Imagine you and your friends are trapped by a super-villain in a cage. There is a giant gear with a diameter half the length of a football field in the center. The only way to open the cage door, get out, and stop the villain’s evil plans will be to rotate this gear by one full revolution. This is a daunting task for one person — but if you have enough friends, you can grab the gear’s teeth and push it together to escape. An analogous task is faced by flocks of tiny bacteria in today’s two featured papers. In “Bacterial ratchet motors”, Di Leonardo and colleagues discuss the mechanics of bacteria pushing a single gear, and in “Swimming bacteria power microscopic gears”, Sokolov and colleagues discuss how bacteria can interact to power more than one gear.
Two types of bacteria were used in these studies — B. Subtilis and a harmless strain of E. Coli. A single bacterium is tiny, with a pill-shaped cell body only a couple of microns in length. One bacterium has no hope of pushing a gear one hundred times its size. It swims around in a random, “run-and-tumble” motion. During a “run” the bacterium swims straight. It then stops and “tumbles”, changing its direction randomly, and then swims straight, or “runs” for a while longer. While bacteria swimming together in large aggregations can align and make interesting flow patterns, up to now their motion has been hard to harness to provide useful work. If this technique were perfected, bacteria-powered gears could be used to power micro-devices, such as very small robots, without using an external power source.
The bacteria used in both studies swam in a liquid medium, which contained the nutrients and oxygen that they need to survive, together with one or two gears. In both of today’s articles, the gears were resting on the bottom of the liquid medium suspended above air. In Di Leonardo’s study, the drop of medium hung from a concave part of a glass slide with 48-80 micron diameter gears; in Sokolov’s study, the medium was stretched in a film between two wires with 380-micron diameter gears. The two setups are shown in Figure 1.
A swarm of bacteria can’t push just any kind of gear. Di Leonardo and colleagues show that if the gear is symmetric, the bacteria can’t rotate it. In this case, there will be an equal chance of bacteria pushing on the left and the right of the gear tooth, not generating an overall rotation. To generate continuous spinning, more bacteria need to push on one side of the tooth than the other. To achieve this, the gears had asymmetric teeth, as in Figure 2a. When bacteria swim towards the corner (like the left bacterium in Figure 2a), they get stuck in the corner. The bacteria can’t escape by swimming straight, so they rotate the gear until “tumbling” and breaking free. When bacteria encounter a tooth while swimming away from the corner (like the right bacterium in Figure 2a), they swim straight off of it. This way, the gear only rotates in one direction. When several bacteria are trapped in the same corner, they spontaneously align and push the gear together, as shown in Figure 2b. This results in a larger force on the gear. The rotation of a single gear is shown in Figure 2c.
Sokolov and colleagues investigated different shapes and arrangements of gears. They showed that gears with teeth either on the inside or on the outside will rotate, as in Figure 3, A-H. They then added a second gear for bacteria to spin. If two gears are close enough to each other, then their teeth ‘catch’ as in Figure 3, I and J.
Bacteria turning a gear are an example of a non-equilibrium system. A system at equilibrium doesn’t consume any energy and doesn’t produce useful work. This might be surprising, but if a gear was placed in an equilibrium system, such as atoms in a gas, it would never rotate. An atom encountering a wall or a corner of a gear will simply bounce off, and the net torque produced by all the atoms bouncing off the gear is zero, no matter what shape it is. The difference between atoms in a gas and bacteria in a fluid is that bacteria have their own internal source of energy, and hence are not at thermodynamic equilibrium. A “running” bacterium will not just bounce off of the wall of a gear corner. Instead, its swimming will rotate the gear by the corner until the next time the bacterium “tumbles” and reorients.
Are gears rotated by bacteria actually a useful system? Sokolov and colleagues estimate that the power generated by the bacteria is watts. Most electronic components, such as the ones in a cell phone, require power on the order of watts. These bacteria are not — as yet — generating nearly enough power for real-world machines. Although the rotation of the gear is not powerful enough to be useful, it is amazing that such small creatures are able to do so at all.