Robot E. Coli

19 Apr 2016

The ‘Robot E. Coli’ project demonstrates how bacteria swim. It was one of several demonstration experiments which formed “The Bacterial Circus” at the Edinburgh International Science Festival. The video show the swimmers in action.

Most bacteria live in an environment that is mostly water. Due to their small size, bacteria experience water very differently from the way we do. Large swimmers, such as fish, typically swim by wagging a tail fin from side to side. For large animals this works, because their inertia propels them along, even if the motion of the tail fin stops. Large animals are in the high Reynolds number regime, where inertia dominates friction. A single kick will propel you through the water. Microscopic animals such as bacteria, are in the low Reynolds number regime, where friction dominates inertia. This means that bacteria, cannot coast on their momentum. A consequence of living at low Reynolds number is that flapping a tail does not lead to net motion: for every flap to the left causing the animal to move forward, the flap to the right causes it to move backward by exactly the same amount. This is why bacteria do not have a fish-like tail, but rather a bundle of hair-like appendages, called flagella, that they spin around in a cork-screw like manner. The cork-screw motion breaks the time-reversal symmetry as it only turns one way allowing the bacteria to move. This feature of low Reynolds number swimmers was explored by E.M. Purcell in 1977 and he formulated the ‘Scallop Theorem’ based on his findings.

We drew inspiration from Taylor’s film ‘Low Reynolds Number Flow' made in the 1960s, in which he used life-sized model bacteria driven by rubber bands. For our project, we used two models: a life-sized fish and an E. coli bacterium with a body about 10 cm in length (compared to the 2 micrometers of an actual E. coli). Both are battery powered. The fish is a simple bath toy, whereas the E. coli model is home built. Our model E. coli consists of an electric motor, housed in a neutrally buoyant and water-tight container, which rotates a metal helix that represents the cork-screw formed by the flagella. As our model E. coli is much larger than its real-world counterpart, it is placed in a highly viscous fluid (glycerol) to ensure that it is in the low Reynolds number regime. Here, it is shown to move slowly, but surely from one end of the ‘aquarium’ to the other. For comparison, we show that the fish moves fine in water, but is completely stationary in the glycerol, despite flapping its tail. This nicely illustrates the point made by Taylor and Purcell