water droplet hanging from a glass slide and found that the bacteria near the air swim to the left, or counterclockwise, as they reported online 19 January in Physical Review Letters.
Why the reversal? The physicists knew that the water-air surface is a "perfect-slip" boundary, along which water can flow without resistance. To figure out what that meant for a nearby E. coli, they used a standard problem-solving trick. As it churns away near the perfect-slip boundary, the single bacterium makes the water flow in exactly the same way it would if the water continued past the surface and an identical-but-opposite bacterium were swimming on the other side, like a reflection in a mirror. The swirling tail of such an "image bacterium" would produce a sideways flow in the vicinity of the tail of real E. coli that would pull the real microbe's flagellum with it in the direction opposite to the push felt near a solid surface. So the real bacterium circles to the left. (College physics textbooks use similar reasoning to explain why charged particles are attracted to a metal surface.)
Researchers had expected that changing the type of surface would change the direction of turning, but "it's the first time that [the reversal] has been observed and the first time it's been explained in terms of a whole image bacteria," Di Leonardo says. Even with the image technique, the hydrodynamic problem is mathematically demanding, says Eric Lauga, the biophysicist and applied mathematician at the University of California, San Diego, who led the analysis of the clockwise turning in the 2006 paper. "They do a great job of quantifying the experiment, which is not a trivial thing to do," he says.
Di Leonard notes that by patterning them on the nanometer scale, researchers can now make surfaces either stickier or slipperier to liquids, so the two effects might serve to steer bacteria through tiny devices. Conversely, Lauga suggests, researchers could get useful information about an unknown surface by plopping E. coli down on it and watching which way microbes swim.